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A more reliable design for biodiversity study?
Abstract
Naeem and Li present the results of a microcosm study in which species
diversity of organisms within trophic groups was varied. They conclude
that the existence of multiple species within these groups enhanced the
``reliability'' of these systems, that is, the increased likelihood of a
consistent level of performance over a given unit of time. But there are
problems with their study. For the least diverse communities, one
predator species was randomly selected from a selection of two, one
autotroph from a selection of three, one consumer of bacteria from a
selection of five, and one omnivore from a selection of six. Meanwhile,
with the most diverse communities, both predator species and all three
autotroph species were used, three consumer bacteria were chosen from
the five and three omnivores from the six.
... These studies have often been unable to distinguish between alternative mechanisms to explain possible biodiversity effects identified in the experiment. This problem has created a lively discussion among ecologists on what constitutes a biodiversity effect, what mechanisms may be responsible for such effects if they exist, and what experimental design can adequately detect these mechanisms (e.g., André et al. 1994, Givnish 1994, Aarssen 1997, Garnier et al. 1997, Grime 1997, Huston 1997, Wardle et al. 1997, Doak et al. 1998, Hector 1998, Hodgson et al. 1998, Lawton et al. 1998, Loreau 1998, 2000a, Tilman et al. 1998, Wardle 1998, Allison 1999, Naeem 1999, Schläpfer and Schmid 1999, Van der Heijden et al. 1999, Hector et al. 2000, Hulot et al. 2000, Petchey 2000, Tilman 2000. ...
... As one such mechanism, we focus on the degree of similarity in species composition among local communities (hereafter ''similarity'' unless otherwise specified; conceptually the inverse of Whittaker's [1972] beta di-versity; see also Loreau 2000b). The potential effect of similarity on ecosystem functioning has only recently begun to be recognized (Huston 1997 [''variance reduction effect''], Tilman et al. 1997, Wardle 1998, Tilman 1999 and has not been well investigated conceptually. We attempt to show that similarity has important implications for understanding the relationship between biodiversity and ecosystem functioning. ...
... We show that the experiment by Naeem and Li (1997) provides at least circumstantial evidence for the hypothesis. Li (1997, 1998) and Wardle (1998) recently discussed the interpretation of the results from Naeem and Li's (1997) microcosm experiment originally designed to test the insurance hypothesis. Naeem and Li varied S/F in their microcosms, measured ecosystem reliability, and concluded that their results supported the insurance hypothesis. ...
As a potential mechanism to explain how biodiversity loss may influence variability in ecosystem functioning, we examine the hypothesis that biodiversity loss lowers similarity in species composition among local communities and that this decreased similarity in turn lowers ecosystem reliability. Ecosystem reliability refers to the probability that a system will provide a consistent level of performance over a given unit of time. This hypothesis is compared with other hypotheses that make similar predictions, including the sampling effect, insurance, and resource use complementarity hypotheses. We provide evidence for the similarity hypothesis through a reanalysis of a recent experiment and show that a key assumption of the hypothesis may be robust through computer simulations. We also address problems and possible solutions regarding how to separately test the similarity and other hypotheses in biodiversity experiments.
... Initial research claimed to demonstrate benefits to ecosystem function from higher biodiversity (Tilman & Downing 1994; Naeem & Li 1998; Griffiths et al. 2000; Loreau 2001; Loreau et al. 2001; Bengtsson 2002; Lynch 2002; Aoki 2003). However, the positive correlation between diversity and resilience (and stability) already received criticisms in the 70s: May, in 1973 observed that an ecosystem depending on more species would be less stable (May 1988; Givnish et al. 1994; Andren et al. 1995; Ulanowicz 2003). ...
... Ecosystems are resilient when ecological interactions reinforce one another and dampen disruptions. Such situations of " biological insurance " may arise due to compensation when a species with an ecological function similar to another species (redundant species) increases in abundance as the other declines (Holling 1996a; Naeem & Li 1998; Peterson et al. 1998). The observation that ecological functions of different species can overlap gave rise to new models. ...
... The insurance hypothesis proposes that biodiversity provides insurance or a buffer against environmental fluctuations, because different species respond differently to these fluctuations, leading to more predictable aggregate communities or ecosystem properties. In this sense, species that are redundant for an ecosystem at a given time may not be redundant at a later point in time (Naeem & Li 1998; Loreau et al. 2001; Hastings 2004; Loreau 2004). Nevertheless, this hypothesis does not infer that diversity actively promotes stability (Loreau 2004). ...
In this paper we aim at summarizing the current definitions of resilience in systems ecology with particular attention towards microbial systems. The recent advances of biomolecular techniques have provided scientists with new tools to investigate these systems in greater detail and with higher resolution. Therefore existing concepts and hypotheses have been revisited and discussed with respect to their applicability for ecosystems ruled by microbial processes. This review has also led to some reflections on the suitability of the term "resilience" as a general goal in environmental policies.
... However, community composition at the time the experiment was set up was also more similar for replicates of the higher than the lower diversity treatments (as a consequence of "sampling effect"). This makes the outcome that they observed inevitable, and we believe that this invalidates the results of their study (Wardle 1998). ...
... Experimental research on the relationship between biodiversity and ecosystem functioning is less than a decade old, and publications of experimental results appeared only in 1994 (Naeem et al. 1994, Tilman andDowning 1994). Wardle et al. are concerned primarily with recent exchanges among authors , Huston 1997, Wardle et al. 1997b, Wardle 1998, Naeem 1999, Wardle 1999. Although known to Wardle et al., their response did not reference replies to these opinions and other related issues (Naeem et al. 1995, Allison et al. 1996, Tilman 1997, Tilman et al. 1997b, Doak et al. 1998, Hector 1998, Lawton et al. 1998, Loreau 1998, Naeem and Li 1998, Allison 1999. ...
... Wardle et al. feel that this exchange is not reflected in the Issues in Ecology brochure on biodiversityecosystem functioning (henceforth, the BD-EF Issues). The majority of articles Wardle et al. refer to , Wardle et al. 1997b, Wardle 1998, Naeem 1999) are commentaries. Most of these are short (1-4 pages) opinion pieces that express concerns over interpretations of findings. ...
... Despite these theoretical advances, there were a number of potential issues with the design of the early experiments that could not be ruled out a posteriori, e.g. that monocultures were available for only a subset of species, or that some covariates had not been measured, such as initial soil conditions, so that their influence on the results could not be tested (Huston 1997(Huston , 2000Doak et al. 1998;Wardle 1998;Schmid et al. 2002;Schmid & Hector 2004). The critiques of the early experiments have been important promoters of more refined studies and methods of analysis, and have led to higher awareness of potential artefacts . ...
... In contrast to the main experiment, species identity effects could be tested in the dominance experiment where all of the nine species and all possible species pairs were equally represented at all diversity levels. The dominance experiment showed that strong effects of individual species, and an overall biodiversity effect, do not need to be mutually exclusive, or be confounded such that the biodiversity effect becomes an artefact of the higher probability of a dominant (and productive) species to occur in more diverse communities, as has been suggested repeatedly (Wardle 1998). In the dominance experiment, five species had higher, and four species had lower observed than expected relative yields averaged across all mixtures. ...
In the past two decades, a large number of studies have investigated the relationship between biodiversity and ecosystem functioning, most of which focussed on a limited set of ecosystem variables. The Jena Experiment was set up in 2002 to investigate the effects of plant diversity on element cycling and trophic interactions, using a multi-disciplinary approach. Here, we review the results of 15 years of research in the Jena Experiment, focussing on the effects of manipulating plant species richness and plant functional richness. With more than 85,000 measures taken from the plant diversity plots, the Jena Experiment has allowed answering fundamental questions important for functional biodiversity research.
First, the question was how general the effect of plant species richness is, regarding the many different processes that take place in an ecosystem. About 45% of different types of ecosystem processes measured in the ‘main experiment’, where plant species richness ranged from 1 to 60 species, were significantly affected by plant species richness, providing strong support for the view that biodiversity is a significant driver of ecosystem functioning. Many measures were not saturating at the 60-species level, but increased linearly with the logarithm of species richness. There was, however, great variability in the strength of response among different processes. One striking pattern was that many processes, in particular belowground processes, took several years to respond to the manipulation of plant species richness, showing that biodiversity experiments have to be long-term, to distinguish trends from transitory patterns. In addition, the results from the Jena Experiment provide further evidence that diversity begets stability, for example stability against invasion of plant species, but unexpectedly some results also suggested the opposite, e.g. when plant communities experience severe perturbations or elevated resource availability. This highlights the need to revisit diversity–stability theory.
Second, we explored whether individual plant species or individual plant functional groups, or biodiversity itself is more important for ecosystem functioning, in particular biomass production. We found strong effects of individual species and plant functional groups on biomass production, yet these effects mostly occurred in addition to, but not instead of, effects of plant species richness.
Third, the Jena Experiment assessed the effect of diversity on multitrophic interactions. The diversity of most organisms responded positively to increases in plant species richness, and the effect was stronger for above- than for belowground organisms, and stronger for herbivores than for carnivores or detritivores. Thus, diversity begets diversity. In addition, the effect on organismic diversity was stronger than the effect on species abundances.
Fourth, the Jena Experiment aimed to assess the effect of diversity on N, P and C cycling and the water balance of the plots, separating between element input into the ecosystem, element turnover, element stocks, and output from the ecosystem. While inputs were generally less affected by plant species richness, measures of element stocks, turnover and output were often positively affected by plant diversity, e.g. carbon storage strongly increased with increasing plant species richness. Variables of the N cycle responded less strongly to plant species richness than variables of the C cycle.
Fifth, plant traits are often used to unravel mechanisms underlying the biodiversity–ecosystem functioning relationship. In the Jena Experiment, most investigated plant traits, both above- and belowground, were plastic and trait expression depended on plant diversity in a complex way, suggesting limitation to using database traits for linking plant traits to particular functions.
Sixth, plant diversity effects on ecosystem processes are often caused by plant diversity effects on species interactions. Analyses in the Jena Experiment including structural equation modelling suggest complex interactions that changed with diversity, e.g. soil carbon storage and greenhouse gas emission were affected by changes in the composition and activity of the belowground microbial community. Manipulation experiments, in which particular organisms, e.g. belowground invertebrates, were excluded from plots in split-plot experiments, supported the important role of the biotic component for element and water fluxes.
Seventh, the Jena Experiment aimed to put the results into the context of agricultural practices in managed grasslands. The effect of increasing plant species richness from 1 to 16 species on plant biomass was, in absolute terms, as strong as the effect of a more intensive grassland management, using fertiliser and increasing mowing frequency. Potential bioenergy production from high-diversity plots was similar to that of conventionally used energy crops. These results suggest that diverse ‘High Nature Value Grasslands’ are multifunctional and can deliver a range of ecosystem services including production-related services.
A final task was to assess the importance of potential artefacts in biodiversity–ecosystem functioning relationships, caused by the weeding of the plant community to maintain plant species composition. While the effort (in hours) needed to weed a plot was often negatively related to plant species richness, species richness still affected the majority of ecosystem variables. Weeding also did not negatively affect monoculture performance; rather, monocultures deteriorated over time for a number of biological reasons, as shown in plant-soil feedback experiments.
To summarize, the Jena Experiment has allowed for a comprehensive analysis of the functional role of biodiversity in an ecosystem. A main challenge for future biodiversity research is to increase our mechanistic understanding of why the magnitude of biodiversity effects differs among processes and contexts. It is likely that there will be no simple answer. For example, among the multitude of mechanisms suggested to underlie the positive plant species richness effect on biomass, some have received limited support in the Jena Experiment, such as vertical root niche partitioning. However, others could not be rejected in targeted analyses. Thus, from the current results in the Jena Experiment, it seems likely that the positive biodiversity effect results from several mechanisms acting simultaneously in more diverse communities, such as reduced pathogen attack, the presence of more plant growth promoting organisms, less seed limitation, and increased trait differences leading to complementarity in resource uptake. Distinguishing between different mechanisms requires careful testing of competing hypotheses. Biodiversity research has matured such that predictive approaches testing particular mechanisms are now possible.
... Further merits of the presented analyses (and data sets) become evident in the light of the harsh criticism that research focused on the biodiversity-productivity relationship has received. Major issues of concern have been the artificiality and immaturity of assembled plant communities and the possibility to create artefacts through an inappropriate experimental design (Givnish 1994;Garnier et al. 1997;Doak et al. 1998;Wardle 1998;Thompson et al. 2005). In the Jena Experiment, the large plot size and the long time scale reduce the artificiality of these created grassland plots. ...
... However, I observed consistently positive net biodiversity effects which clearly rejected the hypothesis of biodiversity effects being transient (Thompson et al. 2005 Tilman et al. 1996;, they were criticized for not allowing a separation of the effects of particular species traits (e.g. nitrogen fixation by legumes) from effects of species interactions Wardle 1998) or a rigid test of the occurrence of transgressive overyielding (Garnier et al. 1997). In the Jena Experiment, these issues were tackled by a careful combination of species richness levels with different functional group compositions. ...
Biodiversity is declining world-wide due to land-use change, urbanization, global warming and other anthropogenic transformations of the environment. Accumulating empirical evidence suggests that this ongoing pauperization of ecosystems impairs ecosystem functioning and thereby threatens human well-being. For assessing the consequences of species extinctions as well as for a prioritization of conservation efforts, a thorough understanding of the relationships between biodiversity and ecosystem functioning is required. In the past, numerous experiments have shown that an increase in biodiversity usually enhances community productivity but we are only beginning to understand why. In this thesis, I used data from a large-scale grassland biodiversity experiment (Jena Experiment) to explore mechanisms underlying positive relationships between plant diversity and aboveground primary productivity.
... However a number of problems about the experimental design were highlighted, for example the ecosystem functioning effects observed could be influenced by the particular size and species of the plants chosen rather than diversity per se. This was shown in the experiment by including fast growing plant species that were only present in high-diversity systems and not present in the low-diversity systems; this has been named the 'selection probability effect' (Grime 1997;Wardle 1998). Therefore it could be explained that the high production shown in species rich assemblages could be due to these fast growing plants (Andre et al 1994). ...
... Therefore it could be explained that the high production shown in species rich assemblages could be due to these fast growing plants (Andre et al 1994). The experimental design also suffered from pseudoreplication, as sub-sets of species were sampled within bigger sets, this resulted in lower levels of diversity being nested in sets of higher diversity levels (Wardle 1998;Fukami et al 2001). ...
... Recognition that loss of species may affect the functioning of ecosystems has led to an impressive accumulation of literature on the effects of diversity loss over the last 10 yr. This work has revealed much about biotic controls on ecosystem functioning, especially on productivity (Naeem et al. 1995, Tilman et al. 1996, Symstad et al. 1998, Hector et al. 1999, Engelhardt and Ritchie 2001) and nutrient retention (Tilman et al. Manuscript received 12 September 2001; revised 4 February 2002; accepted 13 February 2002. 1 Present address: University of Maryland, Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, Maryland 21532-2307 USA. ...
... Potamogeton pectinatus and P. crispus can use bicarbonate when free CO 2 in the water is limiting, whereas P. nodosus has access to atmospheric CO 2 . By choosing morphologically dissimilar species with different effects on resources, we were effectively manipulating species richness and functional diversity at the same time, thereby preventing or minimizing any confounding effects between species richness and functional group richness (Vitousek and Hooper 1993, Hooper and Vitousek 1997, 1998, Huston 1997, Tilman et al. 1997a, Naeem and Li 1998). Choosing dissimilar species also allowed us to understand the diversity of species traits (Engelhardt 2000) that may produce complementary interactions among species, as well as competitive interactions that may prevent complementarity of resource use (Engelhardt and Ritchie 2001). ...
Rapid environmental changes have fostered debates and motivated research on how to effectively preserve or restore ecosystem processes. One such debate deals with the effects of biodiversity, and the loss thereof, on ecosystem processes. Recent studies demonstrate that resource-use complementarity, now known as the "niche-differentiation effect," and the presence of a competitive species with strong effects on ecosystem processes, now known as the "sampling effect," can explain why productivity and nutrient retention are sometimes enhanced with increasing species richness. In a well-replicated outdoor mesocosm experiment, we tested these and other alternative mechanisms that could explain the effects of submersed aquatic plant (macrophyte) diversity on wetland ecosystem processes. Algal biomass increased and phosphorus loss decreased as species richness increased. This result can best be explained by an indirect sampling effect caused by one of the weakest competitors, which appeared to facilitate algal growth and thereby filtering of particles, and thus phosphorus, from the water column. The dominant competitor also appeared to decrease phosphorus loss through direct effects on phosphorus availability in the soil and water. Thus, the effects by one of the weakest and the most dominant competitors combine to produce a diversity effect on phosphorus loss. Macrophyte biomass was not enhanced, but converged toward the intermediate biomass of the most competitive species. Such an "inverse sampling effect" may be produced when the most competitive species is not the most productive species owing to species-specific feedbacks and adaptations to the wetland environment. In summary, we reject the niche-differentiation effect as the dominant mechanism in our macrophyte communities and expand on the role of sampling effects in explaining the relationship between plant communities and ecosystem processes. In particular, indirect and inverse sampling effects combine to drive the relationship between species richness and wetland ecosystem processes. Thus, we demonstrate that plant diversity may affect wetland ecosystem processes when inferior competitors drive system productivity and nutrient retention. To ensure coexistence of such species with superior competitors, wetland systems may need to be maintained in a nonequilibrium state, such as with hydrologic disturbances, which would maintain both higher diversity and enhance ecosystem functioning.
... Constructed communities offer the advantage of directly establishing target richness levels, but they have also been criticised because it can be difficult to isolate the effects of species richness from those of the identity of the particular species used in the experiment (Aarssen 1997;Huston 1997;Hodgson et al. 1998;Loreau 1998;Wardle 1998;Wardle 1999; but see Naeem et al. 1994b;Tilman 1997;Lawton et al. 1998;Naeem & Li 1998;van der Heijden et al. 1999). Communities can be constructed either randomly or nonrandomly from a species pool; choice of species for this pool is an often unrecognised source of bias. ...
... Thus, our initial scanning of titles and abstracts did not reveal the degree to which the methods used to evaluate the effects of species richness relationships have been contested. Second, as with other topics of considerable ecological interest, journals may favour studies that claim to demonstrate positive results over studies that report no definitive relationship (Wardle 1998). Such differential publication might have helped fuel our expectation that relationships between biodiversity and temporal variability have already been conclusively established. ...
The effect of biodiversity on natural communities has recently emerged as a topic of considerable ecological interest. We review studies that explicitly test whether the number of species in a community (species richness) regulates the temporal variability of aggregate community (total biomass, productivity, nutrient cycling) and population (density, biomass) properties. Theoretical studies predict that community variability should decline with increasing species richness, while population variability should increase. Many, but not all, empirical studies support these expectations. However, a closer look reveals that several empirical studies have either imperfect experimental designs or biased methods of calculating variability. Furthermore, most theoretical studies rely on highly unrealistic assumptions. We conclude that evidence to support the claim that biodiversity regulates temporal variability is accumulating, but not unequivocal. More research, in a broader array of ecosystem types and with careful attention to methodological considerations, is needed before we can make definitive statements regarding richness-variability relationships.
... 1, 20 -22 Several studies have indicated that there is a strong dependency of stability on the traits of dominant species present in a given ecosystem. 23,24 In addition, the significance of species composition and identity has been highlighted in some experiments. The presence of a particular species, for instance, nitrogen-fixing legumes, seems to cause nutrient enrichment, leading to the expression of complex responses by communities to perturbation. ...
... Diversity response may therefore originate from a combination of factors, some of which remain hidden. 1,20,23,24 Relative yield ...
The effect of species richness and spatial aggregation on the stability of community productivity in response to drought perturbation was investigated with experimental plant communities. Communities comprising all single- and three-species combinations of the ruderal species, Capsella bursa-pastoris , Tripleurospermum inodorum , Poa annua , and Stellaria media , were established in glasshouse. Habitat patchiness was manipulated by applying different seed-sowing patterns, either aggregated or random. After the establishment of communities, 8 days of drought treatment was imposed. Followed by a week of recovery with a regular watering regime, aboveground biomass was harvested. Community biomass was not affected by species richness or by aggregation, but was affected by perturbation. When multi-species community productivity was compared with monocultures in relative terms, species mixtures performed better in drought-induced conditions. This suggests that the positive effect of species richness may be enhanced under the perturbed condition. Sampling effects were evident under perturbation favouring the least productive species, P. annua and drought-tolerant S. media . All species except C. bursa-pastoris showed reduced productivity in species mixtures, but this may be mitigated under perturbed environments by species complementarity. Lack of clear responses to aggregation may suggest that the revealed diversity effect is not related to spatial structure. While competition predominates in communities in the resource-rich environment, drought perturbation enhance overall community productivity via a shift in relative significance of species interactions from competition to sampling and complementarity effects.
... All of these issues, as well as several others, have been thoroughly discussed in published comments on earlier experiments of this type (4,5,7,13,14,(17)(18)(19). We agree with Hector et al. that environmental conditions have a major effect on plant productivity and that overyielding does occur in some multispecies mixtures, particularly those containing nitrogen-fixing legumes. ...
... Our experiment was not intended to test a particular scenario; it was a general investigation of the effects of changing biodiversity. The "proper" experimental design suggested by Huston et al. does not include orders of assembly or disassembly other than the nonrandomness requirement, and is inconsistent with other suggested designs (18,19). The research they cite (their references 3 through 5) refers to particular scenarios only, and we suspect that good general evidence is currently lacking on orders of loss in relation to a broader set of extinction drivers. ...
Hector et al . ([1][1]) reported on BIODEPTH, a major international experiment on the response of plant productivity to variation in the number of plant species. They found “an overall log-linear reduction of average aboveground biomass with loss of species,” leading to what the accompanying
... O encontro resultou em um livro (Schulze & Mooney, 1993) e diversas outras publicações durante os anos 90 (e.g., , Tilman & Downing, 1994Naeem et al., 1995;Tilman et al., 1996;Chapin et al., 1997;Naeem & Li, 1997). Esses estudos, juntamente com as críticas em relação a validade das abordagens experimentais e interpretação dos resultados Huston, 1997;Wardle, 1998;, serviram de base para o refinamento não apenas do desenho experimental e dos métodos de análise empregados nos estudos B-EF, como também dos modelos propostos para explicar os mecanismos ecológicos subjacentes a relação entre a biodiversidade e os processos ecossistêmicos (Hooper & Vitousek 1997;Tilman et al., 1997;Loreau, 1998a;1998b;Hector, 1998;. ...
Given the unprecedented and growing threats to inland waters — eutrophication, cyanobacterial blooms, over-exploitation, and climate change — from multiple human activities, biodiversity is decreasing at faster rates in freshwater ecosystems than in marine or terrestrial. Since the early 1990s, hundreds of studies attempted to explain how ecosystems respond to biodiversity loss and how changes in biodiversity scale up to affect ecosystem functioning, as well as the provision of goods and services to humans. Recent studies have demonstrated that such biodiversity responses are commonly trait-mediated and the effects of communities on ecosystem functioning also depend on species traits. However, it remains unclear to what extent such biodiversity responses translate into changes in the rates of many ecosystem processes in naturally assembled communities. In this doctoral dissertation, I aimed at evaluating the effects of nutrient availability and cyanobacteria dominance on structure and composition of plankton communities (phytoplankton and zooplankton), and on two important ecosystem functions in aquatic systems: phytoplankton resource use efficiency (RUE) of limiting nutrients — phosphorus and nitrogen — and zooplankton top-down control of algae. For this, I structured this doctoral dissertation in three chapters to explore the mechanisms that underlie biodiversity-ecosystem functioning (B-EF) relationships, using a combination of experimental and fieldwork approaches, together with multiple aspects of biodiversity (i.e., taxonomic and functional diversity). In the first chapter, I and my coauthors analyzed the relationship between different measures of phytoplankton diversity, temporal turnover and RUE using 8-years monitoring data set from a cyanobacteria-dominated subtropical lake, which is now experiencing a shift in the trophic state from oligo-mesotrophic to eutrophic. Additionally, we aimed at evaluating the effect of resource availability on phytoplankton community structure and RUE. In the second chapter, using 1-year monitoring data set from the same lake, we evaluated the relative importance of size-based and taxon-based approaches in explaining the strength of zooplankton top-down control on algae, and also aimed at disentangling the mechanism by which zooplankton body size drives such ecosystem function. Finally, in the third chapter, we used an experimental metacommunity approach that simulated typical gradients of productivity and plant structural complexity to test how zooplankton body size diversity and composition responded to such gradients and whether and how such trait responses impacted top-down control of algae. Through these three chapters, we demonstrated that under environmental changes (i.e., nutrient increase and prolonged cyanobacteria dominance) approaches based on body size and taxonomic richness complement each other in explaining variation in zooplankton top-down control. Our results clearly indicate that zooplankton body size explains a substantial and independent part of the variance in top-down control, which corroborates several studies demonstrating the role of zooplankton body size to control phytoplankton biomass. But contrary to our expectations, species richness also plays a role, indicating that species richness may adequately represent some unmeasured traits that also influence ecosystem functioning. Moreover, we demonstrated that different aspect of biodiversity might have divergent responses and divergent effects on ecosystem functioning depending on environmental perturbation, which highlight the importance of considering multiple aspects of biodiversity — taxonomic and functional approaches — in B-EF research. Overall, our results illustrated the potential for trait-based approaches to reveal biodiversity responses to environmental change and their generalizable effects on ecosystems. Furthermore, given the lack of large grazers in tropical and subtropical regions, and the evidence that Cyanobacteria dominance will increase in freshwater ecosystems under the predicted future climate, the results herein highlight the concern about the energy flow in aquatic systems dominated by Cyanobacteria.
... As a consequence, experiments that manipulate species richness (the number of species) and measure ecosystem functioning (e.g., productivity) have become common over the past decade (Balvanera et al. 2006;Cardinale et al. 2006). The original experiments in the field generated some controversy surrounding the design and statistical analysis of the experiments and the degree to which mechanisms can be inferred from the results (Aarssen 1997;Huston 1997;Tilman et al. 1997;Wardle 1998Wardle , 1999Huston et al. 2000). When the species richness of a community is manipulated, it is not independent of the manipulation of the presence or absence of particular species in the community (Schmid et al. 2002a). ...
Experiments that manipulate species richness and measure ecosystem functioning attempt to separate the effects of species richness (the number of species) from those of species identity. We introduce an experimental design that ensures that each species is selected the same number of times at each level of species richness. In combination with a linear model analysis, this approach is able to unambiguously partition the variance due to different species identities and the variance due to nonlinear species richness, a proxy measure for interactions among species. Our design and analysis provide several advantages over methods that are currently used. First, the linear model method has the potential to directly estimate the role of various ecological mechanisms (e.g., competition, facilitation) rather than the consequences of those mechanisms (e.g., the “complementarity effect”). Second, unlike other methods that are currently used, this one is able to estimate the impact of diversity when the contribution of individual species in a mixture is unknown.
... Andre et al. sumptions may be difficult to defend. A study utilising (1994), Givnish (1994), Huston (1997), Hodgson et al. a design in which SE is likely to influence the outcome (1998), Wardle (1998)l. The only other major published would therefore need to demonstrate, at a minimum, experimental studies making similar claims about the that the community is assembled at random with regard positive effects of diversity either report further data to the relative contribution of the different species to from one of these six studies, or report additional the function being investigated, as well as other func-experiments designed to complement one of these studtions likely to be important in driving that function. ...
Increasingly, those studies which are aiming to study the relationship between biodiversity and ecosystem function are utilising experimental designs in which species diversity is varied, with the species composition at each level of diversity being determined randomly from a predetermined species pool. Studies utilising such designs have been criticised on the basis that they are confounded by 'sampling effect' (SE) or 'selection probability effect', i.e. that the treatments which have the highest diversity have a greater probability of being dominated by the most productive species of the entire species pool; however it has also been claimed that SE is a legitimate mechanism by which diversity effects may express themselves in nature. Firstly I show, using an example of a recently published study claiming to show a diversity effect, how SE can result in the identification of apparent relationships between diversity and ecosystem properties which have little meaning in the real world. I then point out that if we accept SE is a diversity mechanism operating in nature, it is firstly necessary to assume that biological communities are randomly assembled with regard to the ecosystem property being measured; this assumption is inconsistent with conventional concepts about how biological communities are organised. Finally I discuss other experimental approaches which may remove the likelihood of results of biodiversity studies from being confounded by problems associated with SE. None of those studies in which SE may contribute to the observed outcome have successfully shown a result which cannot be ascribed to artifact, and alternative experimental approaches are required in order to better understand how biodiversity loss affects ecosystems in nature.
... One part of the critique is on experimental designs (e.g. Huston 1997, Naeem and Li 1998, Wardle 1998, Allison 1999) and what actually is tested when the number of species is manipulated -is it species richness per se, effects of particular species, or something in between? One could argue that using non-random assemblages merely tests for species richness effects among the organisms used, but at the same time, if all possible combinations are investigated, it allows for statistical separation of species identity and species richness effects (e.g. ...
The work in this thesis deals with effects of changed species richness on process rates among stream-living macroinvertebrates. Global biodiversity is decreasing rapidly and it is poorly known what the consequences of this loss may be for ecosystems and the services they provide. Hence, it is important to investigate the potential effects of losing species. In streams, deforestation, introduction of non-native species, pollution and channelization are examples of events that may affect species richness negatively. In this thesis emphasis is on changes in species richness within functional feeding groups (FFGs) of stream-living macroinvertebrates. The FFGs used were shredding detritivores, grazers, filter feeders and predators - all of which uphold important ecological processes in streams. Along with an observational field study, species richness was manipulated in laboratory and field experiments to investigate the effects of changed species richness on process rates and thus ecosystem functioning.
The results show that effects of changed species richness on process rates may be dramatic. Among the shredding detritivores there were negative effects on leaf mass loss, regardless whether fixed, random or predicted sequences of species loss was investigated. These effects could be attributed to either species richness per se or species composition. However, among the other FFGs the relationship between species richness and process rates was less consistent. In filter feeders, there was no or a negative effect of decreasing species richness while both grazers and predators showed positive effects of species loss.
The results also show that the most important interactions between species in an experiment, thus potentially in a natural community, are likely to determine what the effect of species loss on process rates will be. Facilitation and niche differentiation lead to reduced process rates if species are lost, while mechanisms, such as inter- specific resource or interference competition, produce the opposite effect. Furthermore, in systems with a diminishing resource, the first two mechanisms may become more important over time enhancing the effect of species loss in the long term.
In conclusion, effects of species loss may be dramatically negative or positive even if lost species are classified as redundant. The effect in the short term most likely depends on which species are lost, on the original species composition and on the underlying mechanisms. Questions remaining to be answered are how important the observed effects are in more complex systems and if they are persistent over time? Future studies will tell.
... Because most biotic groups in soil, and food webs as a whole, generally have a species richness much greater than 10, we chose this to define low compared with high-diversity manipulations. We note that there is considerable debate about the benefits and drawbacks of various experimental designs employed in biodiversity research (Naeem & Li, 1998;Wardle, 1998;Hector et al., 2000;Huston et al., 2000;Loreau et al., 2001;Swift et al., 2004), and that most, if not all, of the biodiversity papers that we cite have some drawbacks, such as potential 'hidden' treatments (Huston, 1997). However, scrutinizing the design of soil biodiversity experiments is beyond the scope of our paper. ...
Biodiversity and carbon (C) cycling have been the focus of much research in recent decades, partly because both change as a result of anthropogenic activities that are likely to continue. Soils are extremely species-rich and store approximately 80% of global terrestrial C. Soil organisms play a key role in C dynamics and a loss of species through global changes could influence global C dynamics. Here, we synthesize findings from published studies that have manipulated soil species richness and measured the response in terms of ecosystem functions related to C cycling (such as decomposition, respiration and the abundance or biomass of decomposer biota) to evaluate the impact of biodiversity loss on C dynamics. We grouped studies where one or more biotic groups had been manipulated to include a richness of ≤10 species or >10 species in order to reflect ‘low’ and ‘high’ extents of diversity manipulations. There was a positive relationship between species richness and C cycling in 77–100% of low-diversity experiments, even when the richness of just one biotic group was manipulated, whereas positive relationships occurred less frequently in studies with greater richness (35–64%). Moreover, when positive relationships were observed, these often indicated functional redundancy at low extents of diversity or that community composition had a stronger influence on C cycling than did species richness. Initial reductions in soil species richness resulting from global changes are unlikely to alter C dynamics significantly unless particularly influential species are lost. However, changes in community composition, and the loss of species with an ability to facilitate specialized soil processes related to C cycling, as a result of global changes, may have larger impacts on C dynamics.
... Exactly what form the relationship should take, if any, and what approach(es) should be adopted to reveal its mechanisms are at the heart of the debate. Results from previous studies are varied, with some finding significant positive relationships (Naeem et al. 1994, 1996, Tilman et al. 1996, Hector et al. 1999, Troumbis and Memtsas 2000, Loreau and Hector 2001a, b) and others finding negative (Rusch and Oesterheld 1997, Wardle et al. 1997b, Grime 1998) or inconsistent relationships (Hooper and Vitousek 1997, Wardle et al. 1997a, Hooper 1998, Kenkel et al. 2000). Studies demonstrating a positive relationship have received the most attention (Wardle 1999), perhaps because of their implications for conservation efforts (Hector et al. 2001), and the appealing notion that the potential benefits of high biodiversity (e.g. ...
Using a habitat templet model, we predict that the productivity (total biomass) of plots within a plant community may be positively, negatively or not at all related to variation in the number of species per plot, depending on successional stage (time since major disturbance) and habitat carrying capacity (reflect- ing the total resource supplying power of the habitat). For plots of a given size, a positive relationship between productivity and species richness is predicted in recently disturbed habitats be- cause local neighbourhoods here will have been assembled largely stochastically, usually from a pool of available species with a right-skewed size frequency distribution. Hence, in the earliest stages of succession, plots will have relatively high total biomass only if they contain at least some of the relatively uncommon larger species which will, in turn, be more likely in those neighbourhoods that contain more species (the sampling effect). Among these will also be some of the more common smaller species; hence, these high biomass, species-rich plots should have relatively low species evenness, in contrast to what is predicted under effects involving species complementarity. In late succession, the plots with high total biomass will still be those that contain relatively large species but these plots will now contain relatively few species owing to increased competitive exclusion over time (the competitive dominance effect). In inter- mediate stages of succession, no relationship between plot pro- ductivity and species richness is predicted because the opposing sampling and competitive dominance effects cancel each other out. We predict that the intensity of both the sampling and competitive dominance effects on the productivity/species rich- ness relationship will decrease with decreasing habitat carrying capacity (e.g. decreasing substrate fertility) owing to the inher- ently lower variance in between-plot productivity that is pre- dicted for more resource-impoverished habitats.
... Some authors have indicated that species composition (the consequence of species' biology) controls ecosystem functioning (Huston 1997;Huston et al. 2000;Grime 1997;Wardle 1998;Tilman et al. 1997a;Tilman 1999). Soil moisture content was highest in the Kobresia tibetica swamp meadow (the lowest-altitude site); excessive water would lead to decreased spatial heterogeneity in nutrient resources, to decreased spatial complexity in resource ratios and to increased intraspecific and interspecific competition. ...
During the growing seasons of 2002 and 2003, biomass productivity and diversity were examined along an altitudinal transect on the south-western slope of Beishan Mountain, Maqin County (33 degrees 43'-35 degrees 16'N, 98 degrees 48'-100 degrees 55'E), Qinghai-Tibetan Plateau. Six altitudes were selected, between 3840 and 4435 m. Soil organic matter, soil available N and P and environmental factors significantly affected plant-species diversity and productivity of the alpine meadows. Aboveground biomass declined significantly with increasing altitude (P < 0.05) and it was positively and linearly related to late summer soil-surface temperature. Belowground biomass (0 - 10-cm depth) was significantly greater at the lowest and highest altitudes than at intermediate locations, associated with water and nutrient availabilities. At each site, the maximum belowground biomass values occurred at the beginning and the end of the growing seasons (P < 0.05). Soil organic matter content, and available N and P were negatively and closely related to plant diversity (species richness, Shannon-Wiener diversity index, and Pielou evenness index).
... While species richness or HЈ was statistically significant in all these experiments, species composition (where investigated) had at least an equally strong effect on stability. In some experiments, effects of diversity on temporal variability via compensation or portfolio effects were confounded with effects of compositional similarity among replicates at higher levels of diversity (Wardle 1998). The correlation between compositional similarity and species richness may resemble situations resulting from species loss in real communities (Naeem 1998, Fukami et al. 2001), but determining mechanisms responsible for patterns of ecosystem response becomes problematic. ...
... These studies have considerably advanced our understanding of the role of biological diversity in maintaining fundamental ecological processes. While generating novel understanding, experiments have also drawn attention to the difficulties inherent in identifying the underlying causal mechanisms associated with changes in biodiversity (Aarssen 1997;Huston 1997;Wardle 1999aWardle , 1999b. A widely discussed problem is that of separating effects due to changes in number of species (or other taxonomic categorizations) from effects due to changes in identity of species. ...
Manipulative experiments are often used to identify causal linkages between biodiversity and productivity in terrestrial and aquatic habitats.
Most studies have identified an effect of biodiversity, but their interpretation has stimulated considerable debate. The main difficulties lie in separating the effect of species richness from those due to changes in identity and relative density of species.
Various experimental designs have been adopted to circumvent problems in the analysis of biodiversity. Here I show that these designs may not be able to maintain the probability of type I errors at the nominal level (α = 0·05) under a true null hypothesis of no effect of species richness, in the presence of effects of density and identity of species.
Alternative designs have been proposed to discriminate unambiguously the effects of identity and density of species from those due to number of species. Simulations show that the proposed experiments may have increased capacity to control for type I errors when effects of density and identity of species are also present. These designs have enough flexibility to be useful in the experimental analysis of biodiversity in various assemblages and under a wide range of environmental conditions.
... There are two ways to resolve this apparent contradiction: either the experiments are flawed, or the theory is inadequate to explain such results. There are problems with the experiments (Givnish 1994, Huston 1997, Wardle 1998). In particular, no experiment so far has controlled temporal variability or perturbations directly . ...
The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today. As experiments progress, there is a growing need for adequate theories and models to provide robust interpretations and generalisations of experimental results, and to formulate new hypotheses. This paper provides an overview of recent theoretical advances that have been made on the two major questions in this area: (1) How does biodiversity affect the magnitude of ecosystem processes (short‐term effects of biodiversity)? (2) How does biodiversity contribute to the stability and maintenance of ecosystem processes in the face of perturbations (long‐term effects of biodiversity)?
Positive short‐term effects of species diversity on ecosystem processes, such as primary productivity and nutrient retention, have been explained by two major types of mechanisms: (1) functional niche complementarity (the complementarity effect), and (2) selection of extreme trait values (the selection effect). In both cases, biodiversity provides a range of phenotypic trait variation. In the complementarity effect, trait variation then forms the basis for a permanent association of species that enhances collective performance. In the selection effect, trait variation comes into play only as an initial condition, and a selective process then promotes dominance by species with extreme trait values. Major differences between within‐site effects of biodiversity and across‐site productivity–diversity patterns have also been clarified. The local effects of diversity on ecosystem processes are expected to be masked by the effects of varying environmental parameters in across‐site comparisons.
A major reappraisal of the paradigm that has dominated during the last decades seems necessary if we are to account for long‐term effects of biodiversity on ecosystem functioning. The classical deterministic, equilibrium approaches to stability do not explain the reduced temporal variability of aggregate ecosystem properties that has been observed in more diverse systems. On the other hand, stochastic, nonequilibrium approaches do show two types of biodiversity effects on ecosystem productivity in a fluctuating environment: (1) a buffering effect, i.e., a reduction in the temporal variance; and (2) a performance‐enhancing effect, i.e., an increase in the temporal mean. The basic mechanisms involved in these long‐term insurance effects are very similar to those that operate in short‐term biodiversity effects: temporal niche complementarity, and selection of extreme trait values. The ability of species diversity to provide an insurance against environmental fluctuations and a reservoir of variation allowing adaptation to changing conditions may be critical in a long‐term perspective.
These recent theoretical developments in the area of biodiversity and ecosystem functioning suggest that linking community and ecosystem ecology is a fruitful avenue, which paves the way for a new ecological synthesis.
... Second, the design of diversity manipulations can influence interpretation of results (Huston 1997, Wardle 1998, Allison 1999. We selected species randomly, within some minor constraints, in order to experimentally remove effects of composition on variability. ...
Theory and empirical results suggest that high biodiversity should often cause lower temporal variability in aggregate community properties such as total community biomass. We assembled microbial communities containing 2 to 8 species of competitors in aquatic microcosms and found that the temporal change in total community biomass was positively but insignificantly associated with diversity in a constant temperature environment. There was no evidence of any trend in variable temperature environments. Three non-exclusive mechanisms might explain the lack of a net stabilising effect of species richness on temporal change. (1) A direct destabilising effect of diversity on population level variances caused some populations to vary more when embedded in more diverse communities. (2) Similar responses of the different species to environmental variability might have limited any insurance effect of increased species richness. (3) Large differences in the population level variability of different species (i.e., unevenness) could weaken the relation between species richness and community level stability. These three mechanisms may outweigh the stabilising effects of increases in total community biomass with diversity, statistical averaging, and slightly more negative covariance in more diverse communities. Our experiment and analyses advocate for further experimental investigations of diversity-variability relations.
... It is therefore hard to evaluate whether decreasing variance is the effect of stabilizing properties of increasing species richness or an effect of the ''variance reduction effect''. It should be noted, however, that it can be ecologically relevant to compare diverse assemblages with their less-diverse counterparts (Naeem and Li 1998) since this reflects patterns of species loss observed in nature. ...
It has been proposed that biodiversity can be important for ecosystem functioning and act as an insurance against perturbations and environmental fluctuations. To date, theoretical work supports this idea but direct experimental evidence is still to some extent ambiguous and debated. The main reason for this debate – and the lack of strong empirical support – is due to unavoidable experimentally and statistically inherent variance reduction effects. Here we present the results of an experimental study that circumvents earlier hidden treatments. By random draw without replacement, we collected 180 full-sibling batches of an amphipod from a large pool of possible parents. Assembled amphipod populations with diversity levels ranging from one to ten were exposed to either a single perturbation (nutrient enrichment) or two combined perturbations (nutrient enrichment and desiccation). The results show that the variance in the number of surviving individuals decreased with increasing diversity in the combined perturbations treatment. Predictability in population survival thus seemed to be higher in more diverse assemblages. Our results, together with a simple model suggest that variance-decreasing effects can be due to actual real world statistical sampling effects of increasing diversity.
... This standardizes the SD at a common biomass in a manner that is equivalent to analysing the coefficients of variation, without being subject to inflation of ratios and controlling for differences in biomass among diversity levels. Differences in variance at different diversity levels can be due to compositional differences (McGrady-Steed et al. 1997;Naeem & Li 1998;Wardle 1998), which we do not separate out here. As such, whether reliability or stability increases or decreases with increasing diversity we cannot answer since we are not partitioning variance associated with compositional differences, only whether the variance of the plots increased or decreased with increasing diversity. ...
In an experiment that factorially manipulated plant diversity, CO2, and N, we quantified the effects of the presence of species on assemblage biomass over 10 time points distributed over 5 years. Thirteen of the 16 species planted had statistically significant effects on aboveground and/or belowground biomass. Species differed dramatically in their effects on biomass without any relationship between aboveground and below-ground effects. Temporal complementarity among species in their effects seasonally, successionally, and in response to a dry summer maintained the diversity–biomass relationships over time and may be the cause behind higher diversity plots having less variation in biomass over time. The response of plant biomass to elevated N, but not CO2, was at times entirely dependent on the presence of a single species.
... Many experiments addressed how the increase of productivity resulted from species richness of communities (Tilman 1996Tilman , 1999 Hector et al. 1999; Lawton 2000; Hector et al. 2001; Mittelbach et al. 2001; Spehn et al. 2005). However, some studies discovered that productivity was mostly controlled by species composition rather than biodiversity or how their relationship was uncertain (Grime 1997; Huston 1997; Wardle 1998 Wardle , 1999 Huston et al. 2000). Although many ecology theories believe that density is a key determinant of population productivity, e.g. the law of constant final yield (Kira et al. 1953; Harper 1977), community density has surprisingly received only little attention in regards to change of productivity; e.g. ...
Productivity of artificial grassland communities was investigated in a 6-year field experiment on the Qinghai-Tibetan Plateau.In the experiment, assemblages varying in seven species compositions and four density gradients were grown in fertilized and non-fertilized subplots. We measured biomass of sown species as an indicator of community productivity. In general, 6-years
of experiments indicated that: (i) species composition had a significant influence on community productivity. During the initial
phase of the experiment, sown density significantly affected community productivity, but the effects disappear with the increase
of grown years. This productivity increased with biodiversity increase and fertilization, while the biodiversity effects disappeared
when the influence of composition was removed. (ii) The increase of community productivity with biodiversity was resulted
from joint effects of selection and complementarity. (iii) With an increase of growth time, the selection effects become weaker
while complementarities become enhanced. Influence of density on both effects was significantly different in early stages,
but ultimately this all became insignificant. Fertilization dramatically increased the complementarity effects in all experiment
processes, but had different influences on selection effects during different experimental period.
KeywordsComplementarity effect–Selection effect–Biodiversity–Fertilization–Tibetan plateau
... In addition to assessment of the magnitude of invasion, we also found a reduction in the overall variability in the invasibility of our assemblages with diversity, which supported the similarity hypothesis (Fukami et al. 2001), i.e., an increase in predictability with diversity. Although the similarity hypothesis was interpreted as an artifact of some experimental designs (Wardle 1998), it is now considered as a widely extended phenomenon and as one of the mechanisms that could explain how biodiversity loss influences variability in ecosystem functioning (Fukami et al. 2001). Our study is one of the first experimental studies on invasibility of macroalgal assemblages using synthetic communities, a procedure already used with sessile marine animals (Stachowicz et al. 2002). ...
Climate change is driving species range shifts worldwide. However, physiological responses related to distributional changes are not fully understood. Oceanogra-phers have reported an increase in ocean temperature in the northwest Iberian Peninsula that is potentially related to the decline in some cold-temperate intertidal macroalgae in the Cantabrian Sea, namely Fucus serratus. Low tide stress could also play a role in this decline. We performed one mensurative (in situ) and two manipulative (in culture) experiments designed to evaluate the interactive eVects of some physical factors. The Wrst experiment analysed Weld response to low tide stress in marginal (mid-Cantabrian Sea and northern Portugal) versus central (Galicia) populations of F. serratus. Then a second experiment was performed that utilized either harsh or mild summer conditions of atmospheric temperature, irradiance, humidity, and wind velocity to compare the responses of individuals from one marginal and one central population to low tide stress. Finally, the combined eVect of sea temperature and the other factors was evaluated to detect interactive eVects. Changes in frond growth, maximal photosynthetic quantum yield (F v /F m), temperature, and desiccation were found. Three additive factors (solar irradiation, ocean and air tem-peratures) were found to drive F. serratus distribution, except under mildly humid conditions that ameliorated atmospheric thermal stress (two additive factors). Mid-Can-tabrian Sea temperatures have recently increased, reaching the inhibitory levels suggested in this study of F. serratus. We also expect an additive secondary contribution of low tide stress to this species decline. On the northern Portugal coast, ocean warming plus low tide stress has not reached this species' inhibition threshold. No signiWcant diVerential responses attributed to the population of origin were found. Mechanistic approaches that are designed to analyse the interactive eVects of physical stressors may improve the levels of conWdence in predicted range shifts of species.
... While species richness or HЈ was statistically significant in all these experiments, species composition (where investigated) had at least an equally strong effect on stability. In some experiments, effects of diversity on temporal variability via compensation or portfolio effects were confounded with effects of compositional similarity among replicates at higher levels of diversity (Wardle 1998). The correlation between compositional similarity and species richness may resemble situations resulting from species loss in real communities (Naeem 1998, Fukami et al. 2001), but determining mechanisms responsible for patterns of ecosystem response becomes problematic. ...
Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the relationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are structured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain. Based on our review of the scientific literature, we are certain of the following conclusions: 1)Species' functional characteristics strongly influence ecosystem properties. Functional characteristics operate in a variety of contexts, including effects of dominant species, keystone species, ecological engineers, and interactions among species (e.g., competition, facilitation, mutualism, disease, and predation). Relative abundance alone is not always a good predictor of the ecosystem-level importance of a species, as even relatively rare species (e.g., a keystone predator) can strongly influence pathways of energy and material flows. 2)Alteration of biota in ecosystems via species invasions and extinctions caused by human activities has altered ecosystem goods and services in many well-documented cases. Many of these changes are difficult, expensive, or impossible to reverse or fix with technological solutions. 3)The effects of species loss or changes in composition, and the mechanisms by which the effects manifest themselves, can differ among ecosystem properties, ecosystem types, and pathways of potential community change. 4)Some ecosystem properties are initially insensitive to species loss because (a) ecosystems may have multiple species that carry out similar functional roles, (b) some species may contribute relatively little to ecosystem properties, or (c) properties may be primarily controlled by abiotic environmental conditions. 5)More species are needed to insure a stable supply of ecosystem goods and services as spatial and temporal variability increases, which typically occurs as longer time periods and larger areas are considered. We have high confidence in the following conclusions: 1)Certain combinations of species are complementary in their patterns of resource use and can increase average rates of productivity and nutrient retention. At the same time, environmental conditions can influence the importance of complementarity in structuring communities. Identification of which and how many species act in a complementary way in complex communities is just beginning. 2)Susceptibility to invasion by exotic species is strongly influenced by species composition and, under similar environmental conditions, generally decreases with increasing species richness. However, several other factors, such as propagule pressure, disturbance regime, and resource availability also strongly influence invasion success and often override effects of species richness in comparisons across different sites or ecosystems. 3)Having a range of species that respond differently to different environmental perturbations can stabilize ecosystem process rates in response to disturbances and variation in abiotic conditions. Using practices that maintain a diversity of organisms of different functional effect and functional response types will help preserve a range of management options. Uncertainties remain and further research is necessary in the following areas: 1)Further resolution of the relationships among taxonomic diversity, functional diversity, and community structure is important for identifying mechanisms of biodiversity effects. 2)Multiple trophic levels are common to ecosystems but have been understudied in biodiversity/ecosystem functioning research. The response of ecosystem properties to varying composition and diversity of consumer organisms is much more complex than responses seen in experiments that vary only the diversity of primary producers. 3)Theoretical work on stability has outpaced experimental work, especially field research. We need long-term experiments to be able to assess temporal stability, as well as experimental perturbations to assess response to and recovery from a variety of disturbances. Design and analysis of such experiments must account for several factors that covary with species diversity. 4)Because biodiversity both responds to and influences ecosystem properties, understanding the feedbacks involved is necessary to integrate results from experimental communities with patterns seen at broader scales. Likely patterns of extinction and invasion need to be linked to different drivers of global change, the forces that structure communities, and controls on ecosystem properties for the development of effective management and conservation strategies. 5)This paper focuses primarily on terrestrial systems, with some coverage of freshwater systems, because that is where most empirical and theoretical study has focused. While the fundamental principles described here should apply to marine systems, further study of that realm is necessary. Despite some uncertainties about the mechanisms and circumstances under which diversity influences ecosystem properties, incorporating diversity effects into policy and management is essential, especially in making decisions involving large temporal and spatial scales. Sacrificing those aspects of ecosystems that are difficult or impossible to reconstruct, such as diversity, simply because we are not yet certain about the extent and mechanisms by which they affect ecosystem properties, will restrict future management options even further. It is incumbent upon ecologists to communicate this need, and the values that can derive from such a perspective, to those charged with economic and policy decision-making.
... Though very recent, these studies have fed an explosion of research on the relationship between ecosystem function and biodiversity (Naeem et al. 1994, 1996, b, Hooper and Vitousek 1997, McGrady-Steed et al. 1997, Naeem and Li 1997, Wardle et al. 1997a, 2000a, Doak et al. 1998, Hooper 1998, Hughes and Roughgarden 1998, Loreau 1998a, Mikola and Setälä 1998, Naeem 1998, Symstad et al. 1998, Van der Heijden et al. 1998, Allison 1999, Hector et al. 1999, Mulder et al. 1999, Yachi and Loreau 1999, Norberg 2000, Symstad 2000). Considerable debate, however, surrounds the interpretations of these studies (André et al. 1994, Givnish 1994, Aarssen 1997, Garnier et al. 1997, Grime 1997, Huston 1997, Wardle et al. 1997b, 2000b, Hector 1998, Hodgson et al. 1998, Lawton et al. 1998, Loreau 1998b, Wardle 1998, Naeem 1999, Hulot et al. 2000, Tilman 2000) and it is here where the sense of déjà vu arises. Consider the following questions being addressed in these debates. ...
The ecosystem consequences of dramatic declines or changes in biodiversity have spurred considerable research and tremendous debate that has rekindled most of the major conflicts in ecology, creating a sense of déjà vu. These conflicts include whether ecosystem or community ecology provides better insights into the workings of nature, the relative importance of biotic vs. abiotic factors in governing community composition and structure, the virtues of phenomenological vs. mechanistic research, the relationship between biodiversity and stability, the relative importance of taxonomic vs. functional diversity, and the relative strengths of observation vs. experimental approaches.
Although the tone of the debate has been regrettable, its magnitude signifies the emergence of a new paradigm, one in a series of debates associated with the dialectic that has structured ecological inquiry over two millennia of Western science. This dialectic concerns the tension between those who seek to explain nature by studying its parts and those who seek to explain nature by studying whole-system behavior. Philosophers and historians argue that such a dialectic generates cycles in which a central tenet is challenged by an emerging paradigm, generating new theories and new data to test the emerging paradigm. The scientific community evaluates the accumulating evidence (and it is here that the debates arise), and if subscription to the emerging paradigm increases sufficiently, the emerging paradigm evolves into a new central tenet. Fractionation within the sciences exacerbates this cycle because subdisciplines often focus on either the parts or the whole. Such splintering can be traced to the abandonment of the holistic approach of Aristotelian science during the Scientific Revolution. While such holism may have lessened debate, some have argued that it stagnated Western science. The dialectic, the cycles of emerging paradigms it generates, and the debates that surround each emergence represent the vehicle by which ecology moves forward. Emerging paradigms force scientists to revisit central tenets, pitting old ideas against new theories and new data, and this revisiting is what generates the sense of déjà vu and the cycles of vigorous debate, but ultimately each cycle leads to synthesis and progress.
The emerging paradigm that biodiversity governs ecosystem function is rapidly evolving. In the words of Thomas Kuhn, its controversial experiments have successfully articulated the paradigm. It has successfully challenged ecology's central tenet that biodiversity is primarily an epiphenomenon of ecosystem function secondarily structured by community processes. In its most extreme form, it claims that the reverse is true. Of course, neither the central tenet of ecology nor the emerging paradigm is correct in an absolute sense, but the dialectic that promoted the emergence of biodiversity and ecosystem function as a paradigm redirected ecology to focus on the feedback between ecosystem function and biodiversity rather than studying them independently. The final stage in the evolution of this emerging paradigm will be explicit tests of synthetic mechanisms that have been proposed. Familiarity with the ecological dialectic provides a framework by which ecologists can understand the origin and utility of paradigms in ecology, provides a proper context for the debate that surrounds paradigms as they emerge, promotes synthesis, and deters intellectual chauvinism that may inadvertently accompany specialization within ecology.
... Such recent empirical progress was largely based on controlled experiments with isolated microcosms or relatively simple modified ecosystems (Lawton 1994, Naeem et al. 1994, Tilman & Downing 1994, McGrady-Steed et al. 1997, Naeem & Li 1997, Tilman et al. 1997, Hector et al. 1999, Hulot et al. 2000, Bradford et al. 2002). However, methods and conclusions of these studies have been subject to vehement critique and were controversially debated in leading scientific journals (Huston 1997, Wardle 1998, Huston et al. 2000, Loreau & Hector 2001). We are still only beginning to understand fundamental processes associated with species coexistence in diverse multispecies communities. ...
Ant communities visiting nectar and honeydew sources were studied in a tropical lowland rainforest in North Queensland, Australia. The study focused on the hypothesis whether the distribution and composition of nectar and honeydew diets influence resource partitioning and competition in the ant community, and thus regulate community composition. Ants were the most common consumers on all extrafloral nectaries, while they constituted only a minority of floral visitors. In total, 43 ant species were observed to consume nectar from extrafloral nectaries (34 plant species) or from flowers (14 plant species), and wound sap exudates (three plant species). Six nectar-foraging ant species attended trophobionts (including at least 12 species of homopterans and two species of lycaenid caterpillars) for honeydew. Ant species showed a significant compartmentalisation of nectar use across plant species, although most ant species visited a broad spectrum of plants that strongly overlapped between different ants. Trophobioses were much more specialised at the study site, and some ant species attended certain trophobionts exclusively. On each plant individual, only a single ant colony was observed attending trophobionts. In contrast, simultaneous co-occurrences between different ant species foraging for nectar on the same plant individuals were common (observed in 23% of the surveys), although these proportions varied strongly across plant and ant species. The two most dominant ant species (Oecophylla smaragdina and Anonychomyrma gilberti) had mutually exclusive territories, and they were each associated with a significantly different assemblage of other ant species on nectar plants. This community pattern corresponds with the concept of ant mosaics that is based on dominance hierarchies. Honeydew and nectar sources varied substantially in carbohydrate and amino acid concentration and composition (HPLC analyses). There was a strong relationship between the composition of these resources and their use by ants, in particular by the dominant O. smaragdina. Among all 32 nectar and honeydew sources analysed, resources actually consumed by this ant were characterised by relatively similar amino acid profiles and higher total sugar concentration. The most common diets of O. smaragdina included two honeydew sources (Sextius kurandae membracids on Entada phaseoloides and Caesalpinia traceyi legume lianas) and two extrafloral nectars (Flagellaria indica and Smilax cf. australis) that had the broadest spectrum of amino acids. Furthermore, these trophobioses on lianas showed a significantly higher per capita recruitment of this ant species (number of workers per individual homopteran) compared to trees. F. indica and S. cf. australis extrafloral nectaries were also commonly monopolised by O. smaragdina in a similar way as trophobioses; co-occurrences were significantly rarer than at other nectar sources. Field experiments on nectar preferences were performed using artificial sugar and amino acid solutions in pairwise comparisons. Preferences among sugars were largely concordant between ant species. For most ant species, sucrose was more attractive than any other sugar, and attractiveness increased with sugar concentration. Most ant species also preferred sugar solutions containing mixtures of amino acids over pure sugar solutions. However, choices between different single amino acids in sugar solutions varied substantially and significantly between species. Preferences between solutions were significantly reduced in the presence of competing ant species. Thus the experiments show that both variability in gustatory preferences, especially for amino acids, and conditional effects of competition may be important for resource selection and partitioning in nectar feeding ant communities. Stable carbon and nitrogen isotope composition was analysed for 50 ant species, and additionally for associated plants, homopterans and other arthropods from the study site. Nitrogen isotope ratios (d15N) of ants were not correlated with those of plant foliage from which the ants were collected. Instead, d15N may represent a powerful indicator of trophic position of omnivorous ants like in other foodweb studies, suggesting that members of the ant community spread out in a continuum between largely herbivorous species, feeding on nectar or honeydew, and predatory taxa. Variability between colonies of the same species was also pronounced. d15N values of O. smaragdina colonies from mature forests, where most of their nectar and honeydew sources are found, indicate lower trophic levels than isotope signatures of colonies from open secondary vegetation. This study demonstrates that the distribution and quality of honeydew and nectar sources have a strong structuring impact in diverse tropical ant communities. Amino acids were found to play a key role for ant species preferences and competition, and for nitrogen fluxes to colonies of the arboreal ant fauna. In dieser Arbeit wurden Ameisengemeinschaften an Nektar- und Honigtauquellen in einem tropischen Tieflandregenwald in Nord-Queensland, Australien, untersucht. Die zentrale Hypothese dieser Arbeit war, ob die Verteilung und die Zusammensetzung von Nektar und Honigtau die Ressourcenpartitionierung und Konkurrenz in der Ameisengemeinschaften beeinflusst und daher die Zusammensetzung der Gemeinschaft reguliert. Ameisen stellten die häufigsten Besucher aller extrafloraler Nektarien, aber nur einen geringen Anteil der Blütenbesucher dar. Insgesamt wurden 43 Ameisenarten beobachtet, die Nektar an extrafloralen Nektarien (34 Pflanzenarten) oder Blüten (14 Pflanzenarten) sowie Pflanzenwundsäfte (drei Pflanzenarten) konsumierten. Von diesen Ameisenarten traten sechs als Nutzer von Honigtau in trophobiotischen Assoziationen auf. Die nektarivoren Ameisengemeinschaften verschiedener Pflanzenarten unterschieden sich signifikant, obwohl alle regelmäßig erfassten Ameisenarten ein breites und stark überlappendes Spektrum an Pflanzenarten nutzten. Trophobiosen zeigten einen stärkeren Grad an Spezialisierung im Untersuchungsgebiet; einige Ameisenarten waren exklusiv mit bestimmten Trophobionten assoziiert. An den Trophobiosen eines Pflanzenindividuums trat in allen beobachteten Fällen nur jeweils eine einzige Ameisenkolonie auf. Pflanzenindividuen mit Nektarien wurden dagegen häufig von mehreren Ameisenarten gleichzeitig genutzt. Territorien der beiden dominanten Ameisenarten (Oecophylla smaragdina und Anonychomyrma gilberti) zeigten keine Überschneidungen. Beide Arten waren jeweils mit einem signifikant unterschiedlichen Artenspektrum an Ameisen assoziiert. Dieses Muster stimmt mit dem Konzept der Ameisen-Mosaike überein, das auf Dominanzhierarchien basiert. Honigtau- und Nektarquellen zeigten eine ausgeprägte Variabilität in der Konzentration und Zusammensetzung von Zuckern und Aminosäuren (HPLC Analysen). Die Zusammensetzung dieser Ressourcen zeigte einen deutlichen Zusammenhang mit ihrer Nutzung durch Ameisen, insbesondere bei O. smaragdina. Von 32 analysierten Nektar- und Honigtauquellen waren die von dieser Ameisenart tatsächlich genutzten Ressourcen durch relativ ähnliche Aminosäureprofile und eine höhere Zuckerkonzentration charakterisiert. Zu den häufigsten Futterquellen von O. smaragdina zählten zwei Honigtauquellen (Sextius kurandae Buckelzirpen an Lianen der Leguminosen Entada phaseoloides und Caesalpinia traceyi) und zwei extraflorale Nektarquellen (Flagellaria indica und Smilax cf. australis), welche jeweils das breiteste Spektrum von Aminosäuren aufwiesen. Diese Trophobiosen an Lianen zeigten außerdem eine signifikant höhere Rekrutierungsrate von O. smaragdina als an Bäumen. Auch die extrafloralen Nektarien von F. indica und S. cf. australis wurden von O. smaragdina häufig in ähnlicher Weise wie die Trophobiosen monopolisiert. Freilandexperimente zu Nektarpräferenzen wurden mit künstlichen Zucker- und Aminosäuregemischen durchgeführt. Zuckerpräferenzen wiesen eine hohe Übereinstimmung zwischen Ameisenarten auf. Saccharose wurde von den meisten Arten gegenüber anderen Zuckern bevorzugt, und die Attraktivität der Zuckerlösungen stieg mit ihrer Konzentration. Die meisten Arten bevorzugten Zuckerlösungen mit einem Gemisch aus Aminosäuren gegenüber reinen Zuckerlösungen. Dagegen variierte die Wahl einzelner Aminosäuren in Zuckerlösungen substanziell und signifikant zwischen den Ameisenarten. Präferenzen waren außerdem signifikant reduziert in Gegenwart konkurrierender Ameisenarten. Diese Experimente deuten einerseits auf den Einfluss geschmacksphysiologischer Unterschiede, insbesondere bei Aminosäuren, für die Wahl von Ressourcen in nektarivoren Ameisengemeinschaften hin. Andererseits ist auch die Konkurrenz von entscheidender Bedeutung. Von 50 Ameisenarten wurde die Zusammensetzung stabiler Kohlenstoff- und Stickstoffisotope analysiert, außerdem für Pflanzen, Homopteren und andere Arthropoden des Untersuchungsgebietes. Die Zusammensetzung von Stickstoffisotopen (d15N) bei Ameisen wiesen auf die jeweilige trophische Position der omnivoren Ameisen hin, wobei ein Kontinuum von größtenteils herbivoren Ameisenarten, die Nektar und Honigtauquellen nutzen, bis hin zu räuberischen Arten vorliegt. Neben interspezifischen Unterschieden war außerdem die Variabilität verschiedener Kolonien derselben Ameisenart sehr ausgeprägt. Bei O. smaragdina deuten d15N-Werte von Kolonien in geschlossenen Waldstadien, in denen die meisten ihrer Nektar- und Honigtauquellen vorkommen, auf niedrigere Trophieebenen hin im Vergleich zu Kolonien in offener Sekundärvegetation. Diese Arbeit belegt, dass die Verteilung und Qualität von Nektar und Honigtauquellen einen starken strukturierenden Einfluss auf artenreiche tropische Ameisengemeinschaften haben. Aminosäuren haben dabei eine Schlüsselfunktion für Präferenzen und Konkurrenz, sowie für den Stickstoffhaushalt von Kolonien der arborealen Ameisenfauna.
... Various experimental studies have provided some evidence that the temporal variability of ecosystem properties decreases with increasing diversity in agreement with theoretical predictions (Tilman and Downing 1994; Tilman 1996; McGrady-Steed et al. 1997; Naeem and Li 1997; McGrady-Steed and Morin 2000). These results have been debated because of potential confounding factors in most of these experiments (Huston 1997; Wardle 1998; Fukami et al. 2001; Morin and McGrady-Steed 2004). Furthermore , some recent theoretical (Loreau and Behera 1999) and experimental (Petchey et al. 2002; Pfisterer and Schmid 2002; Gonzalez and Descamps-Julien 2004) studies suggest more complex relationships between stability and diversity. ...
Several theoretical studies propose that biodiversity buffers ecosystem functioning against environmental fluctuations, but virtually all of these studies concern a single trophic level, the primary producers. Changes in biodiversity also affect ecosystem processes through trophic interactions. Therefore, it is important to understand how trophic interactions affect the relationship between biodiversity and the stability of ecosystem processes. Here we present two models to investigate this issue in ecosystems with two trophic levels. The first is an analytically tractable symmetrical plant-herbivore model under random environmental fluctuations, while the second is a mechanistic ecosystem model under periodic environmental fluctuations. Our analysis shows that when diversity affects net species interaction strength, species interactions--both competition among plants and plant-herbivore interactions--have a strong impact on the relationships between diversity and the temporal variability of total biomass of the various trophic levels. More intense plant competition leads to a stronger decrease or a lower increase in variability of total plant biomass, but plant-herbivore interactions always have a destabilizing effect on total plant biomass. Despite the complexity generated by trophic interactions, biodiversity should still act as biological insurance for ecosystem processes, except when mean trophic interaction strength increases strongly with diversity.
Using a habitat templet model, we predict that the productivity (total biomass) of plots within a plant community may be positively, negatively or not at all related to variation in the number of species per plot, depending on successional stage (time since major disturbance) and habitat carrying capacity (reflecting the total resource supplying power of the habitat). For plots of a given size, a positive relationship between productivity and species richness is predicted in recently disturbed habitats because local neighbourhoods here will have been assembled largely stochastically, usually from a pool of available species with a right-skewed size frequency distribution. Hence, in the earliest stages of succession, plots will have relatively high total biomass only if they contain at least some of the relatively uncommon larger species which will, in turn, be more likely in those neighbourhoods that contain more species (the sampling effect). Among these will also be some of the more common smaller species; hence, these high biomass, species-rich plots should have relatively low species evenness, in contrast to what is predicted under effects involving species complementarity. In late succession, the plots with high total biomass will still be those that contain relatively large species but these plots will now contain relatively few species owing to increased competitive exclusion over time (the competitive dominance effect). In intermediate stages of succession, no relationship between plot productivity and species richness is predicted because the opposing sampling and competitive dominance effects cancel each other out. We predict that the intensity of both the sampling and competitive dominance effects on the productivity/species richness relationship will decrease with decreasing habitat carrying capacity (e.g. decreasing substrate fertility) owing to the inherently lower variance in between-plot productivity that is predicted for more resource-impoverished habitats.
The search for self-sustaining, low-input, diversified, and energy-efficient agricultural systems is now a major concern of many researchers, farmers, and policymakers worldwide. A key strategy in sustainable agriculture is to restore functional biodiversity of the agricultural landscape. Biodiversity performs key ecological services and if correctly assembled in time and space can lead to agroecosystems capable of sponsoring their own soil fertility, crop protection and productivity. There is consensus that at least some minimum number of species is essential for ecosystem functioning under constant conditions and that a larger number of species is probably essential for maintaining the stability of ecosystem processes in changing environments. Determining which species have a significant impact on which processes in which ecosystems, however, remains an open empirical question.
A large number of studies have now explicitly examined the relationship between species loss and ecosystem functions. The results from such “biodiversity experiments” have previously been collated and analyzed by two independent groups of authors. Both data sets show that reductions in species diversity generally result in reduced ecosystem functioning, even though the studies cover a wide range of ecosystems, diversity manipulations, and response variables. In this chapter, we analyze the two data sets in parallel to explain variation in the observed functional effects of biodiversity. The main conclusions are: 1) the functional effects of biodiversity differ among ecosystem types (but not between terrestrial and aquatic systems), 2) increases in species richness enhance community responses but negatively affect population responses, 3) stocks are more responsive than rates to biodiversity manipulations, 4) when diversity reductions at one trophic level affect a function at an adjacent trophic level (higher or lower), the function is often reduced 5) increased biodiversity results in increased invasion resistance. We also analyze the shape of the relationship between biodiversity and response variables, and discuss some consequences of different relationships.
Climate change mitigation initiatives based on biological sequestration of carbon have paid little attention to biodiversity, with important implications both for climate change mitigation and for ecosystem services that depend on biodiversity. Here the chapter reviews the theoretical and empirical evidence for forest biodiversity effects on carbon sequestration. This chapter suggests that protection of primary forests is the most effective option for maximizing carbon sequestration in forest ecosystems, and should be included in future international agreements. Because carbon sequestration is a long term goal, this chapter presents the case that avoidance of losses should be emphasized over short term uptake, and that maintenance of mixtures of dominant and subdominant species and genotypes are the safest option for carbon sequestration in plantations and agroforestry systems. Biodiversity conservation should be included in the development of policy for climate change mitigation initiatives based on carbon sequestration in forested systems, including those related to the Kyoto Protocol.
Meta-analysis of the first generation of biodiversity experiments has revealed that there is a general positive relationship between diversity and ecosystem processes that is consistent across trophic groups and ecosystem types. However, the mechanisms generating these general patterns are still under debate. While there are unresolved conceptual issues about the nature of diversity and complementarity, the debate is partly due to the difficulty of performing a full-factorial analysis of the functional effects of all species in a diverse community. However, there are now several different analytical approaches that can address mechanisms even when full factorial analysis is not possible. This chapter presents an overview and users' guide to these methods. This chapter concludes that the current toolbox of methods allows investigation of the mechanisms for most, if not all, biodiversity and ecosystem functioning experiments conducted to date that manipulate species within a single trophic level (e.g. plant biodiversity experiments). Methods that can address mechanisms in multitrophic studies are a key need for future research.
* A graduate level text which incorporates the latest developments in the field of biodiversity and ecosystem functioning, one of the most controversial and high profile areas of ecological research
* The first volume to explore the economics of biodiversity and ecosystem services
* Summarizes the eagerly anticipated findings of two large and highly respected scientific networks, BioMERGE and DIVERSITAS
* Builds on the success and influence of the highly cited Biodiversity and Ecosystem Functioning (OUP, 2002)
* The first volume advancing the scientific foundation of the United Nation's global environmental assessment, Millennium Ecosystem Assessment, that links human well-being with the conservation of biodiversity
Decomposition of plant litter is a key process for the flow of energy and nutrients in ecosystems that may be sensitive to the loss of biodiversity. Two hypothetical mechanisms by which changes in plant diversity could affect litter decomposition are (1) through changes in litter species composition, and (2) by altering the decomposition microenvironment. We tested these ideas in relation to the short-term decomposition of herbaceous plant litter in experimental plant assemblages that differed in the numbers and types of plant species and functional groups that they contained to simulate loss of plant diversity. We used different litterbag experiments to separate the two potential pathways through which diversity could have an effect on decomposition. Our two litterbag trials showed that altering plant diversity affected litter breakdown differently through changes in decomposition microenvironment than through changes in litter composition. In the decomposition microenvironment experiment there was a significant but weak decline in decomposition rate in relation to decreasing plant diversity but no significant effect of plant composition. The litter composition experiment showed no effect of richness but significant effects of composition, including large differences between plant species and functional groups in litter chemistry and decomposition rate. However, for a nested subset of our litter mixtures decomposition was not accurately predicted from single-species bags; there were positive, non-additive effects of litter mixing which enhanced decomposition. We critically assess the strengths and limitations of our short-term litterbag trials in predicting the longer-term effects of changes in plant diversity on litter decomposition rates.
The ecological consequences of biodiversity loss have aroused considerable interest and controversy during the past decade.
Major advances have been made in describing the relationship between species diversity and ecosystem processes, in identifying
functionally important species, and in revealing underlying mechanisms. There is, however, uncertainty as to how results obtained
in recent experiments scale up to landscape and regional levels and generalize across ecosystem types and processes. Larger
numbers of species are probably needed to reduce temporal variability in ecosystem processes in changing environments. A major
future challenge is to determine how biodiversity dynamics, ecosystem processes, and abiotic factors interact.
The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence that individual species effects are important in modifying ecosystem properties. We conducted a field experiment in a New Zealand perennial grassland ecosystem, creating artificial vegetation gaps and imposing manipulation treatments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or “functional groups” of the flora. We monitored vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C3 grass Lolium perenne L. enhanced vegetative cover, biomass, and species richness of both the C4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season. Treatments significantly affected total plant cover and biomass; in particular, C4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C3 grasses reduced total root biomass and drastically enhanced overall shoot-to-root biomass ratios. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different functional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematodes, and predaceous nematodes. However, as long as plants were present, we did not find strong effects of removal treatments, NPP, or plant biomass on these trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, were unresponsive to any factor other than removal of all plants, which reduced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microflora, microbe-feeding nematodes, predaceous nematodes) and two herbivore groups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitrophic effects at higher levels of taxonomic resolution. The principal ordination axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes of soil-associated herbivorous arthropods and microbial-feeding nematodes being related to ordination axes representing plant community structure at earlier measurement dates. Taxonomic diversity of some soil organism groups was linked to plant removals or to plant diversity. For herbivorous arthropods, removal of C4 grasses enhanced diversity; there were negative correlations between plant and arthropod diversity, presumably because of negative influences of C4 species in the most diverse treatments. There was evidence of lag relationships between diversity of plants and that of the three decomposer groups, indicating multitrophic effects of altering plant diversity. Relatively small effects of plant removal on the decomposer food web were also apparent in soil processes regulated by this food web. Decomposition rates of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient dynamics were identified. Although some treatments affected temporal variability (and thus stability) of soil biotic properties (particularly CO2 release) throughout the experiment, there was no evidence of destabilizing effects of plant removals. Our data provide evidence that permanent exclusion of plant species from the species pool can have important consequences for overall vegetation composition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The nature of many of these effects is driven by which plant species are lost from the system, which depends on the various attributes or traits of these species.
The relationship between productivity and species diversity was investigated at the quadrat level for three old-field plant communities that varied in time since the last major disturbance from cultivation. A positive relationship between productivity (estimated by above-ground dry biomass) and the species richness component of diversity was detected only for quadrats from the most recently disturbed community. The communities that experienced longer post-disturbance time showed no significant relationship between productivity and species richness. Quadrat productivity, however, was negatively related to the species evenness component of diversity in all three communities, i.e., regardless of time since disturbance. Furthermore, analyses of covariance using log biomass, species richness and species evenness as response variables, log soil nitrate concentration as a covariate and time since disturbance as a factor revealed that residual log biomass was significantly negatively correlated with residual species evenness, but was not significantly correlated with residual species richness. These results support the view that the productivity of plots within natural vegetation is related more predictably to the relative composition of species (reflected by evenness) than to the number of species present, especially as succession progresses.
While recent theoretical work has demonstrated several mechanisms whereby more diverse communities can exhibit greater temporal stability, empirical examinations have been few and the subject of much debate. We show that the temporal stability of natural summer and winter annual plant communities, at spatial scales of 0.25 m2 and 0.25 ha, tends to increase with community richness. Furthermore, more diverse communities exhibited greater stability because they contained a greater abundance of individuals (overyielding effect). Statistical averaging (the portfolio effect) and negative covariances between species (insurance and competition effects) did not enhance stability. Relationships between diversity and stability tended to be weak and were significant only at the smaller spatial scale. Because more diverse communities contained higher densities of individuals, the effect of diversity per se on stability was unclear and likely small. If overyielding is common in other ecological systems, the loss of individuals and biodiversity may often result in increased variation in ecological communities.
Studies of microbial communities from aquatic ecosystems provide important insights into relations between various aspects of ecosystem functioning and changes in biodiversity. Aquatic microbial systems provide a valuable counterpoint to studies of terrestrial systems, because patterns reflect consequences of interactions occurring over many generations of community development, and are unlikely to represent artifacts of the initial conditions established in experimental communities. In this paper we re-analyse our previously published data to separate the contributions of temporal and spatial variation to overall variation in ecosystem functioning. A new analysis based on re-sampling confirms a negative relationship between richness and the variability of one ecosystem process, carbon dioxide flux. The negative relationship reflects high variation among communities of low species richness, rather than high temporal variation within communities of low richness. We also review the various transformations and summary statistics proposed as alternate measures of variability in ecosystem functioning, to point out that different measures are often appropriate for different kinds of data. Finally, we conclude that arguments about the cosmopolitan distribution of microbes do not preclude the existence of important relations between microbial species richness and ecosystem functioning.
Cannibalistic behavior among Ochlerotatus triseriatus larvae was studied to determine whether cannibals are able to alter their attack behavior based on their relatedness to newly hatched conspecific prey larvae. Fourth instar larvae, reared in one of four different initial densities, were placed with newly emerged first instars and, after 48 h, the number of first instar larvae remaining was recorded. Our data suggest a Type III functional response of non-kin fourth instar larvae to first instar prey density. Several significant effects emerged from our analysis model, namely that the number of first instar prey available, the relationship of the first instar larvae with the cannibals, and the density at which the fourth instar cannibals were reared all affected the number of first instar larvae consumed.
Extensive research has been devoted to understanding the role of biodiversity as a driver of ecosystem functioning. However, no previous study has evaluated the relative contribution of complementarity and selection to productivity in shrublands. We have attempted to do this for a Mediterranean shrubland dominated by Quercus coccifera, Cistus albidus, Ulex parviflorus and Rosmarinus officinalis. We found a highly significant and linear positive relationship between productivity and species richness. No selection effect was apparent, but both the complementarity and net effects were highly significant. The magnitude of these effects increased from two to three species, but became non-significant in the four-species mixtures. Analysis of pairwise interactions revealed that legumes did not promote overyielding. Complementarity was mostly driven by Cistus, which always performed better when growing with other species than when growing with conspecifics. Our results are an addition to the still scarce literature dealing with diversity–productivity relationships in communities dominated by woody species, and show that methodologies commonly used to assess complementarity may not provide a precise estimation when a given species has negative effects on its conspecifics.
The insurance hypothesis states that communities with greater numbers of species will be more stable than communities with fewer species. Various theoretical realisations of this hypothesis and some empirical evidence have been interpreted as general support for the hypothesis. Here, I suggest that very few studies can claim to accurately test the hypothesis. In particular, this is because few studies are sufficiently long in duration, and because sometimes community level variability is measured inappropriately. There are very few experiments (perhaps only six) that meet the assumptions of the theoretical models closely. Across these, around a half of all tests indicate support for the insurance hypothesis, and half show no support. This is probably too small a number of experiments to draw any solid conclusions and should encourage more experimental and observational tests. In particular, there have been very few investigations of how environmental variability interacts with diversity to determine community stability. Other directions for future research include replacing species richness with diversity of responses to environmental change, and developing a more mechanistic framework for understanding empirical results.
This paper uses theory and experiments to explore the effects of diversity on stability, productivity, and susceptibility to invasion. A model of resource competition predicts that increases in diversity cause community stability to increase, but population stability to decrease.. These opposite effects are, to a great extent, explained by how temporal variances in species abundances scale with mean abundance, and by the differential impact of this scaling on population vs. community stability. Community stability also depends on a negative covariance effect (competitive compensation) and on overyielding (ecosystem productivity increasing with diversity). A longterm study in Minnesota grasslands supports these predictions.
Models of competition predict, and field experiments confirm, that greater plant diversity leads to greater primary productivity. This diversity-stability relationship results both from the greater chance that a more productive species would be present at higher diversity (the sampling effect) and from the better "coverage" of habitat heterogeneity caused by a broader range of species traits in amore diverse community(the niche differentiation effect). Both effects cause more complete utilisation of limiting resources at higher diversity, which increases resource retention, further increasing productivity. Finally, lower levels of available limiting resources at higher diversity are predicted to decrease the susceptibility of an ecosystem to invasion, supporting the diversity-stability hypothesis. This mechanism provides rules for community assembly and invasion resistance.
In total, biodiversity should be added to species composition, disturbance, nutrient supply, and climate as a major controller of population and ecosystem dynamics and structure. By their increasingly great directional impacts on all of these controllers, humans are likely to cause major longterm changes in the functioning of ecosystems worldwide. A better understanding of these ecosystem changes is needed if ecologists are to provide society with the knowledge essential for wise management of the Earth and its biological resources.
The world is witnessing a decline in biodiversity which may be greater in magnitude than even previous mass-extinction events. This has rekindled interest in the relationships between biodiversity and the stability of community and ecosystem processes that have been reported in some empirical studies. Diversity has been linked with community and ecosystem processes, but disputes remain over whether it is diversity, environmental factors or the variety of functional groups in a community that drive these patterns. Furthermore, it remains unclear whether variation in diversity resulting from species loss within communities has similar effects on stability as natural variation in diversity associated with gradients in factors that regulate diversity. We believe that, across larger ecological scales, extrinsic determinants of biodiversity such as disturbance regimes and site history may be the primary determinants of certain measures of community stability. Here we use controlled field experiments in savanna grasslands in southern India to demonstrate and explain how low-diversity plant communities can show greater compositional stability when subject to experimental perturbations characteristic of their native environments. These results are best explained by the ecological history and species characteristics of communities rather than by species diversity in itself.
Experiments assessing the effects of biodiversity on ecosystem functioning initially aimed at establishing whether such relationships exist (e.g. Naeem et al. 1995; Tilman et al. 1996; Jonsson and Malmqvist 2000; Engelhardt and Ritchie 2001). These phenomenological studies were useful for helping to identify patterns and articulate further questions. However, using species richness as a simple mea sure of biotic diversity, as they did, had no explicit explanatory power: ecosystem level processes are affected by the functional characteristics of organ isms involved, rather than by taxonomic identity (Odum 1969; Pugh 1980; Grime 1988).
Animal locomotion is controlled, in part, by a central pattern generator (CPG), which is an intraspinal network of neurons capable of generating a rhythmic output. The spatio-temporal symmetries of the quadrupedal gaits walk, trot and pace lead to plausible assumptions about the symmetries of locomotor CPGs. These assumptions imply that the CPG of a quadruped should consist of eight nominally identical subcircuits, arranged in an essentially unique matter. Here we apply analogous arguments to myriapod CPGs. Analyses based on symmetry applied to these networks lead to testable predictions, including a distinction between primary and secondary gaits, the existence of a new primary gait called 'jump', and the occurrence of half-integer wave numbers in myriapod gaits. For bipeds, our analysis also predicts two gaits with the out-of-phase symmetry of the walk and two gaits with the in-phase symmetry of the hop. We present data that support each of these predictions. This work suggests that symmetry can be used to infer a plausible class of CPG network architectures from observed patterns of animal gaits.
Because the goal of ecology is to understand the causes of patterns observed in the natural world, studies that document novel patterns in natural ecosystems are of central importance. The report “The influence of island area on ecosystem properties” by David A. Wardle et al. ([29 Aug., p. 1296
Interactions between biotic and abiotic processes complicate the design and interpretation of ecological experiments. Separating causality from simple correlation requires distinguishing among experimental treatments, experimental responses, and the many processes and properties that are correlated with either the treatments or the responses or both. When an experimental manipulation has multiple components, but only one of them is identified as the experimental treatment, erroneous conclusions about cause and effect relationships are likely because the actual cause of any observed response may be ignored in the interpretation of the experimental results. This unrecognised cause of an observed response can be considered a "hidden treatment". Three types of hidden treatments are potential problems in biodiversity experiments:
(1) abiotic conditions, such as resource levels, or biotic conditions, such as predation, which are intentionally or unintentionally altered in order to create differences in species numbers for"diversity" treatments;
(2) non-random selection of species with particular attributes that produce treatment differences that exceed those due to "diversity" alone;
(3) the increased statistical probability of including a species with dominant negative or positive effect (e.g. dense shade, or nitrogen fixation) in randomly selected groups of species of increasing number or "diversity".
In each of these cases, treatment responses that are actually the result of the "hidden treatment" may be inadvertently attributed to variation in species number. Case studies re-evaluating three different types of biodiversity experiments demonstrate that the increases found in such ecosystem properties as productivity, nutrient use efficiency, and stability (all of which were attributed to higher level of species diversity) were actually caused by "hidden treatments" that altered plant biomass and productivity.
One of the tenets of the conservation movement has been that areas containing many species--those of high biodiversity--are
particularly worth saving, since ecosystems of high diversity show improved ecosystem function. As Grime explains in his Perspective,
this tenet is being replaced by another view, which is bolstered by three reports in this week's issue (pages
1296,
1300, and
1302). These studies all show that it is actually the specific features of the species in an ecosystem that determine its function--not
their number.
Biodiversity may represent a form of biological insurance against the
loss or poor performance of selected species. If this is the case, then
communities with larger numbers of species should be more predictable
with respect to properties such as local biomass. That is, larger
numbers of species should enhance ecosystem reliability, where
reliability refers to the probability that a system will provide a
consistent level of performance over a given unit of time. The validity
of this hypothesis has important ecological, management and economic
implications given the large-scale substitution of diverse natural
ecosystems with less diverse managed systems. No experimental evidence,
however, has supported this hypothesis. To test this hypothesis we
established replicated microbial microcosms with varying numbers of
species per functional group. We found that as the number of species per
functional group increased, replicate communities were more consistent
in biomass and density measures. These results suggest that redundancy
(in the sense of having multiple species per functional group) is a
valuable commodity, and that the provision of adequate redundancy may be
one reason for preserving biodiversity.