Seasonal variation in top-down and bottom-up processes in a grassland arthropod community

Both top-down and bottom-up processes are common in terrestrial ecosystems, but how these opposing forces interact and vary over time is poorly understood. We tested the variation of these processes over seasonal time in a natural temperate zone grassland, a field site characterized by strong seasonal changes in abiotic and biotic conditions. Separate factorial experiments manipulating nutrients and cursorial spiders were performed in the wet and dry seasons. We also performed a water-addition experiment during the summer (dry season) to determine the degree of water limitation during this time. In the spring, nutrient addition increased plant growth and carnivore abundance, indicating a bottom-up control process. Among herbivores, sap-feeders were significantly enhanced while grazers significantly declined resulting in no net change in herbivore abundance. In the summer, water limitation was predominant increasing plants and all herbivores while nutrient (N) effects were non-significant. Top-down processes were present only in the spring season and only impacted the guild of grazing herbivores. These results show that bottom-up limitation is present throughout the season in this grassland, although the specific limiting resource changes as the season progresses. Bottom-up processes affected all trophic levels and many different guilds, while top-down effects were limited to a select group of herbivores and did not extend to the plant trophic level. Our results show that the relative strengths of top-down and bottom-up processes can shift over relatively short periods of time in habitats with a strong seasonal component.     

Intraguild predation,invertebrate predators,and trophic cascades in lake food webs

The top-down and bottom-up properties of model food webs that include intraguild predation and self-limiting factors such as cannibalism are investigated. Intraguild predation can dampen or even reverse the top-down effects predicted by food chain theory. The degree of self-limitation among the intraguild prey is a key factor in determining the direction and strength of the top-down response. Intraguild predation and self-limiting factors can also substantially alter the bottom-up effects of enrichment. These results can help explain the disparate results of trophic cascade experiments in lakes, where cascades are usually seen when large Daphnia are the primary herbivores, but not when smaller-bodied herbivores are dominant. Top-down manipulations should cascade at least modestly to phytoplankton in those lakes whose food web can be reasonably approximated by a chain (typically, those where Daphnia is the dominant herbivore), as predicted by food chain theory. On the other hand, smaller-bodied zooplankton are often preyed upon heavily by invertebrate predators as well as by planktivorous fish, thereby introducing elements of intraguild predation into these food webs. In this case, conventional food chain theory is likely to give incorrect predictions. Very large cascade effects may be due primarily to regime shifts between intraguild predation-dominated food webs and those that more resemble food chains, rather than due to the simple food chain cascade usually considered.     

森林生态系统中植食性昆虫与寄主的互作机制、假说与证据

围绕"多样性稳定性"假说、"联合抗性假说"、"生长势假说"、"胁迫假说"、以及下调、上调和推拉等机制与假说提出的背景与实验验证的证据,力图辨析其概念以及它们之间的相互关系。作者认为,多样性-稳定性机制关注森林生态系统的功能,是基于群落甚至景观层次。多样性条件下的联合抗性机制和联合易感性应属于稳定性中的抵抗力范畴。联合抗性机制的主要基础是基于资源集中假说和天敌假说,这些观点在种群层次上更易理解;上调力和下调力机制是以食物网底部的资源与顶端的天敌来探讨这种互作关系。因此,资源集中与上调力有着对应关系,而天敌假说只是下调力机制中的一个层面而已。植物生长势假说和植物胁迫假说力图从植物个体或种的群体的生长状态出发解析植食性动物的对寄主的选择趋势。上述有关植食性昆虫与寄主互作的机制、假说与证据是基于不同的层面提出的,因而在解析研究目标时,由于基本面的差异有可能会得出不同的结论。以近年来的研究进展和研究成果为依据有针对性地阐述这些理论对森林有害生物生态调控技术的指导作用,其中,联合抗性和联合易感性理论对指导森林有害生物生态控制具有更直接的指导作用。进一步提出了相应的亟待解决的科学问题。     

Assessing top-down and bottom-up control in a litter-based soil macroinvertebrate food chain

The relative importance of top-down and bottom-up control in setting the equilibrium abundances within trophic levels is examined in a comparative study on the litter-based food chain of a temperate deciduous forest. During two consecutive years, we estimated the abundances of macroinvertebrate detritivores and their predators on a natural gradient of annual litterfall. Detritus-based food chains are thought to be classical examples of donor-controlled systems. Indeed, both trophic levels showed higher abundances on sites with higher annual litterfall. Therefore, they appear to be bottom-up controlled. Using the Errors-in-Variables regression technique, we quantitatively compared our data with the equilibrium predictions of a set of simple trophic chain models including bottom-up effects with different types of functional responses (Beddington-DeAngelis, Hassell-Varley, and ratio-dependent). The model with a Hassell-Varley type functional response yielded the best adjustment to the data, although with a very high value of the mutual interference parameter suggesting the existence of overcompensating density dependence. Several changes to the structure of this model were considered. Their adjustment to the data consistently yielded such high values of the interference parameter.     

Complex food webs prevent competitive exclusion among producer species

Herbivorous top-down forces and bottom-up competition for nutrients determine the coexistence and relative biomass patterns of producer species. Combining models of predator-prey and producer-nutrient interactions with a structural model of complex food webs, I investigated these two aspects in a dynamic food-web model. While competitive exclusion leads to persistence of only one producer species in 99.7% of the simulated simple producer communities without consumers, embedding the same producer communities in complex food webs generally yields producer coexistence. In simple producer communities, the producers with the most efficient nutrient-intake rates increase in biomass until they competitively exclude inferior producers. In food webs, herbivory predominantly reduces the biomass density of those producers that dominated in producer communities, which yields a more even biomass distribution. In contrast to prior analyses of simple modules, this facilitation of producer coexistence by herbivory does not require a trade-off between the nutrient-intake efficiency and the resistance to herbivory. The local network structure of food webs (top-down effects of the number of herbivores and the herbivores' maximum consumption rates) and the nutrient supply (bottom-up effect) interactively determine the relative biomass densities of the producer species. A strong negative feedback loop emerges in food webs: factors that increase producer biomasses also increase herbivory, which reduces producer biomasses. This negative feedback loop regulates the coexistence and biomass patterns of the producers by balancing biomass increases of producers and biomass fluxes to herbivores, which prevents competitive exclusion.     

Effects of Multi-chain Omnivory on the Strength of Trophic Control in Lakes

Omnivory has been implicated in both diffusing and intensifying the effects of consumer control in food chains. Some have postulated that the strong, community level, top-down control apparent in lakes is not expressed in terrestrial systems because terrestrial food webs are reticulate, with high degrees of omnivory and diverse plant communities. In contrast, lake food webs are depicted as simple linear chains based on phytoplankton-derived energy. Here, we explore the dynamic implications of recent evidence showing that attached algal (periphyton) carbon contributes substantially to lake primary and secondary productivity, including fish production. Periphyton production represents a cryptic energy source in oligotrophic and mesotrophic lakes that is overlooked by previous theoretical treatment of trophic control in lakes. Literature data demonstrate that many fish are multi-chain omnivores, exploiting food chains based on both littoral and pelagic primary producers. Using consumer-resource models, we examine how multiple food chains affect fourth-level trophic control across nutrient gradients in lakes. The models predict that the stabilizing effects of linked food chains are strongest in lakes where both phytoplankton and periphyton contribute substantially to production of higher trophic levels. This stabilization enables a strong and persistent top down control on the pelagic food chain in mesotrophic lakes. The extension of classical trophic cascade theory to incorporate more complex food web structures driven by multi-chain predators provides a conceptual framework for analysis of reticulate food webs in ecosystems.     

Habitat isolation affects plant–herbivore–enemy interactions on cherry trees

Understanding the interactions between herbivores and natural enemies in fragmented landscapes is essential for conservation biological control. Studies including multiple enemies affecting multiple herbivores, plant damage and growth are needed. Here, we separated independent effects of (1) isolation of cherry trees from woody habitat and (2) the amount of woody habitat in the surrounding landscape (500 m buffers) on interactions between different groups of herbivores with their natural enemies and resulting changes in the growth of young cherry trees. Most predatory arthropods declined with habitat isolation, except some aphid predators (ladybeetles and hoverflies). Herbivores either increased with isolation (herbivorous beetles) or showed no significant response (aphids). In contrast, the amount of woody habitat in the landscape was not relevant for herbivore–enemy interactions at the investigated scale. Plant growth was affected by bottom-up (nutrient availability) and top-down (aphid density) forces but did not change significantly with habitat amount or isolation. We conclude that herbivores can be released from natural enemies at isolated sites, in accordance with the hypothesis that habitat connectivity improves pest control. However, each herbivore group responded differently to the landscape context and had contrasting effects on the same host plant, demonstrating the difficulty to predict landscape effects on plant growth.     

From bottom-up to top-down control of invertebrate herbivores in a retrogressive chronosequence

In the long-term absence of disturbance, ecosystems often enter a decline or retrogressive phase which leads to reductions in primary productivity, plant biomass, nutrient cycling and foliar quality. However, the consequences of ecosystem retrogression for higher trophic levels such as herbivores and predators, are less clear. Using a post-fire forested island-chronosequence across which retrogression occurs, we provide evidence that nutrient availability strongly controls invertebrate herbivore biomass when predators are few, but that there is a switch from bottom-up to top-down control when predators are common. This trophic flip in herbivore control probably arises because invertebrate predators respond to alternative energy channels from the adjacent aquatic matrix, which were independent of terrestrial plant biomass. Our results suggest that effects of nutrient limitation resulting from ecosystem retrogression on trophic cascades are modified by nutrient-independent variation in predator abundance, and this calls for a more holistic approach to trophic ecology to better understand herbivore effects on plant communities.     

A cross-system synthesis of consumer and nutrient resource control on producer biomass

Nutrient availability and herbivory control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that (i) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems and (ii) bottom-up and top-down forces are interdependent, with statistical interactions that synergize or dampen relative influences on producer biomass. We used simple dynamic models to review ecological mechanisms that generate independent vs. interactive responses of community-level biomass. We calibrated these mechanistic predictions with the metrics of factorial meta-analysis and tested their prevalence across freshwater, marine and terrestrial ecosystems with a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients. Our analysis showed that producer community biomass increased with fertilization across all systems, although increases were greatest in freshwater habitats. Herbivore removal generally increased producer biomass in both freshwater and marine systems, but effects were inconsistent on land. With the exception of marine temperate rocky reef systems that showed positive synergism of nutrient enrichment and herbivore removal, experimental studies showed limited support for statistical interactions between nutrient and herbivory treatments on producer biomass. Top-down control of herbivores, compensatory behaviour of multiple herbivore guilds, spatial and temporal heterogeneity of interactions, and herbivore-mediated nutrient recycling may lower the probability of consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently multiple producer resources (e.g. nitrogen, phosphorus, light), multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates) and multiple trophic levels; and - in addition to measuring producer biomass - assess the responses of species diversity, community composition and nutrient status.     

Multitrophic effects of nutrient addition in upland grassland

Although the effects of nutrient enhancement on aquatic systems are well documented, the consequences of nutritional supplements on soil food webs are poorly understood, and results of past research examining bottom-up effects are often conflicting. In addition, many studies have failed to separate the effects of nutrient enrichment and the physical effects of adding organic matter. In this field study, we hypothesised that the addition of nitrogen to soil would result in a trophic cascade, through detritivores (Collembola) to predators (spiders), increasing invertebrate numbers and diversity.Nitrogen and lime were added to plots in an upland grassland in a randomised block design. Populations of Collembola and spiders were sampled by means of pitfall traps and identified to species.Seventeen species of Collembola were identified from the nitrogen plus lime (N+L) and control plots. Species assemblage, diversity, richness, evenness and total number were not affected by nutrient additions. However, there was an increase in the number of Isotomidae juveniles and Parisotoma anglicana trapped in the N+L plots.Of the 44 spider species identified, over 80% were Linyphiidae. An effect on species assemblage from the addition of N+L to the plots was observed on two of the four sampling dates (July 2002 and June 2003). The linyphiid, Oedothorax retusus, was the only species significantly affected by the treatments and was more likely to be trapped in the control plots.The increased number of juvenile Collembola, and change in community composition of spiders, were consequences of the bottom-up effect caused by nutrient inputs. However, despite efforts to eliminate the indirect effects of nutrient inputs, a reduction in soil moisture in the N+L plots cannot be eliminated as a cause of the invertebrate population changes observed. Even so, this experiment was not confounded by the physical effects of habitat structure reported in most previous studies. It provides evidence of moderate bottom-up influences of epigeic soil invertebrate food webs and distinguishes between nutrient addition and plant physical structure effects. It also emphasises the importance of understanding the effects of soil management practices on soil biodiversity, which is under increasing pressure from land development and food production.     

Small mammalian herbivore determines vegetation response to patchy nutrient inputs

Nutrient inputs to plant communities are often spatially heterogeneous, for example those deriving from the dung and urine of large grazing animals. The effect of such localised elevation of nutrients on plant growth and composition has been shown to be modified by the grazing of large herbivores. However, there has been little work on interactions between small mammalian herbivores and such patchy nutrient inputs, even though these interactions are potentially of major significance for plant performance and community structure.
We examined the effect of simulated cattle urine deposition on the vegetation structure, above-ground biomass and species composition of chalk grassland within enriched patches. Short-term exclosures were used to determine whether a small herbivore (rabbit) would preferentially graze the vegetation in enriched patches and what impact this interaction would have on the performance of plants in such patches. Rabbit grazing pressure determined whether nutrient inputs had a negative or positive effect on plant biomass. Nutrients increased plant biomass in the absence of grazing, but when exposed to grazing, plants in nutrient-rich patches had more biomass consumed by herbivores than neighbouring plants. Further, nutrients increased the relative palatability of a less preferred forage species ( Brachypodium pinnatum ), contributing to changes in plant community composition. We conclude that a small herbivore can drive plant responses to patchily distributed nutrients.     

Assessment of Top-Down and Bottom-Up Controls on the Collapse of Alewives (<Emphasis Type="Italic">Alosa pseudoharengus</Emphasis>) in Lake Huron

Food web models are powerful tools to inform management of lake ecosystems, where top-down (predation) and bottom-up (resource) controls likely propagate through multiple trophic levels because of strong predator–prey links. We used the Ecopath with Ecosim modeling approach to assess these controls on the Lake Huron main basin food web and the 2003 collapse of an invasive pelagic prey fish, alewife (Alosa pseudoharengus). We parameterized two Ecopath models to characterize food web changes occurring between two study periods of 1981–1985 and 1998–2002. We also built an Ecosim model and simulated food web time-dynamics under scenarios representing different levels of top-down control by Chinook salmon (Oncorhynchus tshawytscha) and of bottom-up control by quagga mussels (Dreissena rostriformis bugensis) and nutrients. Ecopath results showed an increase in the relative importance of bottom-up controls between the two periods, as production decreased across all trophic levels. The production of non-dreissenid benthos decreased most, which could cause decreases in production of pelagic prey fishes feeding on them. Ecosim simulation results indicated that the alewife collapse was caused by a combination of top-down and bottom-up controls. Results showed that while controls by Chinook salmon were relatively constant before alewife collapse, controls by quagga mussels and nutrients increased jointly to unsustainable levels. Under current conditions of low nutrients and high quagga mussel biomass, simulation results showed that recovery of alewives is unlikely regardless of Chinook salmon biomass in Lake Huron, which implies that the shrinking prey base cannot support the same level of salmonine predators as that prevailed during the 1980s.     

The dynamics of top-down and bottom-up effects in food webs of varying prey diversity, composition, and productivity

Prey diversity is thought to mediate the strength of top-down and bottom-up effects, but few experiments directly test this hypothesis. I assembled food webs of bacteria and bacterivorous protist prey in laboratory microcosms with all combinations of five productivity levels, two top predator treatments (present or absent), and three prey compositions. Depauperate food chains contained one of two edible prey species, while more diverse food webs contained both edible prey species plus two additional less-edible/inedible prey. Equilibrium theory predicts that prey diversity should weaken the top-down and bottom-up effects on trophic level biomasses, due to density compensation among prey species. Top-down effects should increase with productivity in food chains, but decrease with productivity in food webs. Results revealed highly dynamic top-down effects, the strength of which varied more over time than among treatments. Further, top-down effects did not merely vary in absolute strength over time, but also in relative strength across different prey compositions and productivity levels. It might be expected that equilibrium models would qualitatively reproduce time-averaged results. However, time-averaged data largely failed to support equilibrium predictions. This failure may reflect strong temporal variability in treatment effects combined with nonlinear density dependence of species' per-capita growth rates. Strong temporal variability in the strength of top-down effects has not previously been demonstrated, but likely is common in nature as well.     

Relative strengths of top-down and bottom-up forces in a tropical forest community

We tested integrative bottom-up and top-down trophic cascade hypotheses with manipulative experiments in a tropical wet forest, using the ant-plant Piper cenocladum and its associated arthropod community. We examined enhanced nutrients and light along with predator and herbivore exclusions as sources of variation in the relative biomass of plants, their herbivores (via rates of herbivory), and resident predaceous ants. The combined manipulations of secondary consumers, primary consumers, and plant resources allowed us to examine some of the direct and indirect effects on each trophic level and to determine the relative contributions of bottom-up and top-down cascades to the structure of the community. We found that enhanced plant resources (nutrients and light) had direct positive effects on plant biomass. However, we found no evidence of indirect (cascading through the herbivores) effects of plant biomass on predators or top predators. In contrast, ants had indirect effects on plant biomass by decreasing herbivory on the plants. This top-down cascade occurred whether or not plant resources were enriched, conditions which are expected to modify top-down forces. Received: 9 August 1998 / Accepted: 1 December 1998     

Cascading effects of artificial light at night: resource-mediated control of herbivores in a grassland ecosystem

Artificial light at night has a wide range of biological effects on both plants and animals. Here, we review mechanisms by which artificial light at night may restructure ecological communities by modifying the interactions between species. Such mechanisms may be top-down (predator, parasite or grazer controlled), bottom-up (resource-controlled) or involve non-trophic processes, such as pollination, seed dispersal or competition. We present results from an experiment investigating both top-down and bottom-up effects of artificial light at night on the population density of pea aphids Acyrthosiphon pisum in a diverse artificial grassland community in the presence and absence of predators and under low-level light of different spectral composition. We found no evidence for top-down control of A. pisum in this system, but did find evidence for bottom-up effects mediated through the impact of light on flower head density in a leguminous food plant. These results suggest that physiological effects of light on a plant species within a diverse plant community can have detectable demographic effects on a specialist herbivore.     

Comparative and experimental approaches to top-down and bottom-up regulation of bacteria

The regulation of bacterial community biomass and productivity by resources and predators is a central concern in the study of microbial food webs. Resource or bottom-up regulation refers to the limitation of bacteria by carbon and nutrients derived from allocthonous inputs, primary production, and heterotrophic production. Predatory or top-down regulation refers to the limitation of bacteria below levels supportable by resources alone. Large scale comparative studies demonstrate strong correlations between bacterial productivity and biomass, suggesting significant resource regulation. Comparisons of the abundances of heterotrophic flagellates and bacteria, however, imply that in some cases there may be top-down regulation of bacteria in eutrophic environments. Experimental studies in lakes support the importance of resource regulation and reveal little top-down control from protozoans. Increases in bacterial abundance and production with nutrient enrichment were limited in enclosure experiments with high abundances of the cladoceran, Daphnia. Regulation of bacteria by Daphnia may occur in many lakes seasonally and prevail in some lakes throughout the year where these animals sustain dense populations. In most situations, however, bacteria appear to be limited primarily by resources.     

Relative importance of top-down and bottom-up forces in food webs of Sarracenia pitcher communities at a northern and a southern site

The relative importance of resources (bottom-up forces) and natural enemies (top-down forces) for regulating food web dynamics has been debated, and both forces have been found to be critical for determining food web structure. How the relative importance of top-down and bottom-up forces varies between sites with different abiotic conditions is not well understood. Using the pitcher plant inquiline community as a model system, I examine how the relative importance of top-down and bottom-up effects differs between two disparate sites. Resources (ant carcasses) and top predators (mosquito larvae) were manipulated in two identical 4 × 4 factorial press experiments, conducted at two geographically distant sites (Michigan and Florida) within the range of the purple pitcher plant, Sarracenia purpurea, and the aquatic community that resides in its leaves. Overall, top predators reduced the density of prey populations while additional resources bolstered them, and the relative importance of top-down and bottom-up forces varied between sites and for different trophic levels. Specifically, top-down effects on protozoa were stronger in Florida than in Michigan, while the opposite pattern was found for rotifers. These findings experimentally demonstrate that the strength of predator–prey interactions, even those involving the same species, vary across space. While only two sites are compared in this study, I hypothesize that site differences in temperature, which influences metabolic rate, may be responsible for variation in consumer–resource interactions. These findings warrant further investigation into the specific factors that modify the relative importance of top-down and bottom-up effects.     

Herbivore and nutrient control of lawn and bunch grass distributions in a southern African savanna

In African savannas, bottom-up soil nutritional factors and top-down herbivory have both been suggested to control the distribution patterns of bunch and lawn grasses. We tested the separate and combined roles of these factors on grassland distribution in a South African savanna by focusing on three main objectives, namely (1) are grazing patches and lawns restricted to specific soils or sites, (2) does herbivory alter rates of nutrient cycling which facilitates lawns and (3) are there any differences in foliage quality between lawn and bunch grass-dominated sites that might influence animals in choosing to feed on lawns? We set up ten sites along a grazing gradient in the Hluhluwe-iMfolozi Park in KwaZulu-Natal, South Africa. At these sites, we measured total soil nutrients, soil nutrient turnover rates and grass foliage nutrient concentrations. We found that in the Hluhluwe-iMfolozi Park the spatial distribution of lawn and bunch grassland types appears to be an animal-driven phenomenon and not dependent on specific soil properties. The short-structured and distinct species found in lawns do not appear to be restricted to nutrient-enriched patches. However, the grasses of these lawns had significantly higher nutrient concentrations, in their foliage, which might explain the high attraction to these patches by herbivores. We also did not find any animal-induced stimulation of nutrient cycling rates that are often associated with lawn grass species.     

Differential responses of size‐based functional groups to bottom–up and top–down perturbations in pelagic food webs: a meta‐analysis

We performed a meta‐analysis of 31 lake mesocosm experiments to investigate differences in the responses of pelagic food chains and food webs to nutrient enrichment and fish presence. Trophic levels were divided into size‐based functional groups (phytoplankton into highly edible and poorly edible algae, and zooplankton into small herbivores, large herbivores and omnivorous zooplankton) in the food webs. Our meta‐analysis shows that 1) nutrient enrichment has a positive effect on phytoplankton and zooplankton, while fish presence has a positive effect on phytoplankton and a negative effect on zooplankton in the food chains; 2) nutrient enrichment has a positive effect on highly edible algae and small herbivores, but no effect on poorly edible algae, large herbivores and omnivorous zooplankton in the food webs. Planktivorous fish have a positive effect on highly edible algae and small herbivores, a negative effect on large herbivores and omnivorous zooplankton, and no effect on poorly edible algae. Our meta‐analysis confirms that nutrient enrichment and planktivorous fish affect functional groups differentially within trophic levels, revealing important changes in the functioning of food webs. The analysis of fish effects shows the well‐described trophic cascade in the food chain and reveals two trophic cascades in the food web: one transmitted by large herbivores that benefit highly edible phytoplankton, and one transmitted by omnivorous zooplankton that benefit small herbivores. Comparison between the responses of food webs and simple food chains also shows consistent biomass compensation between functional groups within trophic levels.     

Predator diet breadth influences the relative importance of bottom-up and top-down control of prey biomass and diversity

We investigated the effects of predator diet breadth on the relative importance of bottom-up and top-down control of prey assemblages, using microbial food webs containing bacteria, bacterivorous protists and rotifers, and two different top predators. The experiment used a factorial design that independently manipulated productivity and the presence or absence of two top predators with different diet breadths. Predators included a "specialist" predatory ciliate Euplotes aediculatus, which was restricted to feeding on small prey, and a "generalist" predatory ciliate Stentor coeruleus, which could feed on the entire range of prey sizes. Both total prey biomass and prey diversity increased with productivity in the predator-free control and specialist predator treatments, a pattern consistent with bottom-up control, but both remained unchanged by productivity in the generalist predator treatment, a pattern consistent with top-down control. Linear food chain models adequately described responses in the generalist predator treatment, whereas food web models incorporating edible and inedible prey (which can coexist in the absence of predators) adequately described responses in the specialist predator treatment. These results suggest that predator diet breadth can play an important role in modulating the relative strength of bottom-up and top-down forces in ecological communities.