Terrestrial trophic cascades: how much do they trickle?

Although more consensus is now emerging on the magnitude and frequency of cascading trophic effects in aquatic communities, the debate over their terrestrial counterparts continues. We used meta-analysis to analyze field experiments on trophic cascades in terrestrial arthropod-dominated food webs to evaluate the overall magnitude of trophic cascades and conditions affecting their occurrence and strength. We found extensive support for the presence of trophic cascades in terrestrial communities. In the majority of experiments, predator removal led to increased densities of herbivorous insects and higher levels of plant damage. Cascades in which removing predators led to decreased herbivory also were detected but were less frequent and weaker, suggesting a predominantly three-trophic-level behavior of arthropod-dominated terrestrial food webs. Despite the clear evidence that cascades often decreased plant damage, residual effects of predation produced either no or only minimal changes in overall plant biomass. Agricultural systems and natural communities exhibited similarly strong effects of predation on herbivore abundance. However, resulting effects on plant damage and community-wide effects of trophic cascades on plant biomass usually were highly variable, and only in the managed agricultural systems did predators occasionally have strong indirect effects on plant biomass. Our meta-analysis suggests that the effects of trophic cascades on the biomass of primary producers are weaker in terrestrial than aquatic food webs.     

Trophic cascades in the bryosphere: the impact of global change factors on top‐down control of cyanobacterial N2‐fixation

Trophic cascades in which predators regulate densities of organisms at lower trophic levels are important drivers of population dynamics, but effects of trophic cascades on ecosystem‐level fluxes and processes, and the conditions under which top‐down control is important, remain unresolved. We manipulated the structure of a food web in boreal feather mosses and found that moss‐inhabiting microfauna exerted top‐down control of N₂‐fixation by moss‐associated cyanobacteria. However, the presence of higher trophic levels alleviated this top‐down control, likely through feeding on bacterivorous microfauna. These effects of food‐web structure on cyanobacterial N₂‐fixation were dependent on global change factors and strongly suppressed under N fertilisation. Our findings illustrate how food web interactions and trophic cascades can regulate N cycling in boreal ecosystems, where carbon uptake is generally strongly N‐limited, and shifting trophic control of N cycling under global change is therefore likely to impact ecosystem functioning.     

Birds help plants: a meta-analysis of top-down trophic cascades caused by avian predators

The tritrophic interactions between plants, herbivores and avian predators are complex and prone to trophic cascades. We conducted a meta-analysis of original articles that have studied birds as predators of invertebrate herbivores, to compare top-down trophic cascades with different plant responses from different environments and climatic areas. Our search found 29 suitable articles, with a total of 81 separate experimental study set-ups. The meta-analysis revealed that plants benefited from the presence of birds. A significant reduction was observed in the level of leaf damage and plant mortality. The presence of birds also positively affected the amount of plant biomass, whereas effects on plant growth were negligible. There were no differences in the effects between agricultural and natural environments. Similarly, plants performed better in all climatic areas (tropical, temperate and boreal) when birds were present. Moreover, both mature plants and saplings gained benefits from the presence of birds. Our results show that birds cause top-down trophic cascades and thus they play an integral role in ecosystems.     

Assembly rules for functional groups of North American shrews: effects of geographic range and habitat partitioning

Trophic cascades exist in numerous terrestrial systems, including many systems with ants as the top predator. Many studies show how behavioral modifications of herbivores are especially important in mediating species interactions and trophic cascades. Although most studies of trophic cascades focus on predator-herbivore-plant links, the trophic cascades concept could be applied to almost any level of trophic interactions. Especially considering the importance of parasites we consider here the interactions between the parasitic phorid fly, Pseudacteon sp. (Diptera: Phoridae), its ant host, Azteca instabilis (F. Smith) (Hymenoptera: Formicidae), and the herbivore, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) in the coffee agroecosystem. We investigated the effects of phorid flies on ant behavior by monitoring ant recruitment to tuna baits over a 30-min period in the presence or absence of phorid flies. To study the indirect effects of phorids on larvae, we placed baits on coffee plants to elevate ant foraging levels to levels near to ant nests, placed larvae near baits, and recorded the effects of ants on larvae either in the presence or absence of phorid flies. We found that phorid fly presence significantly reduced ant ability to recruit to baits through behavioral modifications and also significantly lessened ant ability to attack, carry away, or force herbivores off plants. We conclude there is a behaviorally-modified species-level trophic cascade in the coffee agroecosystem, with potentially important effects in ant and herbivore communities as well as for coffee production.     

Bottom-up trophic cascades and material transfer in terrestrial food webs

In contrast to top-down trophic cascades, few reviews have appeared of bottom-up trophic cascades. We review the recent development of research on bottom-up cascades in terrestrial food webs, focusing on tritrophic systems consisting of plants, herbivorous insects, and natural enemies, and attempt to integrate bottom-up cascade and material transfer among trophic levels. Bottom-up cascades are frequently reported in various tritrophic systems, and are important to determine community structure, population dynamics, and individual performance of higher trophic levels. In addition, we highlight several features of bottom-up cascades. Accumulation or dilution of plant nutritional and defensive materials by herbivorous insects provides a mechanistic base for several bottom-up cascades. Such a stoichiometric approach has the potential to improve our understanding of bottom-up cascading effects in terrestrial food webs. We suggest a future direction for research by integration of bottom-up cascades and material transfer among trophic levels.     

Predators indirectly protect tundra plants by reducing herbivore abundance

The role of predators in controlling herbivores and indirectly affecting plant abundance is controversial, and some have argued that such trophic cascades are rare in terrestrial habitats. To examine the potential of trophic cascades in a shrubby tundra ecosystem, vole densities, plant damage and plant cover were examined in areas with and without small mammal predators. On islands without predators, vole densities and plant damage were upto five times higher compared with predator-rich mainland habitats. As a consequence, the abundance of three out of five dwarf shrub species was substantially reduced on predator-free islands, providing evidence for strong cascading effects in a rather large-scale terrestrial system. Herbs on the other hand were not affected by the increased herbivory on islands. This suggests that the strength of trophic cascades also depends on the interaction between plant type and seasonality.     

Planktonic dispersal dampens temporal trophic cascades in pond metacommunities

Trophic cascades, in which changes in predation affect the biomass of lower trophic levels, vary substantially in strength and incidence. Most work to explain this variation has focused on local factors and has ignored larger regional effects. To study how metacommunity dynamics can alter trophic cascades, we constructed mesocosm metacommunities consisting of three pond communities with heterogeneous levels of fish predation and examined how planktonic dispersal rate (5–140% per week) affected biomass partitioning. Two of the three communities differed continually in the occurrence of fish and supported different but constant environments in a 'spatial trophic cascade,' while the third community supported temporally variable fish occurrence in a 'temporal trophic cascade.' We find that the presence, but the not the magnitude, of dispersal dampens temporal trophic cascades through an increase in grazer biomass. In contrast, dispersal has no effect on the strength of spatial cascades due to strong sorting pressures in the communities with constant presence or absence of fish as top predators.     

The biogeography of trophic cascades on US oyster reefs

Predators can indirectly benefit prey populations by suppressing mid‐trophic level consumers, but often the strength and outcome of trophic cascades are uncertain. We manipulated oyster reef communities to test the generality of potential causal factors across a 1000‐km region. Densities of oyster consumers were weakly influenced by predators at all sites. In contrast, consumer foraging behaviour in the presence of predators varied considerably, and these behavioural effects altered the trophic cascade across space. Variability in the behavioural cascade was linked to regional gradients in oyster recruitment to and sediment accumulation on reefs. Specifically, asynchronous gradients in these factors influenced whether the benefits of suppressed consumer foraging on oyster recruits exceeded costs of sediment accumulation resulting from decreased consumer activity. Thus, although predation on consumers remains consistent, predator influences on behaviour do not; rather, they interact with environmental gradients to cause biogeographic variability in the net strength of trophic cascades.     

Exploitation ecosystems and trophic cascades in non‐equilibrium systems: pasture – red kangaroo – dingo interactions in arid Australia

The exploitation ecosystems hypothesis (EEH) proposes that 1) plant biomass reflects the primary productivity of an ecosystem modified by the regulating effect of herbivory, and 2) herbivore abundance reflects the productivity of plants modified by the regulating effect of predation. Primary productivity thus determines the number of trophic levels in an ecosystem and the extent to which bottom–up and top–down regulation influence the biomass ratios of adjacent and non‐adjacent trophic levels (i.e. trophic cascading). We constructed an interactive model of plant (pasture), herbivore (red kangaroo Macropus rufus) and predator (dingo Canis lupus dingo), a system in which trophic cascades have been suggested to occur, and used it to test the effects of increasing stochastic variation in primary productivity and dingo culling on predictions of the EEH. The model contained four feedback loops: the predator–herbivore and herbivore–plant feedback loops, and the predator and plant density‐dependent feedback loops. The equilibrium conditions along the primary productivity gradient reproduced the three zones of trophic dynamics predicted by the EEH, plus an additional zone at productivities above which the maximum density of a predator is achieved due to social regulation: that zone is characterized by increasing herbivore density and decreasing plant biomass. Culling dingoes produced trophic cascades that were strongly attenuated at primary productivities below which the maximum density of dingoes was attained. Results were robust to uncertainty in kangaroo off‐take by dingoes and to the efficacy of dingo culling, but prey switching by dingoes from red kangaroos to reptiles would weaken trophic cascades. We conclude that social regulation of carnivores has important implications for expression of the EEH and trophic cascades, and that attenuation of trophic cascades increases with increasing stochasticity in primary productivity. Our model also provides a framework for understanding the conditions in which dingo‐mediated trophic cascades might be expected to occur, and generates testable predictions about the effects of higher dingo densities (e.g. by stopping culling or reintroduction to former range) on kangaroo and pasture dynamics.     

Fishing indirectly structures macroalgal assemblages by altering herbivore behavior

Fishing has clear direct effects on harvested species, but its cascading, indirect effects are less well understood. Fishing disproportionately removes larger, predatory fishes from marine food webs. Most studies of the consequent indirect effects focus on density-mediated interactions where predator removal alternately drives increases and decreases in abundances of successively lower trophic-level species. While prey may increase in number with fewer predators, they may also alter their behavior. When such behavioral responses impact the food resources of prey species, behaviorally mediated trophic cascades can dramatically shape landscapes. It remains unclear whether this pathway of change is typically triggered by ocean fishing. By coupling a simple foraging model with empirical observations from coral reefs, we provide a mechanistic basis for understanding and predicting how predator harvest can alter the landscape of risk for herbivores and consequently drive dramatic changes in primary producer distributions. These results broaden trophic cascade predictions for fisheries to include behavioral changes. They also provide a framework for detecting the presence and magnitude of behaviorally mediated cascades. This knowledge will help to reconcile the disparity between expected and observed patterns of fishing-induced cascades in the sea.     

Competitive displacement alters top-down effects on carbon dioxide concentrations in a freshwater ecosystem

Climate change and invasive species have the potential to alter species diversity, creating novel species interactions. Interspecific competition and facilitation between predators may either enhance or dampen trophic cascades, ultimately influencing total predator effects on communities and biogeochemical cycling of ecosystems. However, previous studies have only investigated the effects of a single predator species on CO₂ flux of aquatic ecosystems. In this study, we measured and compared the individual and joint effects of predatory damselfly larvae and diving beetles on total prey biomass, leaf litter processing, and dissolved CO₂ concentrations of experimental bromeliad ecosystems. Damselfly larvae created strong trophic cascades that reduced CO₂ concentrations by ~46 % relative to no-predator treatments. Conversely, the effects of diving beetles on prey biomass, leaf litter processing, and dissolved CO₂ were not statistically different to no-predator treatments. Relative to multiplicative null models, the presence of damselfly larvae and diving beetles together resulted in antagonistic relations that eliminated trophic cascades and top-down influences on CO₂ concentrations. Furthermore, we showed that the antagonistic interactions between predators occurred due to a tactile response that culminated in competitive displacement of damselfly larvae. Our results demonstrate that predator identity and predator–predator interactions can influence CO₂ concentrations of an aquatic ecosystem. We suggest that predator effects on CO₂ fluxes may depend on the particular predator species removed or added to the ecosystem and their interactions with other predators.     

Predator–prey interactions in a grassland food chain vary with temperature and food quality

Because species interactions are often context‐dependent, abiotic factors such as temperature and biotic factors such as food quality may alter species interactions with potential consequences to ecosystem structure and function. For example, altered predator–prey interactions may influence the dynamics of trophic cascades, affecting net primary production. In a three‐year field experiment, we manipulated a plant–grasshopper–spider food chain in mesic tallgrass prairie to investigate the effects of temperature and food quality on grasshopper performance, and to understand the direct and indirect tritrophic interactions that contribute to trophic cascades. Because spiders are active at cooler temperatures than grasshoppers in our system, we hypothesized that predator effects would be strongest in cooled treatments, and weakest in warmed treatments. Grasshopper spider interactions were highly context‐dependent and varied significantly with food quality, temperature treatment and year. Spiders most often reduced grasshopper survival in the cooled and ambient temperature treatments, but had little to no effect on grasshopper survival in the warmed treatments, as hypothesized. In some years, plants compensated for grasshopper herbivory and trophic cascades were not observed despite significant effects of predators on grasshopper survival. However, in the year they were observed, trophic cascades only occurred in cooled treatments where predator effects on grasshoppers were strongest. Predicting ecosystem responses to climate change will require an understanding of how temperature influences species interactions. Our results demonstrate that changes in daily temperature regimes can alter predator–prey interactions among arthropods with consequences for ecosystem processes such as primary production and the relative importance of top–down and bottom–up processes.     

Most soil trophic guilds increase plant growth: a meta‐analytical review

Trophic cascades are important drivers of plant and animal abundances in aquatic and aboveground systems, but in soils trophic cascades have been thought to be of limited importance due to omnivory and other factors. Here we use a meta‐analysis of 215 studies with 1526 experiments that measured plant growth responses to additions or removals of soil organisms to test how different soil trophic levels affect plant growth. Consistent with the trophic cascade hypothesis, we found that herbivores and plant pathogens (henceforth pests) decreased plant growth and that predators of pests increased plant growth. The magnitude of this trophic cascade was similar to that reported for aboveground systems. In contrast, we did not find evidence for trophic cascades in decomposer‐ and symbiont‐based (henceforth mutualist) food chains. In these food chains, mutualists increased plant growth and predators of mutualists also increased plant growth, presumably by increasing nutrient cycling rates. Therefore, mutualists, predators of mutualists and predators of pests all increased plant growth. Further, experiments that added multiple organisms from different trophic levels also increased plant growth. As a result, across the dataset, soil organisms increased plant growth 29% and non‐pest soil organisms increased plant growth 46%. Omnivory has traditionally been thought to confound soil trophic dynamics, but here we suggest that omnivory allows for a simplified perspective of soil food webs – one in which most soil organisms increase plant growth by preying on pests or increasing nutrient cycling rates. An implication of this perspective is that processes that decrease soil organism abundance (e.g. soil tillage) are likely to decrease aboveground productivity. Synthesis Soil foodwebs have resisted generalizations due to their diversity and interconnectedness. Here we use results from a meta‐analysis to inform a simplified perspective of soil foodwebs: one in which most soil trophic guilds increase plant growth. Our review also includes the first widespread support for the presence of trophic cascades in soils.     

Intraguild indirect effects through trophic cascades between stream-dwelling fishes

1. To examine the strength and role of indirect effects through trophic cascades by omnivorous fish on algal biomass in streams, we introduced one of four fish species (ayu Plecoglossus altivelis altivelis, pike gudgeon Pseudogobio esocinus esocinus, Japanese dace Tribolodon hakonensis and pale chub Zacco platypus) in experimental pools. We also investigated the indirect effects of gudgeon, dace and chub on the growth of ayu. 2. We chose the four fish species across a continuum of omnivory. Ayu fed mainly on benthic algae, and gudgeon predominantly on invertebrates. Dace and chub fed on both algae and invertebrates, but dace preyed on invertebrates more than chub. 3. The presence of gudgeon, dace and chub reduced the number of algal-feeding invertebrates and increased the algal biomass through trophic cascades. Consequently, ayu growth rate over the experimental period in pools with one of the three fish species was 25.9-44.1 times greater than the growth rate in pools with only ayu. 4. The positive indirect effect on ayu growth was large for gudgeon and dace and small for chub, whereas the addition of ayu reduced ayu growth considerably due to intraspecific competition. 5. The relative intensity of indirect effects on ayu growth through trophic cascades was predictable from the food overlap between ayu and the other fishes. However, the similar strength of indirect effects by gudgeon and dace that fed differently on algae and invertebrates suggests that feeding behaviour, prey preference and trait-mediated indirect interactions were also important in the prediction.     

Effects of experimental duration and density of Japanese dace <Emphasis Type="Italic">Tribolodon hakonensis</Emphasis> on the strength of trophic cascades on benthic algae

To analyze density-mediated indirect effects through trophic cascades caused by Japanese dace Tribolodon hakonensis on the algal biomass and growth of ayu Plecoglossus altivelis altivelis, (freshwater fish belonging to Salmoniformes), I introduced zero, five, ten, and 20 dace with and without ayu into experimental pools. Ayu fed predominantly on benthic algae, whereas dace fed on both invertebrates and algae. The percentage of algae in the food content of dace was correlated with the number of dace in a pool. In the experiment lasting 8 days, trophic cascades on algae were not clear. In contrast, when the experimental duration was extended to 20 days, the number of gastropods and ephemeropteran nymphs was negatively correlated with the number of dace, consequently increasing the algal biomass and ayu growth through trophic cascades. Compared with pools with five dace, the presence of ten and 20 dace had similar gross effects on the number of algivorous invertebrates and algal biomass and greater gross effects on the ayu growth. The significant increase in ayu growth in pools with ten and 20 dace strongly suggests that small positive effects on algal growth by dace accumulated and extended to ayu during a 20-day experimental period. These results indicate the importance of analyzing interactions between the density and behavior of the top predator and the experimental duration on the intensity of trophic cascades.     

Aridity promotes bet hedging via delayed hatching: a case study with two temporary pond crustaceans along a latitudinal gradient

Removal of predators is often hypothesized to alter community structure through trophic cascades. However, despite recent advances in our understanding of trophic cascades, evidence is often circumstantial on coral reefs because fishing pressure frequently co-varies with other anthropogenic effects, such as fishing for herbivorous fishes and changes in water quality due to pollution. Australia’s outer Great Barrier Reef (GBR) has experienced fishing-induced declines of apex predators and mesopredators, but pollution and targeting of herbivorous fishes are minimal. Here, we quantify fish and benthic assemblages across a fishing-induced predator density gradient on the outer GBR, including apex predators and mesopredators to herbivores and benthic assemblages, to test for evidence of trophic cascades and alternative hypotheses to trophic cascade theory. Using structural equation models, we found no cascading effects from apex predators to lower trophic levels: a loss of apex predators did not lead to higher levels of mesopredators, and this did not suppress mobile herbivores and drive algal proliferation. Likewise, we found no effects of mesopredators on lower trophic levels: a decline of mesopredators was not associated with higher abundances of algae-farming damselfishes and algae-dominated reefs. These findings indicate that top-down forces on coral reefs are weak, at least on the outer GBR. We conclude that predator-mediated trophic cascades are probably the exception rather than the rule in complex ecosystems such as the outer GBR.     

Morphological drivers of trophic cascades

Worldwide, local anthropogenic extinctions have recently been reported to induce trophic cascades, defined as perturbations of top consumers that propagate along food chains down to primary producers. This focus on the effects of top‐consumer extinction (i.e. of species presence) ignores potential cascading effects of the rapid morphological changes that may precede extinction. Here, we show in an experimental, three‐level food chain including medaka fish, herbivorous zooplankton and unicellular algae that varying body length of a single fish from large (36.3 mm) to small (11.5 mm) induced a stronger trophic cascade than varying an average‐sized (23.8 mm) fish from being present to absent. The strength of fish predation on zooplankton scaled quasi linearly (not with a power exponent) with fish body length and associated gape width, suggesting that the resultant trophic cascade was morphology (not metabolism)‐dependent. The effect of fish body length was stronger on phyto‐ than on zooplankton, because large‐sized fish had the unique ability to suppress large‐sized herbivores, which in turn had high grazing capacities. Hence, our results show that consumer body size, by setting diet breadth, can both drive and magnify the strength of trophic cascades. In contrast, fish body shape had no significant effect on fish predatory performances when its allometric component (the effect of size on shape) was removed. In the wild, human‐induced body downsizing of top consumers is widespread, and mitigating the resultant perturbations to ecosystem function and services will require a paradigm shift from preserving species presence towards preserving species size structure.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

相似文献   

Dynamics of Reciprocal Pulsed Subsidies in Local and Meta-Ecosystems

Temporally variable and reciprocal subsidies between ecosystems are ubiquitous. These spatial flows can generate a suite of direct and indirect effects in local and meta-ecosystems. The focus of most subsidy research, however, has been on the response of consumers in recipient ecosystems to constant subsidies over very short or very long time scales. We derive a meta-ecosystem model to explicitly consider the dynamic feedbacks between local ecosystems coupled through reciprocal pulsed subsidies. We predict oscillating reinforcing and dampening effects of reciprocal pulsed herbivore flows. Maximum reinforcing effects between reciprocal pulsed herbivore flows occur when these flows are in phase with the dynamics of neighboring predators. This prediction is robust to a range of pulse quantities and frequencies. Reciprocal pulsed herbivore subsidies lead to spatial and temporal variability in the strength of trophic cascades in local and meta-ecosystems but these cascading effects are the strongest when reciprocal pulsed subsidies are temporally concentrated. When predators demonstrate a behavioral response to prey abundance, reciprocal pulsed subsidies dampen the strength of local trophic cascades but lead to strong trophic cascades across local ecosystems. The timing of reciprocal pulsed subsidies is a critical component that determines the cascading effects of spatial flows. We show that spatial and temporal variabilities in resources and consumers can have a significant influence on the strength of cascading trophic interactions; therefore, our ability to detect and understand trophic cascades may depend on the scale of inquiry of ecological studies.     

Effects of environmental stress cascade up through four trophic levels in a salt marsh study system

1. Although in recent years there have been a number of studies demonstrating trophic cascades in terrestrial systems, it is still unclear what environmental conditions enable or enhance such cascades, especially among four trophic levels. 2. In this study, the influence of environmental stress (increased soil pore water salinity) on a four trophic level study system in a Florida salt marsh was examined by experimentally increasing soil pore water salinity. Effects of increased salinity on the quality of the host plant, Batis maritima, were assessed, as were resulting effects on the lepidopteran herbivore Ascia monuste, and the primary parasitoids and hyperparasitoids of its caterpillars. 3. Increased salinity altered host‐plant quality, which subsequently affected the consumer species. These effects of altered plant quality cascaded up through the herbivore and primary parasitoid to the hyperparasitoid Hypopteromalus inimicus, influencing its density, sex ratio, body size, and initial egg load. 4. These results demonstrate how heterogeneity in environmental stress can result in effects that cascade up through four trophic levels. We suggest that such strong effects at higher trophic levels may be more likely in systems in which relationships are more specific and intimate such as those among hosts, parasitoids, and hyperparasitoids.