Environmental Control of Food Web Structure

Trends in Ecology & Evolution, 2012

Glossary Assembly: the set of processes by which a food web is rebuilt after disturbance or the creation of new habitat. Bioenergetics: the flow and transformation of energy in and between living organisms and between living organisms and their environment. Cascade model: a food-web model which assumes hierarchical feeding along a single niche axis, with each species allocated a probability of feeding on taxa below it in the hierarchy. Community web: a food web intended to include all species and trophic links that occur within a defined ecological community. 'Species' in this sense might involve various degrees of aggregation or division of biological species. Compartmentalisation: the property by which one subset of species within a food web operates, to varying degrees, independently of other parts. Ecological stoichiometry: how the balance of multiple chemical elements within organisms influences their interactions in food webs. Ecosystem function: the physical, chemical, and biological processes or attributes that contribute to the self-maintenance of the ecosystem; including energy flow, nutrient cycling, filtering, buffering of contaminants, and regulation of populations. Interaction strength: a measure of how much a predator alters population size, biomass, or production of its prey. In food webs these are associated with energy flows while noting that predators can affect prey non-trophically. Niche model: a food-web model which assigns each consumer a feeding distribution on the niche axis that can overlap with itself (cannibalism) and permits trophic loops but still generates realistic patterns whereby species tend to feed in a linear hierarchy. Quantitative food web: a food web where the interaction strengths or trophic flows are quantified. Scale-independence and/or dependence: the degree to which attributes of a food web change with food-web size. Stability: measures of the ability to and/or speed with which a food web regains its structure following a disturbance (resilience) or resists change in food web structure (resistance).

Freshwater Biology, 2012

Artificial Life and Robotics, 2009

Ecology, 2016

Habitat size and climate are known to affect the trophic structure and dynamics of communities, but their interactive effects are poorly understood. Organisms from different trophic levels vary in terms of metabolic requirements and heat dissipation. Indeed, larger species such as keystone predators require more stable climatic conditions than their prey. Likewise, habitat size disproportionally affects large-sized predators, which require larger home ranges and are thus restricted to larger habitats. Therefore, food web structure in patchy ecosystems is expected to be shaped by habitat size and climate variations. Here we investigate this prediction using natural aquatic microcosm (bromeliad phytotelmata) food webs composed of litter resources (mainly detritus), detritivores, mesopredators, and top predators (damselflies). We surveyed 240 bromeliads of varying sizes (water retention capacity) across 12 open restingas in SE Brazil spread across a wide range of tropical latitudes (-12.6° to -27.6°, ca. 2,000 km) and climates (Δ mean annual temperature = 5.3°C). We found a strong increase in predator-to-detritivore mass ratio with habitat size, which was representative of a typical inverted trophic pyramid in larger ecosystems. However, this relationship was contingent among the restingas; slopes of linear models were steeper in more stable and favorable climates, leading to inverted trophic pyramids (and top-down control) being more pronounced in environments with more favorable climatic conditions. By contrast, detritivore-resource and mesopredator-detritivore mass ratios were not affected by habitat size or climate variations across latitudes. Our results highlight that the combined effects of habitat size, climate and predator composition are pivotal to understanding the impacts of multiple environmental factors on food web structure and dynamics.

The American Naturalist, 1998

In nature, fluxes across habitats often bring both nuory, which plays a central role in consumer-resource intrient and energetic resources into areas of low productivity from areas of higher productivity. These inputs can alter consumption teractions and food web dynamics. They further rates of consumer and predator species in the recipient food webs, suggested that multichannel omnivory can dampen or fathereby influencing food web stability. Starting from a well-studied cilitate trophic cascades. McCann and Hastings (1997) tritrophic food chain model, we investigated the impact of allochrecently found that food web dynamics were stabilized by thonous inputs on the stability of a simple food web model. We weak to moderate amounts of trophic omnivory, one considered the effects of allochthonous inputs on stability of the component of multichannel omnivory. They did not, model using four sets of biologically plausible parameters that rephowever, examine the influence of other types of multiresent different dynamical outcomes. We found that low levels of allochthonous inputs stabilize food web dynamics when species channel omnivory on food web dynamics. In this article, preferentially feed on the autochthonous sources, while either inwe address this problem by extending a simple food creasing the input level or changing the feeding preference to favor chain to include a different component of multichannel allochthonous inputs, or both, led to a decoupling of the food omnivory: allochthonous inputs (inputs entering from chain that could result in the loss of one or all species. We argue another habitat). We demonstrate that allochthonous that allochthonous inputs are important sources of productivity in sources can also stabilize food web dynamics, further many food webs and their influence needs to be studied further. suggesting that donor control may be an important fac-This is especially important in the various systems, such as caves, headwater streams, and some small marine islands, in which more tor in community dynamics and that trophic cascades energy enters the food web from allochthonous inputs than from may be weakened in systems that have relatively large alautochthonous inputs. lochthonous inputs.

Physical Review Letters, 2002

We analyze the properties of seven community food webs from a variety of environments--including freshwater, marine-freshwater interfaces and terrestrial environments. We uncover quantitative unifying patterns that describe the properties of the diverse trophic webs considered and suggest that statistical physics concepts such as scaling and universality may be useful in the description of ecosystems. Specifically, we find that several quantities characterizing these diverse food webs obey functional forms that are universal across the different environments considered. The empirical results are in remarkable agreement with the analytical solution of a recently proposed model for food webs.

Journal of Animal Ecology, 2008

1. Following the development of the relatively successful niche model, several other simple structural food web models have been proposed. These models predict the detailed structure of complex food webs given only two input parameters, the numbers of species and the number of feeding links among them. 2. The models claim different degrees of success but have not been compared consistently with each other or with the empirical data. We compared the performance of five structural models rigorously against 10 empirical food webs from a variety of aquatic and terrestrial habitats containing 25-92 species and 68-997 links. 3. All models include near-hierarchical ordering of species' consumption and have identical distributions of the number of prey of each consumer species, but differ in the extent to which species' diets are required to be contiguous and the rules used to assign feeding links. 4. The models perform similarly on a range of food-web properties, including the fraction of top, intermediate and basal species, the standard deviations of generality and connectivity and the fraction of herbivores and omnivores. 5. For other properties, including the standard deviation of vulnerability, the fraction of cannibals and species in loops, mean trophic level, path length, clustering coefficient, maximum similarity and diet discontinuity, there are significant differences in the performance of the different models. 6. While the empirical data do not support the niche model's assumption of diet contiguity, models which relax this assumption all have worse overall performance than the niche model. All the models underestimate severely the fraction of species that are herbivores and exhibit other important failures that need to be addressed in future research.

Global Ecology and Biogeography, 2019

Aim: How do factors such as space, time, climate and other ecological drivers influence food web structure and dynamics? Collections of well-studied food webs and replicate food webs from the same system that span biogeographical and ecological gradients now enable detailed, quantitative investigation of such questions and help integrate food web ecology and macroecology. Here, we integrate macroecology and food web ecology by focusing on how ecogeographical rules [the latitudinal diversity gradient (LDG), Bergmann's rule, the island rule and Rapoport's rule] are associated with the architecture of food webs. Location: Global. Time period: Current. Major taxa studied: All taxa. Methods: We discuss the implications of each ecogeographical rule for food webs, present predictions for how food web structure will vary with each rule, assess | 1205 BAISER Et Al.

A B S T R A C T Determining the spatial scale is a crucial stage in any ecological study. However, knowledge on the effect of spatial scale on food web network properties is still lacking in published literature. In this work, the effect of spatial scale on the network properties was determined for the Tagus estuary ecosystem. The food web of the Tagus estuary was assembled at three spatial scales, in terms of grain (resolution) and extent. In terms of grain, the three scales defined were: small grain (30 sites), medium grain (15 sites) and large grain (6 sites). In terms of extent, the three scales defined were: extended estuary (30 sites), strictly estuarine (20 sites), nursery area (6 sites). The lists of species for each site were merged and used to construct one major food web containing all species observed, at each of the spatial scales under analysis. It was concluded that the spatial extent of the sampling highly influenced food web networks properties, more than grain, and should thus be carefully chosen when conducting food web studies in estuaries, as well as in other ecosystems that exhibit strong environmental gradients. Studies that aim to characterize estuarine food webs should encompass the full extent of the salinity gradient.

Ecology, 2005

We analyze the properties of model food webs and of fifteen community food webs from a variety of environments -including freshwater, marine-freshwater interfaces and terrestrial environments. We first perform a theoretical analysis of a recently proposed model for food webs-the niche model of Williams and Martinez (2000). We derive analytical expressions for the distributions of species' number of prey, number of predators, and total number of trophic links and find that they follow universal functional forms. We also derive expressions for a number of other biologically relevant parameters which depend on these distributions. These include the fraction of top, intermediate, basal, and cannibal species, the standard deviations of generality and vulnerability, the correlation coefficient between species' number of prey and number of predators, and assortativity. We show that our findings are robust under rather general conditions; a result which could not have been demonstrated without treating the problem analytically. We then use our analytical predictions as a guide to the analysis of fifteen of the most complete empirical food webs available. We uncover quantitative unifying patterns that describe the properties of the model food webs and most of the trophic webs considered. Our results support a strong new hypothesis that the empirical distributions of number of prey and number of predators follow universal functional forms that, without free parameters, match our analytical predictions. Further, we find that the empirically observed correlation coefficient, assortativity, and fraction of cannibal species are consistent with our analytical expressions and simulations of the niche model. Finally, we show that two quantities typically used to characterize complex networks, the average distance between nodes and the average clustering coefficient of the nodes, show a high degree of regularity for both the empirical data and simulations of the niche model. Our findings suggest that statistical physics concepts such as scaling and universality may be useful in the description of natural ecosystems.

Trends in Ecology & Evolution, 2012

Given the unprecedented rate of species extinctions facing the planet, understanding the causes and consequences of species diversity in ecosystems is of paramount importance. Ecologists have investigated both the influence of environmental variables on species diversity and the influence of species diversity on ecosystem function and stability. These investigations have largely been carried out without taking into account the overarching stabilizing structures of food webs that arise from evolutionary and successional processes and that are maintained through species interactions. Here, we argue that the same large-scale structures that have been purported to convey stability to food webs can also help to understand both the distribution of species diversity in nature and the relationship between species diversity and food web stability. Specifically, the allocation of species diversity to slow energy channels within food webs results in the skewed distribution of interactions strengths that has been shown to confer stability to complex food webs. We end by discussing the processes that might generate and maintain the structured, stable and diverse food webs observed in nature.

Food Webs, 2015

Food webs are one of the primary frameworks on which the ecological sciences have been built. Research in this field has burgeoned over recent decades, expanding into diverse sub-disciplines and employing many different methodological approaches. Here we structure a historical review around 14 researchers and the specific contributions they have made to the field. Beginning with Charles Elton's insights into food web structure, and continuing to contemporary ecologists and emerging areas of study, we highlight some of the most important empirical and theoretical advances made over the last century. The review highlights that there are fundamentally different ways in which food webs are depicted and studied. Specifically, when one views systems through mathematical, energy flow or functional lenses, very different perspectives on food web structure and dynamics emerge. The contributions of these scientists illustrate the considerable advances that the field has undergone, and they provide the foundation for expansive on-going research programs that fall under the broad umbrella of food web ecology.

Longitudinal changes in the structure and properties of food webs are known to occur in river systems from mountains to lowland areas. However, it is still unknown how food web structure and properties change along the estuarine-coastal gradient. Highly resolved estuarine food webs were assembled for 30 sites in the Tagus estuary and adjacent coastal waters to investigate changes in food web structure and properties along 4 sections: upper estuary, middle estuary, channel and coast. This work confirmed the highly variable nature of estuarine systems, not only in terms of physicochemical and biological communities, but also in food web structure and network properties. The upper estuary stood out for having the highest variability for many food web properties. Overall, more significant differences were detected within this estuarinecoastal gradient than previously reported for river systems, and over a much smaller spatial scale.

Aquatic Food Webs, 2005

Ecology Letters, 2010

Food web structure plays an important role when determining robustness to cascading secondary extinctions. However, existing food web models do not take into account likely changes in trophic interactions (‘rewiring’) following species loss. We investigated structural dynamics in 12 empirically documented food webs by simulating primary species loss using three realistic removal criteria, and measured robustness in terms of subsequent secondary extinctions. In our model, novel trophic interactions can be established between predators and food items not previously consumed following the loss of competing predator species. By considering the increase in robustness conferred through rewiring, we identify a new category of species – overlap species – which promote robustness as shown by comparing simulations incorporating structural dynamics to those with static topologies. The fraction of overlap species in a food web is highly correlated with this increase in robustness; whereas species richness and connectance are uncorrelated with increased robustness. Our findings underline the importance of compensatory mechanisms that may buffer ecosystems against environmental change, and highlight the likely role of particular species that are expected to facilitate this buffering.