[Show abstract][Hide abstract] ABSTRACT: Agroforestry practices may influence pest incidence and abundance both through increased top-down regulation by natural enemies and via bottom-up factors such as moderation of microclimate, soil nutrients and water content. We conducted a meta-analysis of the effects of agroforestry on the abundance of invertebrate pests, weeds, natural enemies and plant damage due to pests and diseases. We also tested whether effects of agroforestry were dependent on crop type (annual or perennial), type of pest association (above or belowground) and weed type (parasitic Striga weeds or non-parasitic weeds).Agroforestry practices resulted in lower abundances of both parasitic and non-parasitic weeds, and in higher abundances of natural enemies. The effects of agroforestry on invertebrate pests and diseases were dependent on crop type. In perennial crops (e.g. coffee, cocoa and plantain), agroforestry was associated with lower pest abundances and less plant damage. However, the effects were not significant in annual crops (e.g. maize, rice and beans). Despite the limited number of crop-pest systems available for the analyses, overall our results suggest that agroforestry is beneficial in terms of pest, disease and weed management.
Full-text · Article · Aug 2015 · Basic and Applied Ecology
[Show abstract][Hide abstract] ABSTRACT: Disturbances, such as fire and grazing, are often claimed to facilitate plant species richness and plant invasions in particular, although empirical evidence is contradictory. We conducted a meta-analysis to synthesize the literature on how non-native plant species are affected by disturbances. We explored whether the observed impact of disturbance on non-native plant communities is related to its type and frequency, to habitat type, study approach (observational or experimental), and to the temporal and spatial scales of the study. To put the results in a broader context, we also conducted a set of parallel analyses on a data set involving native plant species. The diversity and abundance of non-native plant species were significantly higher at disturbed sites than at undisturbed sites, while the diversity and abundance of native plant species did not differ between the two types of sites. The effect of disturbance on non-native plant species depended on the measure used to evaluate the impact (species diversity or abundance) and on disturbance type, with grazing and anthropogenic disturbances leading to higher diversity and abundance of non-native plant species than other disturbance types examined. The impact of disturbance on non-natives was also associated with study approach, habitat type and temporal scale, but these factors covaried with disturbance type, complicating the interpretation of the results. Overall, our results indicate that disturbance has a positive impact particularly on non-native plant species (at least when they are already present in the community), and that the strength of this impact depends primarily on the disturbance type.
[Show abstract][Hide abstract] ABSTRACT: Tropical forests are important reservoirs of biodiversity, but the processes that maintain this diversity remain poorly understood. The Janzen-Connell hypothesis suggests that specialized natural enemies such as insect herbivores and fungal pathogens maintain high diversity by elevating mortality when plant species occur at high density (negative density dependence; NDD). NDD has been detected widely in tropical forests, but the prediction that NDD caused by insects and pathogens has a community-wide role in maintaining tropical plant diversity remains untested. We show experimentally that changes in plant diversity and species composition are caused by fungal pathogens and insect herbivores. Effective plant species richness increased across the seed-to-seedling transition, corresponding to large changes in species composition. Treating seeds and young seedlings with fungicides significantly reduced the diversity of the seedling assemblage, consistent with the Janzen-Connell hypothesis. Although suppressing insect herbivores using insecticides did not alter species diversity, it greatly increased seedling recruitment and caused a marked shift in seedling species composition. Overall, seedling recruitment was significantly reduced at high conspecific seed densities and this NDD was greatest for the species that were most abundant as seeds. Suppressing fungi reduced the negative effects of density on recruitment, confirming that the diversity-enhancing effect of fungi is mediated by NDD. Our study provides an overall test of the Janzen-Connell hypothesis and demonstrates the crucial role that insects and pathogens have both in structuring tropical plant communities and in maintaining their remarkable diversity.
[Show abstract][Hide abstract] ABSTRACT: An increase in species richness with decreasing latitude is a prominent pattern in nature. However, it remains unclear whether there are corresponding latitudinal gradients in the properties of ecological interaction networks. We investigated the structure of 216 quantitative antagonistic networks comprising insect hosts and their parasitoids, drawn from 28 studies from the High Arctic to the tropics. Key metrics of network structure were strongly affected by the size of the interaction matrix (i.e. the total number of interactions documented between individuals) and by the taxonomic diversity of the host taxa involved. After controlling for these sampling effects, quantitative networks showed no consistent structural patterns across latitude and host guilds, suggesting that there may be basic rules for how sets of antagonists interact with resource species. Furthermore, the strong association between network size and structure implies that many apparent spatial and temporal variations in network structure may prove to be artefacts.
[Show abstract][Hide abstract] ABSTRACT: The coexistence of plant species in species-rich tropical forests can be promoted by specialised enemies acting in a negatively density-dependent manner. While survival of tropical tree seedlings is often negatively density-dependent, the causes have rarely been identified. We tested whether insects and plant pathogens cause density-dependent seedling recruitment and survival in five forest tree species in Belize, Central America. We manipulated densities of seeds or newly germinated seedlings in small (1 m2 or 0.25 m2) plots close to fruiting conspecific trees. Using a factorial design, we excluded enemies from subsets of the plots with fungicides and insecticides. Seed germination (for two species) and early seedling survival (for all species) were monitored at approximately weekly intervals for up to eight weeks, during the period when plants are likely to be most susceptible to natural enemies. In Terminalia amazonia, seed germination was negatively density-dependent and the proportion of seeds germinating increased when insects were excluded. However, the magnitude of the insecticide effect was independent of density. The only significant density effect for survival of young seedlings was in Acacia polyphylla; counter to expectation, seedling survival was higher at high densities. In a few cases pesticide application had a significant effect on seedling survival, but in only one case (Terminalia amazonia) was a significant pesticide × density interaction detected. Our results caution against generalising from studies conducted on a single species at a single time and place and illustrate the challenges of experimentally testing for enemy-mediated negative density-dependence. Experimental outcomes are likely to depend on the spatial scale at which the principal enemies disperse and respond to plant density, and the timescales over which they act. Gathering information on these variables will improve our understanding of the natural histories of tropical forest species and help inform the design of future experiments.
[Show abstract][Hide abstract] ABSTRACT: Ecology Letters (2012)
Although phytophagous insects and plant pathogens frequently share the same host plant, interactions among such phylogenetically distant taxa have received limited attention. Here, we place pathogens and insects in the context of a multitrophic-level community. Focusing on the invasive powdery mildew Erysiphe alphitoides and the insect community on oak (Quercus robur), we demonstrate that mildew–insect interactions may be mediated by both the host plant and by natural enemies, and that the trait-specific outcome of individual interactions can range from negative to positive. Moreover, mildew affects resource selection by insects, thereby modifying the distribution of a specialist herbivore at two spatial scales (within and among trees). Finally, a long-term survey suggests that species-specific responses to mildew scale up to generate landscape-level variation in the insect community structure. Overall, our results show that frequently overlooked cross-kingdom interactions may play a major role in structuring terrestrial plant-based communities.
[Show abstract][Hide abstract] ABSTRACT: 1. The small tortoiseshell butterfly (Aglais urticae L.) is considered to be a widespread and abundant generalist species in Northern Europe. However, it declined sharply in the U.K. between 2003 and 2008, coinciding with the arrival and spread of a parasitoid, Sturmia bella Meig. (Diptera: Tachinidae), which specialises on nymphalid butterflies.
2. Whether the decline in A. urticae is associated with the arrival of S. bella was investigated using data from a large-scale butterfly monitoring scheme, and by collecting larvae to assess parasitoid incidence and parasitism frequency. Similar data were compiled for a related butterfly (Inachis io) which is also parasitised by S. bella but which is not declining.
3. Sturmia bella was recorded as far north as north Lincolnshire (53.53°N). Aglais urticae has declined significantly to the south of this latitude, but not to the north.
4. Sturmia bella was present in 26% and 15% of the larval groups of A. urticae and I. io, respectively, and now kills more individuals of A. urticae (but not I. io) than any native parasitoid.
5. Survival was 25–48% lower in batches of A. urticae larvae where S. bella was present, indicating that S. bella causes host mortality in addition to that caused by native parasitoids.
6. Our results suggest that S. bella may be playing a role in the recent decline of A. urticae. However, further research is needed to establish its effects relative to other potential drivers of trends in the abundance of this butterfly.
No preview · Article · Mar 2011 · Ecological Entomology
[Show abstract][Hide abstract] ABSTRACT: 1. Shared enemies may link the dynamics of their prey. Recently, quantitative food webs have been used to infer that herbivorous insect species attacked by the same major parasitoid species will affect each other negatively through apparent competition. Nonetheless, theoretical work predicts several alternative outcomes, including positive effects.
2. In this paper, we use an experimental approach to link food web patterns to realized population dynamics. First, we construct a quantitative food web for three dominant leaf miner species on the oak Quercus robur. We then measure short- and long-term indirect effects by increasing leaf miner densities on individual trees. Finally, we test whether experimental results are consistent with natural leaf miner dynamics on unmanipulated trees.
3. The quantitative food web shows that all leaf miner species share a minimum of four parasitoid species. While only a small fraction of the parasitoid pool is shared among Tischeria ekebladella and each of two Phyllonorycter species, the parasitoid communities of the congeneric Phyllonorycter species overlap substantially.
4. Based on the structure of the food web, we predict strong short- and long-term indirect interactions between the Phyllonorycter species, and limited interactions between them and T. ekebladella. As T. ekebladella is the main source of its own parasitoids, we expect to find intraspecific density-dependent parasitism in this species.
5. Consistent with these predictions, parasitism in T. ekebladella was high on trees with high densities of conspecifics in the previous generation. Among leaf miner species sharing more parasitoids, we found positive rather than negative interactions among years. No short-term indirect interactions (i.e. indirect interactions within a single generation) were detected.
6. Overall, this study is the first to experimentally demonstrate that herbivores with overlapping parasitoid communities may exhibit independent population dynamics – or even apparent mutualism. Hence, it proves the potential for versatile indirect interactions in nature, and suggests that the link between patterns in food web structure and realized population dynamics should be verified by rigorous experiments.
Preview · Article · Jan 2011 · Journal of Animal Ecology
[Show abstract][Hide abstract] ABSTRACT: Ecology Letters (2010)
The Janzen-Connell hypothesis is a leading explanation for plant-species diversity in tropical forests. It suggests that specialized natural enemies decrease offspring survival at high densities beneath parents, giving locally rarer species an advantage. This mechanism, in its original form, assumes that density dependence is overcompensating: mortality must be disproportionately high at the highest densities, with few offspring recruiting below their parents. We tested this assumption using parallel shadehouse and field density-series experiments on seedlings of a tropical tree, Pleradenophora longicuspis. We found strong, overcompensating mortality driven by fungal pathogens, causing 90% (shadehouse) or 100% (field) mortality within 4 weeks of germination, and generating a negative relationship between initial and final seedling densities. Fungicide treatment led to much lower, density-independent, mortality. Overcompensating mortality was extremely rapid, and could be missed without detailed monitoring. Such dynamics may prevent dead trees from being replaced by conspecifics, promoting coexistence as envisioned by the Janzen-Connell hypothesis.
[Show abstract][Hide abstract] ABSTRACT: Ecology Letters (2010) 13: 383–393
The extent to which behavioural choices reflect fine-tuned evolutionary adaptation remains an open debate. For herbivorous insects, the preference–performance hypothesis (PPH) states that female insects will evolve to oviposit on hosts on which their offspring fare best. In this study, we use meta-analysis to assess the balance of evidence for and against the PPH, and to evaluate the role of individual factors proposed to influence host selection by female insects. We do so in an explicitly bitrophic context (herbivores versus plants). Overall, our analyses offer clear support for the PPH: Offspring survive better on preferred plant types, and females lay more eggs on plant types conducive to offspring performance. We also found evidence for an effect of diet breadth on host choice: female preference for ‘good quality plants’ was stronger in oligophagous insects than in polyphagous insects. Nonetheless, despite the large numbers of preference–performance studies conducted to date, sample sizes in our meta-analysis are low due to the inconsistent format used by authors to present their results. To improve the situation, we invite authors to contribute to the data base emerging from this work, with the aim of reaching a strengthened synthesis of the subject field.
[Show abstract][Hide abstract] ABSTRACT: Summary • The seed-to-seedling transition may be a critical stage in determining the dynamics of plant populations. Insects which kill seeds either before or after dispersal can influence the population dynamics of individual plant species, and ultimately, plant diversity and assemblage composition. • We discuss the potential for insect seed predators to maintain diversity in plant assemblages and to structure their composition, with a particular focus on diverse tropical forest habitats. We suggest that our ability to understand the functional effects of insect seed predators is hampered by a shortage of unbiased information on (i) their responses to the density of prey seeds at different spatial scales, and (ii) their host plant specificity. • Density-dependence and its implications may be best assessed using manipulative field experiments. Such approaches can reveal how insect seed predators respond behaviourally and demographically to the density of individual host species and multiple host species across a range of spatial scales. • Host specificity and its implications may be best addressed through quantitative food web approaches previously applied largely to host–parasitoid interactions. Food webs will allow ecologists to assess the likely importance of indirect interactions such as apparent competition and apparent mutualism in structuring plant assemblages, and the functional consequences of adding or removing individual resource or consumer species. • Fully quantifying the wider effects of seed predators will require studies that better integrate seed stage-specific demographic information, and which quantify the long-term effects of variations in seed predation rates for plant recruitment. • Synthesis and applications. Compared to other functionally important insect groups such as pollinators, seed predators have received relatively little attention in the context of the functioning and sustainability of agro-ecosystems and the consequences of global environmental change for ecological communities. A fuller understanding of the ecology of insect seed predator–plant interactions will be valuable to conservation and management in a range of natural and agricultural systems. For example, seed predator community ecology is relevant to predicting the consequences of deliberately or unintentionally introducing novel resource or consumer species; the process of habitat recovery following local disturbances; and managing the effects of pest or beneficial seed predators in agricultural crops. Furthermore, patterns of insect seed predation on a larger scale are likely to be highly sensitive to global environmental change drivers such as climate change and systematic habitat modification and fragmentation, with implications for the structure and organization of ecological communities more widely.
Full-text · Article · Oct 2008 · Journal of Applied Ecology
[Show abstract][Hide abstract] ABSTRACT: 1. The spatial structure of natural populations may profoundly influence their dynamics. Depending on the frequency of movements among local populations and the consequent balance between local and regional population processes, earlier work has attempted to classify metapopulations into clear-cut categories, ranging from patchy populations to sets of remnant populations. In an alternative, dichotomous scheme, local populations have been classified as self-sustaining populations generating a surplus of individuals (sources) and those depending on immigration for persistence (sinks). 2. In this paper, we describe the spatial population structure of the leaf-mining moth Tischeria ekebladella, a specialist herbivore of the pedunculate oak Quercus robur. We relate moth dispersal to the distribution of oaks on Wattkast, a small island (5 km(2)) off the south-western coast of Finland. 3. We build a spatially realistic metapopulation model derived from assumptions concerning the behaviour of individual moths, and show that the model is able to explain part of the variation in observed patterns of occurrence and colonization. 4. While the species was always present on large trees, a considerable proportion of the local populations associated with small oaks showed extinction-recolonization dynamics. The vast majority of moth individuals occur on large trees. 5. According to model predictions, the dominance of local vs. regional processes in tree-specific moth dynamics varies drastically across the landscape. Most local populations may be defined broadly as 'sinks', as model simulations suggest that in the absence of immigration, only the largest oaks will sustain viable moth populations. Large trees in areas of high oak density will contribute most to the overall persistence of the metapopulation by acting as sources of moths colonizing other trees. 6. No single 'metapopulation type' will suffice to describe the oak-moth system. Instead, our study supports the notion that real populations are often a mix of earlier identified categories. The level to which local populations may persist after landscape modification will vary across the landscape, and sweeping classifications of metapopulations into single categories will contribute little to understanding how individual local populations contribute to the overall persistence of the system.
Full-text · Article · Aug 2008 · Journal of Animal Ecology
[Show abstract][Hide abstract] ABSTRACT: Abstract 1. The distribution, abundance and population dynamics of herbivorous insects may be affected by trophic interactions, by abiotic influences, or by intra-specific processes. Relatively little is known about how trophic influences vary across space. Here, we investigate spatial variation in mortality in the oak-feeding leaf miner Tischeria ekebladella as attributable to individual causal agents.
2. Leaf miners were experimentally introduced on 67 trees on an island 5 km2 in area in south-western Finland. On each tree, some larvae were protected by a muslin bag, others by a glue barrier around the branch and some left exposed.
3. In the bagged transplants, 78.4% of larvae survived, compared with only 9.6% in the other two treatments. Most of the mortality was because of airborne agents: mortality on branches sheltered by a glue barrier was as high as on fully exposed branch tips.
4. We consider mortality caused by parasitoid wasps to be the main source of larval death and the primary factor driving general patterns of survival. The effects of bird predation and premature leaf abscission were negligible.
5. We detected spatial aggregation in larval survival and parasitism rates at the level of individual trees, but not across the landscape.
6. Spatial variation in overall leaf miner survival, parasitism and leaf abscission does not suffice to explain patterns of incidence and abundance of wild T. ekebladella on experimental trees. Rather, we identify metapopulation dynamics as a likely determinant of the spatial distribution of T. ekebladella in the landscape.
Preview · Article · May 2008 · Ecological Entomology
[Show abstract][Hide abstract] ABSTRACT: 1. According to the preference-performance hypothesis, female insects select resources that maximize offspring performance. To achieve high fitness, leaf miner females should then adjust their oviposition behaviour in response to leaf attributes signalling high host quality. 2. Here we investigate resource selection in Tischeria ekebladella, a leaf-mining moth of the pedunculate oak (Quercus robur), in relation to two alternative hypotheses: (1) females select their resources with respect to their future quality for developing larvae; or (2) temporal changes in resource quality prevent females from selecting the best larval resources. 3. Specifically, we test whether females show the strongest selection at the levels at which quality varies the most (shoots and leaves); whether they respond to specific leaf attributes (leaf size, phenolic content and conspecific eggs); and whether female preference is reflected in offspring performance. 4. Female choice of leaves was found to be non-random. Within trees, the females preferred certain shoots, but when the shoots were on different trees the degree of discrimination was about four times larger than when they were on the same trees. 5. While females typically lay more eggs on large leaves, this is not a result of active selection of large leaves, but rather a result of females moving at random and ovipositing at regular intervals. 6. The females in our study did not adjust their oviposition behaviour in response to leaf phenolic contents (as measured by the time of larval feeding). Neither did they avoid leaves with conspecific eggs. 7. Female choice of oviposition sites did not match patterns of offspring performance: there was no positive association between offspring survival and counts of eggs. 8. We propose that temporal variation in resource quality may prevent female moths from evaluating resource quality reliably. To compensate for this, females may adopt a risk-spreading strategy when selecting their resources.
Full-text · Article · Oct 2007 · Journal of Animal Ecology
[Show abstract][Hide abstract] ABSTRACT: Summary • The spatial population structure of parasites may have profound effects on both ecological and evolutionary dynamics. Strong gene flow among local populations has been hypothesized to prevent differential performance on local and foreign hosts. • We studied the spatial population structure of an obligate pathogen, the oak mildew (Microsphaera alphitoides) on the pedunculate oak (Quercus robur). Patterns of distribution and colonization–extinction dynamics were recorded in a set of 113 trees over 3 years, and in an additional set of 77 trees over 2 years. Colonization of uninfected hosts was examined in a set of 100 experimentally transplanted hosts, and differential performance on local and foreign hosts tested by reciprocal inoculations conducted in the laboratory. • Across years, the mildew occupied a high proportion of host trees ( ≥ 2/3). Patterns of mildew infection showed either no spatial aggregation (years 2003, 2005), or aggregation at a large spatial scale (up to 400 m; year 2004). High and even incidence of infection reflects efficient dispersal: during a single year, transplanted uninfected host plants were rapidly colonized in all parts of the landscape. • Local mildew populations still performed best on their original hosts: in reciprocal inoculations conducted on mature foliage, local mildew populations infected a significantly higher proportion of leaves from their original host tree than from other trees. Yet, even on the local host less than half of the leaves were infected. • To account for the observed patterns, two selection processes are invoked. The colonization experiment suggests that mildew spores are spread widely, with selection filtering out the genotypes that cannot infect the focal host. Local mildew populations are subject to further selection events later in the summer, favouring the strains best adapted to phenological changes in the local environment. • In conclusion, the patterns observed and processes inferred in M. alphitoides suggest that complex selection pressures may affect local parasite populations, blurring any clear-cut relation between gene flow and performance on local and foreign hosts. Functional Ecology (2007)doi: 10.1111/j.1365-2435.2007.01315.x
[Show abstract][Hide abstract] ABSTRACT: When examined at any moment in time, different parts of an individual oak tree exhibit almost as large differences in quality as different trees. But how consistent are such patterns in time? In this paper, we use intraclass correlations to assess the temporal consistency of host plant quality at several spatial scales. As measures of quality, we use both individual chemical attributes (phenolic contents) and the overall performance (larval survival) of the host-specific leaf-miner Tischeria ekebladella. Concentrations of 24 phenolic compounds were monitored on seven trees throughout a season. Variation in mine initiation and larval survival rates was assessed for individuals transplanted to another set of trees early versus later in the season, while year-to-year variation in larval survival was studied through stratified surveys of wild individuals during three years. At all time scales considered, measures of host quality were moderately consistent: a tree favourable in quality at one point in time often remained so, but there was abundant variation around this relationship (ρ=0.4–0.6). One hierarchical level deviated from this general pattern: on individual branches, larval survival rates varied randomly among years (ρ=0). Our study suggests that the quality of trees, and in particular of smaller units within trees, may be difficult to predict both in space and in time. To account for this, insects might benefit from adopting a bet-hedging strategy when selecting resources.
[Show abstract][Hide abstract] ABSTRACT: Why is the World green – what keeps herbivores, and herbivorous insects in particular, from consuming all of their food? When this question was first posed, the relative importance of top-down and bottom-up effects was hotly disputed. While modern ecologists may agree that impacts from several different directions will affect local insect densities, the bottom-up vs top-down jargon seems to be stuck in a unidimensional world. Here, we argue that the strength of almost every bottom-up and top-down force is likely to vary in space, and that in itself, spatial structure invokes new processes which defy classification in the traditional bottom-up top-down scheme. To understand the relative importance of different forces keeping herbivore numbers in check, we feel that we need a fresh synthesis between the novel paradigm of spatial ecology and the classical paradigms of top-down and bottom-up studies. This synthesis requires a consideration of forces beyond the standard framework of top-down vs bottom-up effects, and should be based on comparing the relative strength of such forces at several sites in a spatially explicit framework. Overall, we should switch our focus from whether the relative strength of top-down and bottom-up factors vary in space to why there is variation, how much variation there is, and at what spatial scale(s) it occurs.
[Show abstract][Hide abstract] ABSTRACT: From the perspective of a specialist herbivore, how homogenous are individual tree crowns as patches of habitat? We partitioned variation in physical and chemical host leaf traits and in the abundance and performance of a specialist oak leaf miner, Tischeria ekebladella, into variation at different hierarchical levels. For the phenolic contents of the leaves, we examined variation among oak stands, among trees within stands and among branches within trees. For leaf size and water content, we assessed variation among trees within a single stand, among shoots within trees, and among leaves within shoots. For moth abundance and performance, we examined variation across all levels: among oak stands, among trees within stands, among branches within trees, among shoots within branches and among leaves and insect individuals.For measures of phenolic contents, we found little variation among stands but substantial variation among individual trees. Yet, a tree particularly rich in a given compound was often comparatively poor in another. At a finer spatial scale, the phenolic composition of individual parts of a single tree was quite consistent, whereas leaf weight and water content varied widely within individual tree crowns. Moth abundances varied more among shoots within branches than at any other spatial level, whereas moth survival showed equal levels of variation within individual shoots as among separate oak stands. Likewise, for four other measures of larval performance (assessed at the level of trees and lower), we found more variation within than between trees.In conclusion, the large variation observed in the performance of a specialist moth and in the physical traits of the leaves among different parts of single tree crowns refutes the image of an oak tree as an ‘island’ of internally homogeneous quality. Hence, we may expect little evolutionary adaptation of T. ekebladella at the scale of individual trees. The moths may instead evolve to behaviourally select their resource at a very fine scale. Large variation within trees also calls for extensive replication within trees in ecological sampling designs and/or the sampling of maximally similar leaves.