The coordination of green-brown food webs and their disruption by anthropogenic nutrient inputs

Aim

Our goal was to quantify nitrogen flows and stocks in green-brown food webs in different ecosystems, how they differ across ecosystems and how they respond to nutrient enrichment.

Location

Global.

Time period

Contemporary.

Major taxa studied

Plants, phytoplankton, macroalgae, invertebrates, vertebrates and zooplankton.

Methods

Data from >500 studies were combined to estimate nitrogen stocks and fluxes in green-brown food webs in forests, grasslands, brackish environments, seagrass meadows, lakes and oceans. We compared the stocks, fluxes and metabolic rates of different functional groups within each food web. We also used these estimates to build a dynamical model to test the response of the ecosystems to nutrient enrichment.

Results

We found surprising symmetries between the green and brown channels across ecosystems, in their stocks, fluxes and consumption coefficients and mortality rates. We also found that nitrogen enrichment, either organic or inorganic, can disrupt this balance between the green and brown channels.

Main conclusions

Linking green and brown food webs reveals a previously hidden symmetry between herbivory and detritivory, which appears to be a widespread property of natural ecosystems but can be disrupted by anthropogenic nitrogen additions.

Cascading Effects of Birds and Bats in a Shaded Coffee Agroforestry System

Volant vertebrate insectivores, including birds and bats, can be important regulators of herbivores in forests and agro-ecosystems. Their effects can be realized directly through predation and indirectly via intraguild predation. This paper examines data from bird and bat exclosures in coffee farms in Chiapas, Mexico in order to determine their effect on herbivores. Arthropods were sampled in 32 exclosures (with 10 coffee plants in each) and their paired controls three times during 6 months. After 3 months, herbivore and spider abundance increased, underscoring the importance of both intertrophic predation between volant vertebrate insectivores and herbivores and intraguild predation between volant vertebrate insectivores and spiders. After 6 months, herbivore abundance increased in the exclosures, which is indicative of a direct negative effect of birds and bats on herbivores. We suggest that intraguild predation is important in this food web and that seasonality may change the relative importance of intraguild vs. intertrophic predation. Results suggest a dissipating trophic cascade and echo the growing body of evidence that finds birds and bats are regulators of herbivores in agro-ecosystems.

Keystone nonconsumptive effects within a diverse predator community

The number of prey killed by diverse predator communities is determined by complementarity and interference among predators, and by traits of particular predator species. However, it is less clear how predators' nonconsumptive effects (NCEs) scale with increasing predator biodiversity. We examined NCEs exerted on Culex mosquitoes by a diverse community of aquatic predators. In the field, mosquito larvae co-occurred with differing densities and species compositions of mesopredator insects; top predator dragonfly naiads were present in roughly half of surveyed water bodies. We reproduced these predator community features in artificial ponds, exposing mosquito larvae to predator cues and measuring resulting effects on mosquito traits throughout development. Nonconsumptive effects of various combinations of mesopredator species reduced the survival of mosquito larvae to pupation, and reduced the size and longevity of adult mosquitoes that later emerged from the water. Intriguingly, adding single dragonfly naiads to ponds restored survivorship of larval mosquitoes to levels seen in the absence of predators, and further decreased adult mosquito longevity compared with mosquitoes emerging from mesopredator treatments. Behavioral observations revealed that mosquito larvae regularly deployed "diving" escape behavior in the presence of the mesopredators, but not when a dragonfly naiad was also present. This suggests that dragonflies may have relaxed NCEs of the mesopredators by causing mosquitoes to abandon energetically costly diving. Our study demonstrates that adding one individual of a functionally unique species can substantially alter community-wide NCEs of predators on prey. For pathogen vectors like mosquitoes, this could in turn influence disease dynamics.

Ecological-network models link diversity, structure and function in the plankton food-web

A planktonic food-web model including sixty-three functional nodes (representing auto- mixo- and heterotrophs) was developed to integrate most trophic diversity present in the plankton. The model was implemented in two variants - which we named 'green' and 'blue' - characterized by opposite amounts of phytoplankton biomass and representing, respectively, bloom and non-bloom states of the system. Taxonomically disaggregated food-webs described herein allowed to shed light on how components of the plankton community changed their trophic behavior in the two different conditions, and modified the overall functioning of the plankton food web. The green and blue food-webs showed distinct organizations in terms of trophic roles of the nodes and carbon fluxes between them. Such re-organization stemmed from switches in selective grazing by both metazoan and protozoan consumers. Switches in food-web structure resulted in relatively small differences in the efficiency of material transfer towards higher trophic levels. For instance, from green to blue states, a seven-fold decrease in phytoplankton biomass translated into only a two-fold decrease in potential planktivorous fish biomass. By linking diversity, structure and function in the plankton food-web, we discuss the role of internal mechanisms, relying on species-specific functionalities, in driving the 'adaptive' responses of plankton communities to perturbations.

TROPHIC LEVELS AND TROPHIC TANGLES: THE PREVALENCE OF OMNIVORY IN REAL FOOD WEBS

The concept of trophic levels is one of the oldest in ecology and informs our understanding of energy flow and top-down control within food webs, but it has been criticized for ignoring omnivory. We tested whether trophic levels were apparent in 58 real food webs in four habitat types by examining patterns of trophic position. A large proportion of taxa (64.4%) occupied integer trophic positions, suggesting that discrete trophic levels do exist. Importantly however, the majority of those trophic positions were aggregated around integer values of 0 and 1, representing plants and herbivores. For the majority of the real food webs considered here, secondary consumers were no more likely to occupy an integer trophic position than in randomized food webs. This means that, above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores. Omnivory was most common in marine systems, rarest in streams, and intermediate in lakes and terrestrial food webs. Trophic-level-based concepts such as trophic cascades may apply to systems with short food chains, but they become less valid as food chains lengthen.

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.

Complex dynamics in a three-level trophic system with intraspecies interaction

In this paper, we present a three-level trophic food chain, including intraspecies interaction. In contrast with other analyses, we consider the effect on the third trophic level by the first-level parameters. The model shows complex, as well as, chaotic oscillations. Bifurcation diagrams show period doubling route to chaos and crises. Also from the forward and backwards sections of the bifurcation diagrams, we find hysteresis. This result implies the coexistence of attractors for the same parameter values. In particular, we consider the coexistence of a chaotic and a P1 attractors. Our results show that the regulation in the food chain is not exclusive to either a food-prey or prey-predator interaction, but to a more subtle food-prey-predator interaction, where, for some parameter values, a food-prey or a prey-predator regulation may dominate the system's dynamics. Finally, we consider the impact of the intraspecies interaction in the overall dynamics of the food chain.

Limits to Trophic Levels and Omnivory in Complex Food Webs: Theory and Data

While trophic levels have found broad application throughout ecology, they are also in much contention on analytical and empirical grounds. Here, we use a new generation of data and theory to examine long-standing questions about trophic-level limits and degrees of omnivory. The data include food webs of the Chesapeake Bay, U.S.A., the island of Saint Martin, a U.K. grassland, and a Florida seagrass community, which appear to be the most trophically complete food webs available in the primary literature due to their inclusion of autotrophs and empirically derived estimates of the relative energetic contributions of each trophic link. We show that most (54%) of the 212 species in the four food webs can be unambiguously assigned to a discrete trophic level. Omnivory among the remaining species appears to be quite limited, as judged by the standard deviation of omnivores' energy-weighted food-chain lengths. This allows simple algorithms based on binary food webs without energetic details to yield surprisingly accurate estimates of species' trophic and omnivory levels. While maximum trophic levels may plausibly exceed historically asserted limits, our analyses contradict both recent empirical claims that these limits are exceeded and recent theoretical claims that rampant omnivory eliminates the scientific utility of the trophic-level concept.

Are there real differences among aquatic and terrestrial food webs?

Recently, aquatic and terrestrial ecologists have put forward several hypotheses regarding similarities and differences in food-web structure and function among these ecosystem types. Although many of these hypotheses explore why strong top-down effects and trophic cascades might be less common in terrestrial than in aquatic ecosystems, there is little theoretical or empirical evidence available to support or refute these hypotheses. Many unanswered questions remain about potential differences across ecosystem types: progress will require empirical studies designed within a broader context that allows for more direct comparisons.

Top-down and bottom-up community regulation in marine rocky intertidal habitats

Strong top-down control by consumers has been demonstrated in rocky intertidal communities around the world. In contrast, the role of bottom-up effects (nutrients and productivity), known to have important influences in terrestrial and particularly freshwater ecosystems, is poorly known in marine hard-bottom communities. Recent studies in South Africa, New England, Oregon and New Zealand suggest that bottom-up processes can have important effects on rocky intertidal community structure. A significant aspect of all of these studies was the incorporation of processes varying on larger spatial scales than previously considered (10's to 1000's of km). In all four regions, variation in oceanographic factors (currents, upwelling, nutrients, rates of particle flux) was associated with different magnitudes of algal and/or phytoplankton abundance, availability of particulate food, and rates of recruitment. These processes led to differences in prey abundance and growth, secondary production, consumer growth, and consumer impact on prey resources. Oceanographic conditions therefore may vary on scales that generate ecologically significant variability in populations at the bottom of the food chain, and through upward-flowing food chain effects, lead to variation in top-down trophic effects. I conclude that top-down and bottom-up processes can be important joint determinants of community structure in rocky intertidal habitats, and predict that such effects will occur generally wherever oceanographic 'discontinuities' lie adjacent to rocky coastlines. I further argue that increased attention by researchers and of funding agencies to such benthic-pelagic coupling would dramatically enhance our understanding of the dynamics of marine ecosystems.

Food Web Effects of Prey Size Refugia: Variable Interactions and Alternative Stable Equilibria

Predators can have highly variable effects on the abundance and composition of food webs, ranging from strong to weak effects of top predators. Typical food web models assume that individual prey are identical in their susceptibility to predators throughout their lives, but many prey species become less vulnerable to predators through ontogeny. A simple set of models is explored where prey must pass through a vulnerable stage prior to achieving a predator-invulnerable size refuge. As productivity of the environment increases, the proportional impact of predators decreases because more individuals become and remain in the invulnerable adult stage. The addition of a competitor prey species that can not achieve size refuge results in contrasting outcomes. At low productivity, the small species wins in competition, and the system is strongly consumer controlled. At high productivity, the large species wins due to the presence of predators, and the system becomes less consumer controlled. At intermediate productivity, either the small or the large species can win depending on initial conditions, and the system can be either strongly or weakly consumer controlled. Such alternative stable equilibria derived from models with prey size refugia may help to explain many natural situations.

Higher-order predators and the regulation of insect herbivore populations

Empirical research has not supported the prediction that populations of terrestrial herbivorous arthropods are regulated solely by their natural enemies. Instead, both natural enemies (top-down effects) and resources (bottom-up effects) may play important regulatory roles. This review evaluates the hypothesis that higher-order predators may constrain the top-down control of herbivore populations. Natural enemies of herbivorous arthropods generally are not top predators within terrestrial food webs. Insect pathogens and entomopathogenic nematodes inhabiting the soil may be attacked by diverse micro- and mesofauna. Predatory and parasitic insects are attacked by their own suite of predators, parasitoids, and pathogens. The view of natural enemy ecology that has emerged from laboratory studies, where natural enemies are often isolated from all elements of the biotic community except for their hosts or prey, may be an unreliable guide to field dynamics. Experimental work suggests that interactions of biological control agents with their own natural enemies can disrupt the effective control of herbivore populations. Disruption has been observed experimentally in interactions of bacteria with bacteriophages, nematodes with nematophagous fungi, parasitoids with predators, parasitoids with hyperparasitoids, and predators with other predators. Higher-order predators have been little studied; manipulative field experiments will be especially valuable in furthering our understanding of their roles in arthropod communities.