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Experimental exclusion of insectivorous predators results in no responses across multiple trophic levels in a water-limited, sagebrush-steppe ecosystem
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We examined the indirect effects of insectivorous birds on plant growth through consumption of leaf-chewing insects in a Missouri Ozark deciduous forest. Over a period of 2 yr, we compared insect numbers, leaf damage, and resultant plant growth for control saplings of white oak (Quercus alba), vs. saplings that we caged to limit access by insectivorous birds but not herbivorous insects. In a third treatment, we sprayed insecticide on young white oak trees to determine the impact of the insect herbivores on plant growth in the presence of birds. The total number of insects encountered on plants inside of cages was twice that on control plants for both years. Insecticide spray reduced herbivore numbers substantially but did not eliminate them. As a result of the treatments, cage plants suffered 25% leaf area loss, control plants 13%, and spray plants 6% at the end of the first season (34, 24, and 9%, respectively, for the 2nd yr). As a result of the differences in damage, cage plants produced one-third less total aboveground biomass compared to insecticide-treated plants, with control plants producing intermediate value. Differences in biomass production were due mainly to decreased leaf biomass, which in turn was associated with decreased leaf size in subsequent years as a result to high damage during the previous year. This is the first terrestrial ecosystem study to demonstrate a significant impact of insectivorous birds on plant growth. Our results suggest that over the long term observed declines in North American populations of insectivorous birds may reduce forest productivity because of potentially higher numbers of leaf-chewing insects and the concomitant negative effect on plant growth.
We conducted a field experiment to assess interrelationships between leaf-tissue secondary chemistry, avian predation, and the abundance and diversity of arthropods occurring on sagebrush (Artemisia tridentata) in central Oregon. Arthropods were removed from individual shrubs, some of which were then caged to exclude birds. Secondary chemistry and arthropods were sampled at intervals up to 56 wk following the defaunation/caging treatment. Recovery rates differed among arthropod taxa and functional groups. Several sap-sucking homopterans and hemipterans reached control levels within 2-4 wk of the treatment, whereas abundances of parasitoids and predators recovered to match control numbers only 6 wk after defaunation. Abundances of several herbivorous leaf-chewing taxa (primarily lepidopteran larvae) remained significantly depressed even 56 wk after the treatment. Fungivores (oribatid mites) reached greater abundances on defaunated than on control shrubs by the end of the experiment. There were also significant changes in the concentrations or frequency of occurrence of several chemical compounds following the defaunation treatment. Several hydrocarbons, sesquiterpenes, and monoterpenes that were present in most shrubs exhibited sharply reduced concentrations in leaf tissues within 4 wk of the treatment, whereas some alcohols and ketones (linalool, borneol, thujone), which occurred at relatively low frequencies among control plants, increased dramatically in their frequencies of occurrence following arthropod removal. Both changes persisted for the duration of the experiment. We found several significant associations between the abundance or occurrence of arthropod taxa or groups and concentrations or frequencies of secondary compounds, but these were most prevalent among the leaf-chewing lepidopterans. We suggest that the shrubs responded to the removal of herbivorous, leaf-chewing arthropods by altering chemical allocation patterns; the changes in chemistry persisted for over a year because recolonization of the defaunated plants by these herbivores was slow. Effects of the caging treatment were much less obvious. The recovery of the diversity of arthropods known to constitute prey for birds in this system was slower on shrubs protected from avian predation than on exposed shrubs, but a year after defaunation diversity had increased to higher levels on the protected shrubs. Few arthropod taxa or functional groups differed in abundance or frequency of occurrence between caged and uncaged shrubs, although fungivores (which are not eaten by birds) increased to levels on protected shrubs that were nearly twice those on the uncaged controls by the end of the experiment. Between-shrub variance in the abundance and diversity of bird-prey taxa was greater among exposed than protected shrubs, possibly reflecting the effects of area-restricted searching by the birds. There were no differences in leaf-tissue secondary chemistry between caged and uncaged shrubs.
Shoots and roots are autotrophic and heterotrophic organs of plants with different physiological functions. Do they have different metabolomes? Do their metabolisms respond differently to environmental changes such as drought? We used metabolomics and elemental analyses to answer these questions. First, we show that shoots and roots have different metabolomes and nutrient and elemental stoichiometries. Second, we show that the shoot metabolome is much more variable among species and seasons than is the root metabolome. Third, we show that the metabolic response of shoots to drought contrasts with that of roots; shoots decrease their growth metabolism (lower concentrations of sugars, amino acids, nucleosides, N, P, and K), and roots increase it in a mirrored response. Shoots are metabolically deactivated during drought to reduce the consumption of water and nutrients, whereas roots are metabolically activated to enhance the uptake of water and nutrients, together buffering the effects of drought, at least at the short term.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
The study reports estimates of declines of natural ecosystems in the US provides a rationale for ecosystem-level conservation, discusses decline and threat as criteria for conservation, and relates ecosystem losses to endangerment at species and population levels. The methodology for this report consisted of a literature review and a survey of conservation agencies and professionals. The result of this preliminary study indicated significant losses of biodiversity at the ecosystem level in the US. Losses of all kinds of ecosystems have been most pronounced in the South, Northeast, and Midwest, and in California. Integrated conservation plans for all ecosystems should be developed in each ecoregion of the US, starting with types and regions that sustained the greatest losses and are at greatest risk for further loss. Ecosystem conservation need not be restricted to pristine sites, which are now almost nonexistent. -from Authors
Trophic cascades-the indirect effects of carnivores on plants mediated by herbivores-are common across ecosystems, but their influence on biogeochemical cycles, particularly the terrestrial carbon cycle, are largely unexplored. Here, using a (13)C pulse-chase experiment, we demonstrate how trophic structure influences ecosystem carbon dynamics in a meadow system. By manipulating the presence of herbivores and predators, we show that even without an initial change in total plant or herbivore biomass, the cascading effects of predators in this system begin to affect carbon cycling through enhanced carbon fixation by plants. Prolonged cascading effects on plant biomass lead to slowing of carbon loss via ecosystem respiration and reallocation of carbon among plant aboveground and belowground tissues. Consequently, up to 1.4-fold more carbon is retained in plant biomass when carnivores are present compared with when they are absent, owing primarily to greater carbon storage in grass and belowground plant biomass driven largely by predator nonconsumptive (fear) effects on herbivores. Our data highlight the influence that the mere presence of predators, as opposed to direct consumption of herbivores, can have on carbon uptake, allocation, and retention in terrestrial ecosystems.
Using the short-lived isotope (11)C (t(1/2) = 20.4 min) as (11)CO(2), we captured temporal changes in whole-plant carbon movement and partitioning of recently fixed carbon into primary and secondary metabolites in a time course (2, 6, and 24 h) following simulated herbivory with the well-known defense elicitor methyl jasmonate (MeJA) to young leaves of Arabidopsis (Arabidopsis thaliana). Both (11)CO(2) fixation and (11)C-photosynthate export from the labeled source leaf increased rapidly (2 h) following MeJA treatment relative to controls, with preferential allocation of radiolabeled resources belowground. At the same time, (11)C-photosynthate remaining in the aboveground sink tissues showed preferential allocation to MeJA-treated, young leaves, where it was incorporated into (11)C-cinnamic acid. By 24 h, resource allocation toward roots returned to control levels, while allocation to the young leaves increased. This corresponded to an increase in invertase activity and the accumulation of phenolic compounds, particularly anthocyanins, in young leaves. Induction of phenolics was suppressed in sucrose transporter mutant plants (suc2-1), indicating that this phenomenon may be controlled, in part, by phloem loading at source leaves. However, when plant roots were chilled to 5°C to disrupt carbon flow between above- and belowground tissues, source leaves failed to allocate resources belowground or toward damaged leaves following wounding and MeJA treatment to young leaves, suggesting that roots may play an integral role in controlling how plants respond defensively aboveground.
Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem
processes are controversial. Plant species diversity, functional diversity, and functional composition were experimentally
varied in grassland plots. Each factor by itself had significant effects on many ecosystem processes, but functional composition
and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen,
and light penetration. Thus, habitat modifications and management practices that change functional diversity and functional
composition are likely to have large impacts on ecosystem processes.
Understanding and predicting the impacts of anthropogenically-driven climate change on species interactions and ecosystem processes is a critical scientific and societal challenge. Climate change has important ecological consequences for species interactions that occur across multiple trophic levels. In this update, we broadly examine recent literature focused on disentangling the direct and indirect effects of temperature and water availability on plants, phytophagous insects, and the natural enemies of these insects with special attention given to forest ecosystems. We highlight the role of temperature in shaping plant and insect metabolism, growth, development, and phenology. Additionally, we address the complexity involved in determining climate-mediated effects on plant-insect and multi-trophic level interactions as well as the roles of plant ecophysiological processes in driving both bottom-up and top-down controls. Climate warming may exacerbate plant susceptibility to attacks by some insect groups, particularly under reduced water availability. Despite considerable growth in research investigating the effects of climate change on plants and insects, we lack a mechanistic understanding of how temperature and precipitation influence species interactions, particularly with respect to plant defense traits and insect outbreaks. Moreover, a systematic literature review reveals that research efforts to date are highly over-represented by plant studies and suggests a need for greater attention to plant-insect and multitrophic level interactions. Understanding the role of climatic variability and change on such interactions will provide further insight into links between abiotic and biotic drivers of community- and ecosystem-level processes.
We examined the impact of avian predators on the community of insects feeding on Lemmon's willow, Salix lemmonii, at Carman Valley, California. Salix lemmonii supports a diverse assemblage of insect species that feed in either a concealed or an exposed manner on leaves by mining, tying, curling, galling, skeletonizing, or chewing. We tested the hypothesis that there are guild specific differences in the effects of avian predators on concealed and exposed-feeding folivores. We compared the feeding damage and frequency of each guild in the presence and absence of avian predators. Contrary to most studies of the effects of avian predators on folivorous insects, birds failed to reduce significantly the frequency of or feeding damage caused by exposed-feeding insects during 1999 and 2000. Chewing insects, which accounted for the majority of foliar damage by exposed-feeding insects, inflicted slightly more damage and skeletonizing insects slightly less damage in the presence of birds. We also observed no reduction in the frequency of concealed-feeding insects in the presence of avian predators; rather three out of four guilds were slightly more common in the presence of avian predators. While the abundance of foliage gleaning birds at the study site was slightly lower than the average for the past nine years, we interpret our results to indicate a true lack of an effect of avian predators on the community of folivorous insects on S. lemmonii. Future inquiry into the effects of avian predators on folivorous insects should focus on determining under what circumstances birds will or will not suppress guilds of folivorous insects.
Artificial lighting has been used to illuminate the nocturnal environment for centuries and continues to expand with urbanization and economic development. Yet, the potential ecological impact of the resultant light pollution has only recently emerged as a major cause for concern. While investigations have demonstrated that artificial lighting can influence organism behaviour, reproductive success and survivorship, none have addressed whether it is altering the composition of communities. We show, for the first time, that invertebrate community composition is affected by proximity to street lighting independently of the time of day. Five major invertebrate groups contributed to compositional differences, resulting in an increase in the number of predatory and scavenging individuals in brightly lit communities. Our results indicate that street lighting changes the environment at higher levels of biological organization than previously recognized, raising the potential that it can alter the structure and function of ecosystems.
Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems occupy more than one-third of the Earth's land mass, are greatly affected by changes in water availability, and are predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on these globally significant ecosystems.
Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these
animals may be humankind’s most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic
problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater
ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but
also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire,
carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary
research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.
Background:
Herbivory reduces leaf area, disrupts the function of leaves, and ultimately alters yield and productivity. Herbivore damage to foliage typically is assessed in the field by measuring the amount of leaf tissue removed and disrupted. This approach assumes the remaining tissues are unaltered, and plant photosynthesis and water balance function normally. However, recent application of thermal and fluorescent imaging technologies revealed that alterations to photosynthesis and transpiration propagate into remaining undamaged leaf tissue.
Scope and conclusions:
This review briefly examines the indirect effects of herbivory on photosynthesis, measured by gas exchange or chlorophyll fluorescence, and identifies four mechanisms contributing to the indirect suppression of photosynthesis in remaining leaf tissues: severed vasculature, altered sink demand, defence-induced autotoxicity, and defence-induced down-regulation of photosynthesis. We review the chlorophyll fluorescence and thermal imaging techniques used to gather layers of spatial data and discuss methods for compiling these layers to achieve greater insight into mechanisms contributing to the indirect suppression of photosynthesis. We also elaborate on a few herbivore-induced gene-regulating mechanisms which modulate photosynthesis and discuss the difficult nature of measuring spatial heterogeneity when combining fluorescence imaging and gas exchange technology. Although few studies have characterized herbivore-induced indirect effects on photosynthesis at the leaf level, an emerging literature suggests that the loss of photosynthetic capacity following herbivory may be greater than direct loss of photosynthetic tissues. Depending on the damage guild, ignoring the indirect suppression of photosynthesis by arthropods and other organisms may lead to an underestimate of their physiological and ecological impacts.
The significance of soil water redistribution facilitated by roots (an extension of "hydraulic lift", here termed hydraulic redistribution) was assessed for a stand of Artemisia tridentata using measurements and a simulation model. The model incorporated water movement within the soil via unsaturated flow and hydraulic redistribution and soil water loss from transpiration. The model used Buckingham-Darcy's law for unsaturated flow while hydraulic redistribution was developed as a function of the distribution of active roots, root conductance for water, and relative soil-root (rhizosphere) conductance for water. Simulations were conducted to compare model predictions with time courses of soil water potential at several depths, and to evaluate the importance of root distribution, soil hydraulic conductance and root xylem conductance on transpiration rates and the dynamics of soil water. The model was able to effectively predict soil water potential during a summer drying cycle, and the rapid redistribution of water down to 1.5 m into the soil column after rainfall events. Results of simulations indicated that hydraulic redistribution could increase whole canopy transpiration over a 100-day drying cycle. While the increase was only 3.5% over the entire 100-day period, hydraulic redistribution increased transpiration up to 20.5% for some days. The presence of high soil water content within the lower rooting zone appears to be necessary for sizeable increases in transpiration due to hydraulic redistribution. Simulation results also indicated that root distributions with roots concentrated in shallow soil layers experienced the greatest increase in transpiration due to hydraulic redistribution. This redistribution had much less effect on transpiration with more uniform root distributions, higher soil hydraulic conductivity and lower root conductivity. Simulation results indicated that redistribution of water by roots can be an important component in soil water dynamics, and the model presented here provides a useful approach to incorporating hydraulic redistribution into larger models of soil processes.
Terrestrial systems are thought to be organized predominantly from the bottom-up, but there is a growing literature documenting top-down trophic cascades under certain ecological conditions. We conducted an experiment to examine how arthropod community structure on a foundation riparian tree mediates the ability of insectivorous birds to influence tree growth. We built whole-tree bird exclosures around 35 mature cottonwood (Populus spp.) trees at two sites in northern Utah, USA, to measure the effect of bird predation on arthropod herbivore and predator species richness, abundance, and biomass, and on tree performance. We maintained bird exclosures over two growing seasons and conducted nondestructive arthropod surveys that recorded 63652 arthropods of 689 morphospecies representing 19 orders. Five major patterns emerged: (1) We found a significant trophic cascade (18% reduction in trunk growth when birds were excluded) only at one site in one year. (2) The significant trophic cascade was associated with higher precipitation, tree growth, and arthropod abundance, richness, and biomass than other site-year combinations. (3) The trophic cascade was weak or not evident when tree growth and insect populations were low apparently due to drought. (4) Concurrent with the stronger trophic cascade, bird predation significantly reduced total arthropod abundance, richness, and biomass. Arthropod biomass was 67% greater on trees without bird predation. This pattern was driven largely by two herbivore groups (folivores and non-aphid sap-feeders) suggesting that birds targeted these groups. (5) Three species of folivores (Orthoptera: Melanoplus spp.) were strong links between birds and trees and were only present in the site and the year in which the stronger trophic cascade occurred. Our results suggest that this trophic system is predominately bottom-up driven, but under certain conditions the influence of top predators can stimulate whole tree growth. When the most limiting factor for tree growth switched from water availability to herbivory, the avian predators gained the potential to reduce herbivory. This potential could be realized when strong links between the birds and plant, i.e., species that were both abundant herbivores and preferred prey, were present.
Many studies have quantified the indirect effect of hydrocarbon-based economies on climate change and biodiversity, concluding that a significant proportion of species will be threatened with extinction. However, few studies have measured the direct effect of new energy production infrastructure on species persistence.
We propose a systematic way to forecast patterns of future energy development and calculate impacts to species using spatially-explicit predictive modeling techniques to estimate oil and gas potential and create development build-out scenarios by seeding the landscape with oil and gas wells based on underlying potential. We illustrate our approach for the greater sage-grouse (Centrocercus urophasianus) in the western US and translate the build-out scenarios into estimated impacts on sage-grouse. We project that future oil and gas development will cause a 7-19 percent decline from 2007 sage-grouse lek population counts and impact 3.7 million ha of sagebrush shrublands and 1.1 million ha of grasslands in the study area.
Maps of where oil and gas development is anticipated in the US Intermountain West can be used by decision-makers intent on minimizing impacts to sage-grouse. This analysis also provides a general framework for using predictive models and build-out scenarios to anticipate impacts to species. These predictive models and build-out scenarios allow tradeoffs to be considered between species conservation and energy development prior to implementation.
Because a diversity of resources should support a diversity of consumers, most models predict that increasing plant diversity increases animal diversity. We report results of a direct experimental test of the dependence of animal diversity on plant diversity. We sampled arthropods in a well-replicated grassland experiment in which plant species richness and plant functional richness were directly manipulated. In simple regressions, both the number of species planted (log2 transformed) and the number of functional groups planted significantly increased arthropod species richness but not arthropod abundance. However, the number of species planted was the only significant predictor of arthropod species richness when both predictor variables were included in ANOVAs or a MANOVA. Although highly significant, arthropod species richness regressions had low R2 values, high intercepts (24 arthropod species in monocultures), and shallow slopes. Analyses of relations among plants and arthropod trophic groups indicated that herbivore diversity was influenced by plant, parasite, and predator diversity. Furthermore, herbivore diversity was more strongly correlated with parasite and predator diversity than with plant diversity. Together with regression results, this suggests that, although increasing plant diversity significantly increased arthropod diversity, local herbivore diversity is also maintained by, and in turn maintains, a diversity of parasites and predators.
Population growth rate is determined in all vertebrate populations by food supplies, and we postulate bottom-up control as the universal primary standard. But this primary control system can be overridden by three secondary controls: top-down processes from predators, social interactions within the species and disturbances. Different combinations of these processes affect population growth rates in different ways. Thus, some relationships between growth rate and density can be hyperbolic or even have multiple nodes. We illustrate some of these in marsupial, ungulate and rabbit populations. Complex interactions between food, predators, environmental disturbance and social behaviour produce the myriad observations of population growth in nature, and we need to develop generalizations to classify populations. Different animal groups differ in the combination of these four processes that affect them, in their growth rates and in their vulnerability to extinction. Because conservation and management of populations depend critically on what factors drive population growth, we need to develop universal generalizations that will relieve us from the need to study every single population before we can make recommendations for management.
Net CO(2) uptake in full sunlight, total leaf area (TLA), projected leaf area of detached leaves (PLA), and the silhouette area of attached leaves in their natural orientation to the sun at midday on June 1 (SLA) were measured for sun shoots of six conifer species. Among species, TLA/SLA ranged between 5.2 and 10.0 (x bar = 7.3), TLA/PLA ranged between 2.5 and 2.9 (x bar = 2.7) and PLA/SLA ranged between 2.0 and 3.7 (x bar = 2.2). These ratios were reflected in the ratios of net photosynthesis computed on the basis of the three measures of leaf area. The much smaller values for TLA/PLA compared with the values for TLA/SLA indicate that leaf orientation effects, or shading, or both, caused more variation in the interception of solar radiation than did variation in leaf geometry (i.e., cross-section). Silhouette leaf area of lodgepole pine, (Pinus contorta spp. latifolia) and subalpine fir (Abies lasiocarpa) shoots measured at the summer solstice varied almost 2-fold with diurnal changes in solar altitude and azimuth. Sun shoots of both species and shade shoots of lodgepole pine had the greatest SLA during the early morning and late afternoon. The midday decline in SLA was related to the relatively upright orientation of needles of subalpine fir sun shoots and the relatively upright orientation of both sun and shade shoots of lodgepole pine. Shade shoots of subalpine fir reached a maximum in SLA at midday and this was related to the near horizontal orientation both of the shoots and the needles on them.
All terrestrial ecosystems consist of aboveground and belowground components that interact to influence community- and ecosystem-level
processes and properties. Here we show how these components are closely interlinked at the community level, reinforced by
a greater degree of specificity between plants and soil organisms than has been previously supposed. As such, aboveground
and belowground communities can be powerful mutual drivers, with both positive and negative feedbacks. A combined aboveground-belowground
approach to community and ecosystem ecology is enhancing our understanding of the regulation and functional significance of
biodiversity and of the environmental impacts of human-induced global change phenomena.
Airborne communication between individuals, called "eavesdropping" in this paper, can cause plants to become more resistant to herbivores when a neighbor has been experimentally clipped. The ecological relevance of this result has been in question, since individuals may be too far apart for this interaction to affect many plants in natural populations. We investigated induced resistance to herbivory in sagebrush, Artemisia tridentata, caused by experimental clipping of the focal plant and its neighbors. We found no evidence for systemic induced resistance when one branch was clipped and another branch on the same plant was assayed for naturally occurring damage. In this experiment, air contact and plant age were not controlled. Previous work indicated that sagebrush received less damage when a neighboring upwind plant within 15 cm had been experimentally clipped. Here we found that pairs of sagebrush plants that were up to 60 cm apart were influenced by experimental clipping of a neighbor. Furthermore, we observed that most individuals had conspecific neighbors that were much closer than 60 cm. Air contact was essential for communication; treatments that reduced airflow between neighboring individuals, either because of wind direction or bagging, prevented induced resistance. Airflow was also necessary for systemic induced resistance among branches within an individual. Reports from the literature indicated that sagebrush is highly sectorial, as are many desert shrubs. Branches within a sagebrush plant do not freely exchange material via vascular connections and apparently cannot rely on an internal signaling pathway for coordinating induction of resistance to herbivores. Instead, they may use external, volatile cues. This hypothesis provides a proximal explanation for why sagebrush does not demonstrate systemic induced resistance without directed airflow, and why airborne communication between branches induces resistance.
Because a diversity of resources should support a diversity of consumers, most models predict that increasing plant diversity increases animal diversity. We report results of a direct experimental test of the dependence of animal diversity on plant diversity. We sampled arthropods in a well‐replicated grassland experiment in which plant species richness and plant functional richness were directly manipulated. In simple regressions, both the number of species planted ( \landscape $\mathrm{log}\,_{2} transformed) and the number of functional groups planted significantly increased arthropod species richness but not arthropod abundance. However, the number of species planted was the only significant predictor of arthropod species richness when both predictor variables were included in ANOVAs or a MANOVA. Although highly significant, arthropod species richness regressions had low \landscape $R^{2} values, high intercepts (24 arthropod species in monocultures), and shallow slopes. Analyses of relations among plants and arthropod trophic groups indicated that herbivore diversity was influenced by plant, parasite, and predator diversity. Furthermore, herbivore diversity was more strongly correlated with parasite and predator diversity than with plant diversity. Together with regression results, this suggests that, although increasing plant diversity significantly increased arthropod diversity, local herbivore diversity is also maintained by, and in turn maintains, a diversity of parasites and predators.
Elevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climatesoil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha1 yr1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low-N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C-CO2 signature, reflected the turnover times of SOMC pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climateSOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.
Most plants interact with both arbuscular mycorrhizal ( AM ) fungi, which increase nutrient acquisition, and herbivores such as aphids, which drain nutrients from plants. Both AM fungi and aphids can affect plant metabolic pathways and may influence each other by altering the condition of the shared host plant.
This study tests simultaneously the effects of AM fungi on interactions with aphids (bottom‐up effects) and the effects of aphids on interactions with AM fungi (top‐down effects). We hypothesized that: (i) attractiveness of plants to aphids is regulated by induced changes in production of plant volatile organic compounds ( VOC s) triggered by AM fungi or aphids; (ii) aphids reduce AM fungal colonization; and (iii) AM fungal colonization affects aphid development.
Broad beans were exposed to AM fungi, aphids and a combination of both. To test for the strength of bottom‐up and top‐down effects, separate treatments enabled establishment of mycorrhizas either before or after aphids were added to plants. VOC s produced by plants were used to (i) test their attractiveness to aphids and (ii) identify the semiochemicals causing attraction. We also measured plant growth and nutrition, AM fungal colonization and aphid reproduction.
AM fungi increased the attractiveness of plants to aphids, and this effect tended to prevail even for aphid‐infested plants. However, both attractiveness and aphid population growth depended on the timing of AM fungal inoculation. AM fungi suppressed emission of the sesquiterpenes ( E )‐caryophyllene and ( E )‐ β ‐farnesene, and aphid attractiveness to VOC s was negatively associated with the proportion of sesquiterpenes in the sample. Emission of ( Z )‐3‐hexenyl acetate, naphthalene and ( R )‐germacrene D was regulated by an interaction between aphids and AM fungi. Aphids had a negative effect on mycorrhizal colonization, plant biomass and nutrition.
Our data show that below‐ and above‐ground organisms can interact by altering the quality of their shared host plant even though there is no direct contact between them. Plant interactions with herbivores and AM fungi operate in both directions: AM fungi have a key bottom‐up role in insect host location by increasing the attractiveness of plant VOC s to aphids, whereas aphids inhibit formation of AM symbioses.
The leaves of plants in nature are commonly subjected to damage from a wide variety of agents, including herbivory, air pollutants, and simple physical damage. Despite the attention paid to damage effects on living plants, the potential effects on the quality of litter derived from damaged leaves has not been considered. We used controlled laboratory assays of decomposition to show that both ozone (0.2 mL/m(3), 4 h) and mite damage, but not ultraviolet radiation (UV-B) exposure, to living leaves of cottonwood plants resulted in a decrease in decomposition rate of litter derived from damaged leaves. Decomposition rates were approximate to 50% slower for litter from damaged plants, and there was a twofold increase in the refractory fraction. Contrary to expectation, there was a negative relationship between rate of decomposition and litter nitrogen content. Our finding of slow decomposition of high-nitrogen litter is explained by a general mechanism whereby cellular damage causes increases in complex phenolic material. Such materials can lead to reductions in decomposition and binding of available nitrogen. We suggest that this mechanism can translate a common occurrence, damage by a diversity of processes, into long-term and possibly large-scale alterations in detritus processing.
Human welfare is significantly linked to ecosystem services such as the suppression of pest insects by birds and bats. However, effects of biocontrol services on tropical cash crop yield are still largely unknown. For the first time, we manipulated the access of birds and bats in an exclosure experiment (day, night and full exclosures compared to open controls in Indonesian cacao agroforestry) and quantified the arthropod communities, the fruit development and the final yield over a long time period (15 months). We found that bat and bird exclusion increased insect herbivore abundance, despite the concurrent release of mesopredators such as ants and spiders, and negatively affected fruit development, with final crop yield decreasing by 31% across local (shade cover) and landscape (distance to primary forest) gradients. Our results highlight the tremendous economic impact of common insectivorous birds and bats, which need to become an essential part of sustainable landscape management.
Many of the existing parametric and nonparametric tests for homogeneity of variances, and some variations of these tests, are examined in this paper. Comparisons are made under the null hypothesis (for robustness) and under the alternative (for power). Monte Carlo simulations of various symmetric and asymmetric distributions, for various sample sizes, reveal a few tests that are robust and have good power. These tests are further compared using data from outer continental shelf bidding on oil and gas leases.
We used bird-centered vegetation measurements to assess selection of shrub-defined patches by foraging Sage Sparrows (Amphispiza belli) and Brewer's Sparrows (Spizella breweri) over a period of 3 yr. Principal components of habitat variation based on a random sample of 900 patches were patch size, species composition (big sagebrush [Artemisia tridentata] vs. green rabbitbrush [Chrysothamnus viscidiflora] vs. gray rabbitbrush [C. nauseosus]), and plant vigor. Both bird species differed from random (P < 0.001) in their selection of patches in each year, but did not differ from each other. Patch structure varied significantly among years (P < 0.001), as did each species' use of patches. Between-year changes in the two bird species' selection patterns were highly correlated with changes in the habitat, and with each other. These correlations suggest that shrubsteppe sparrows 'track' interannual variation in patch composition and structure (which is presumably induced by precipitation variation) but maintain nonrandom use patterns within years.
Urban and suburban forests provide important recreational values to the public and can also contain high levels of biodiversity even though management regimes of such forests often include the clearing of bushes, shrubs and small trees to make them more attractive to people. In a field experiment, we studied the importance of bush canopies as foraging sites for insectivorous birds. In suburban forests, the canopies of rowan (Sorbus aucuparia) and alder buckthorn (Frangula alnus) were net-enclosed for two months at three study sites in SW Sweden. We compared abundance, biomass and body size distributions of arthropods between experimental and control canopies of rowan and alder buckthorn and found the effects to be strong. Biomass of arthropods was generally an order of magnitude higher on canopies protected from bird predation than on controls. Dermaptera contributed most to the biomass estimated in net-enclosed bushes. In certain taxa, however, the biomass was not significantly affected by the experimental treatment, possibly in part due to on-going reproduction during the experimental period. Furthermore, median body size in net-enclosed bushes was 1.3–3.0 times the control, excluding Araneae. The relative difference in arthropod biomass due to the experimental treatment was related to the density of breeding insectivorous birds at each site. We concluded that bush canopies are important foraging sites for birds in suburban forests, although we cannot rule out trait-mediated effects. Management regimes that include clearing of the ground may affect the bird community negatively.
Bell System Technical Journal, also pp. 623-656 (October)
The respiratory activity of plants in the light, measured as CO:! release from the TCA cycle or O-2 consumption by the respiratory chain, varies between 25 and 100% of the dark respiratory activity. This has been interpreted as evidence for an inhibition of respiration during photosynthesis. However, studies with specific respiratory inhibitiors have shown that oxidative phosphorylation occurs in the Light and provides the cytosol with ATP, which is required for sucrose synthesis. Respiratory activity in the light might also be required to sustain a high photosynthetic capacity and might even prevent photoinhibition. Sources of redox equivalents for oxidative phosphorylation can be photosynthetically generated, externally oxidized NADPH; photorespiratory oxidation of glycine to serine; or a partial activity of the TCA cycle. Fifty to seventy-five percent of the redox equivalents produced in the mitochondria remain in the matrix and can function in ATP syntheses. The other 25-50% are exported via the mitochondrial malate-oxaloacetate shuttle and can function in nitrate reduction in the cytosol or hydroxypyruvate reduction in the peroxisomes.
The enemies hypothesis holds that predatory insects and parasitoids are more effective at controlling populations of herbivores in diverse systems of vegetation than in simple ones. Eighteen studies that tested the enemies hypothesis are reviewed. Of those studies reporting mortality from predation or parasitism, nine found higher mortality rates in diverse systems; two found a lower mortality rate; and two found no difference. The mechanisms that are thought to underlie the enemies hypothesis and directions for future research are discussed. Evidence suggests that the enemies hypothesis and the resource concentration hypothesis (which predicts that herbivores more easily find, stay in, and reproduce in monocultures of host plants than in polycultures) are complementary mechanisms in reducing numbers of herbivores in diverse agricultural systems.
The sagebrush biotype is the largest in the western United States. This vast sagebrush community is thought to harbor equally vast and diverse arthropod communities, but these remain little explored. Our objective was to examine the diversity, abundance, and seasonal phenology of arthropod taxa found on the dominant shrub of the sagebrush ecosystem, big sagebrush (Artemisia tridentata). We wanted to improve understanding of this little-studied arthropod assemblage that may play significant roles in the dynamics of sagebrush populations and the sagebrush ecosystem. We sampled free-living and gall-forming arthropods from a stratified random sample of sagebrush plants at the Barton Road Ecological Research Area, Idaho, resulting in a sample of over 8000 individuals and 232 morphospecies. Species richness and abundance declined from May to August, and abundance of most taxa similarly declined over the summer. A few taxa, including Acari (mites), were notably more abundant in August. Fluid feeders were the most diverse and abundant free-living feeding guild during all months and comprised up to 79% of morphospecies. The gall formers included 4713 individuals of 12 species of gall flies (Rhopalomyia spp.), primarily (97%) R. ampullaria. Abundance of galls increased from small to large (presumably young to old) plants. Overall, A. tridentata was host to a high diversity of arthropods, some of which have potential to cause or mitigate significant damage to their host plant. Arthropods seem likely to have the greatest impact on sagebrush early in the growing season, when they are most diverse and abundant. Documentation of the full diversity of arthropods associated with sagebrush required samples taken throughout the growing season, but a single sample early in the growing season captured a high proportion of taxa.
Many factors can influence the top‐down and bottom‐up dynamics of phytophagous insects. Although interactions between herbivore species have been frequently shown to be ecologically important, the effects of such horizontal trophic interactions on the relative roles of top‐down and bottom‐up forces have gone largely unstudied. In this paper we report on the results of a factorial field experiment in which we examined the effects of within‐trophic‐level interactions on the top‐down and bottom‐up dynamics of a salt marsh planthopper.
We manipulated the bottom‐up effects of plant quality by increasing soil salinity, and manipulated top‐down effects by decreasing the intensity of parasitoid attack with yellow sticky traps that removed hymenopteran parasitoids. We applied these treatments to plots in two patches of the host plant, one with low densities of lepidopteran stem borer larvae, and one with high densities of stem borers. We maintained the treatments and monitored planthopper density for ten months, from March through December 1999. Increased salinity significantly increased planthopper density within one month of the first application of salt. The rapid response of the planthopper to salt treatments suggested a chemical mechanism, perhaps mobilization of bound nitrogen. Yellow sticky traps, although significantly reducing parasitism of planthopper eggs, had little impact on hopper density. The density of lepidopteran stem borers, however, had an even greater impact on planthopper density than did salt treatments, with high stem borer plots supporting much lower densities of hoppers. Stem borer density also reduced the response of the planthopper to other treatments, especially salt supplementation. The results of this study show that the impact of within‐trophic‐level interactions can significantly change herbivore trophic dynamics and can be even more important than either top‐down or bottom‐up effects in determining herbivore density.
Elevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long-term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha−1 yr−1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low-N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C-CO2 signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.
Insect herbivores can reduce growth, seed production, and population dynamics of host plants, but do not always do so. Big
sagebrush (Artemisia tridentata) has one of the largest ranges of any shrub in North America, and is the dominant and characteristic shrub of the extensive
sagebrush steppe ecosystem of the western United States. Nevertheless, the impact of insect herbivores on big sagebrush, its
dominant and characteristic shrub, is largely unknown. Occasional large effects of insect herbivore outbreaks are documented,
but there is little knowledge of the impact of the more typical, nominal herbivory that is produced by the diverse community
of insects associated with big sagebrush in natural communities. In 2008, we removed insects from big sagebrush plants with
insecticide to evaluate whether insect herbivores reduced growth and seed production of big sagebrush. Removal of herbivores
led to significant and substantial increases in inflorescence growth (22%), flower production (325%), and seed production
(1053%) of big sagebrush. Our results showed the impact of insect herbivory in the current growing season on the growth and
reproduction of big sagebrush and revealed an unrecognized, significant role of non-outbreak herbivores on big sagebrush.
KeywordsArthropod-Defoliator-Feeding guild-Trophic dynamics-Sagebrush steppe
Insectivorous birds can increase plant growth by consuming herbivorous insects and reducing insect damage. However, plant
traits such as the level of chemical defense may affect the quantity and quality of insects, and alter the foraging behavior
of birds. Therefore, I predicted that plant traits can also modify the effect of birds on leaf damage and plant growth. This
study compared the effect of insectivorous birds on the herbivory and growth of two chemically different willow species, weakly
defended Salix phylicifolia and strongly defended S. myrsinifolia under two fertilization levels. Half of the willows were protected from birds using a translucent gill-net, which did not
limit access by insects. The effect of birds on the densities of leaf-chewing insects and leaf damage was considerable on
unfertilized S. phylicifolia but less obvious on fertilized ones. The effect of bird predation was negligible on S. myrsinifolia, which had very low insect densities in all treatments. Birds increased the growth of the experimental willows, but the effect
was clear only in unfertilized S. phylicifolia. I suggest that birds avoided foraging on willows with low populations of insects and little visible damage. The study shows
that bird predation can alter the patterns of insect densities we see on willows, emphasizing the importance of considering
multitrophic effects when studying plant-insect interactions.
Differences of collembolan communities within the organo-mineral A layer were studied in relation to physico-chemical changes in humus at nine sites of beech forests (Fagus sylvatica L.) and first generation spruce stands (Picea abies (L.) Karst.), planted on former beech forest 30 years ago (Central Pyrenees, France). Changes in humus form were caused by the spruce plantation and occurred mainly within the fermentation horizon where acidifying litter accumulation increased the horizon depth. The recent replacement of beech by spruce induced a shift from mull towards moder humus forms, which is explained by the decreasing organic matter turnover rate. A significant decrease in the three exchangeable cations and pH under spruce was also observed. Collembolan species diversity within the A horizon was significantly lower under spruce at this early stage of the silvogenetic cycle. Differences between species composition of Collembola between the two forest stands is related to changes in environmental conditions (e.g. nutrient availability, soil porosity, soil moisture). This study shows how forest management practices are susceptible to modify biological activity within the A horizon under exotic conifer plantations.
The biota of the food web of the sand community in the Coachella Valley desert include 174 species of vascular plants, 138 species of vertebrates, >55 species of arachnids, and an unknown (but great) number of microorganisms, insects (2000-3000 estimated species), Acari and nematodes. Trophic relations are presented in a series of nested subwebs and delineations of the community. Complexity arises from the large number of interactive species, the frequency of omnivory, age structure, looping, the lack of compartmentalization, and the complexity of the arthropod and soil faunas. Patterns from the Coachella web are compared with theoretical predictions and "empirical generalizations' derived from catalogs of published webs. The Coachella web differs greatly: chains are longer, omnivory and loops are not rare, connectivity is greater, top predators are rare or nonexistent, and prey-to-predator ratios are >1.0. The evidence argues that actual community food webs are extraordinarily more complex than those webs cataloged by theorists. -from Author
Low precipitation and high temperatures in deserts limit primary productivity, and reduce rates of herbivory and microbial decomposition. As a result, the small amount of plant tissue produced in deserts dies unconsumed and turns to litter that is consumed by macrodetritivorous arthropods, which are preyed upon by small ectothermic arthropods and reptiles. This ectotherm-based food chain is energetically efficient and, despite the low productivity of the desert habitat, is able to support a fourth trophic level of endothermic predators, i.e. mammals and birds, that forage over large areas and locate their prey visually from a distance. This four-link chain results in trophic interactions that run from the large endothermic predators through the ectothermic ones to the macrodetritivores. Thus, macrodetritivores are released from predation and become food-limited. However, the full expression of these interactions occurs only in low productivity habitats with low plant cover. In productive habitats, plant cover blocks the vision of endothermic predators and provides refuge to small ectothermic ones. This results in ectothermic predators becoming abundant in habitats with high plant cover and controlling their prey, the macrodetritivores. Thus, the increase in productivity in deserts decreases the amount of energy that reaches top predators as it has a mainly structural effect: to increase plant cover and mediate predation interactions.
Traditionally, herbivorous insects are thought to exhibit enhanced performance and outbreak dynamics on water-stressed host plants due to induced changes in plant physiology. Recent experimental studies, however, provide mixed support for this historical view. To test the plant-stress hypothesis (PSH), we employed two methods (the traditional vote-counting approach and meta-analysis) to assess published studies that investigated insect responses to experimentally induced water-deficit in plants. For insects, we examined how water deficit affects survivorship, fecundity, density, relative growth rate, and oviposition preference. Responses were analyzed by major feeding guild (sap-feeding insects and chewing insects) and for the subguilds of sap-feeders (phloem, mesophyll, and xylem feeders) and chewing insects (free-living chewers, borers, leaf miners, and gall-formers). Both vote counting and meta-analysis found strong negative effects of water stress on the performance of sap-feeding insects at large and on members of the phloem- and mesophyll-feeding subguilds in particular. Both analytical techniques demonstrated a nonsignificant response for chewing insects at large due to the offsetting effects of water stress on the different subguilds. For example, our analyses found consistent positive responses for borers, negative responses for gall-formers, and in consistent responses for free-living species and leaf miners. Overall, our analyses strongly challenge the historical view that herbivorous insects exhibit. elevated performance and outbreak dynamics on water-stressed plants. Rather, there is widespread evidence that many phytophagous insects, especially sap-feeders, are adversely affected by continuous water stress. Despite enhanced foliar nitrogen during times of plant stress, concurrent reductions in turgor and water content interfere with an herbivore's ability to access or utilize nitrogen. To explain the discrepancy between the observed outbreaks of phytophagous insects on water-stressed plants in nature and the negative effects detected in many experimental studies where plants are continuously stressed, we propose a "pulsed stress hypothesis" whereby bouts of stress and the recovery of turgor allow sap-feeders to benefit from stress-induced increases in plant nitrogen. Our finding that phloem-feeding insects respond positively on intermittently stressed plants but exhibit poor performance on continuously stressed ones is consistent with this hypothesis and suggests that the phenology of water stress as it mediates nitrogen availability may hold the key to understanding how water stress affects the population dynamics of insect herbivores.
Growth of vegetative structures occurred in the spring when water was not limiting, and shrubs in both watering treatments exerted little stomatal control over water loss. Reproductive growth occurred through the summer when water was limiting, and supplemental watering increased reproductive growth. Shrubs conserved water during summer by abscising leaves and lowering stomatal conductance of both vegetative and reproductive modules in response to decreases in xylem pressure potential and increases in evaporative demand. Leaf abscission can occur without decreasing the amount of carbon available for reproductive growth because inflorescences are capable of positive photosynthetic rates comparable to vegetative leaves. Resource limitations were, however, reflected in the efficiency of water use during tissue construction; floral leaves and floral heads of shrubs not receiving supplemental water were produced with higher water-use efficiency. Conservative use of water during production of vegetative modules would offer no advantage to A. tridentata because neighbouring species are also most active at this time. Reproductive growth in A. tridentata occurs during summer when neighbouring species are largely dormant, and so efficient use of water may allow development of reproductive structures to continue throughout the summer even with limited supplies of water. -from Authors
Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.