Dynamics of Simple Food Webs

2006

Summary 1. Successional changes during sequential assembly of food webs were examined. This was carried out by numerical methods, drawing one species at a time from a species pool and obtaining the permanent (persistent) community emerging at each step. Interactions among species were based on some simple rules about body sizes of consumers and their prey, and community dynamics were described in terms of flows of biomass density. 2.

Oecologia, 1998

Traditional ecological theory predicts that the stability of simple food webs will decline with an increasing number of trophic levels and increasing amounts of omnivory. These ideas have been tested using protozoans in laboratory microcosms. However, the results are equivocal, and contrary to expectation, omnivory is common in natural food webs. Two recent developments lead us to re-evaluate these predictions using food webs assembled from protists and bacteria. First, recent modelling work suggests that omnivory is actually stabilizing, providing that interactions are not too strong. Second, it is dicult to evaluate the degree of omnivory of some protozoan species without explicit experimental tests. This study used seven species of ciliated protozoa and a mixed bacterial¯ora to assemble four food webs with two trophic levels, and four webs with three trophic levels. Protist species were assigned a rank for their degree of omnivory using information in the literature and the results of experiments that tested whether the starvation rate of predators was in¯uenced by the amount of bacteria on which they may have fed and whether cannibalism (a form of omnivory) occurred. Consistent with recent modelling work, both bacterivorous and predatory species with higher degrees of omnivory showed more stable dynamics, measured using time until extinction and the temporal variability of population density. Systems with two protist species were less persistent than systems with one protist species, supporting the prediction that longer food chains will be less stable dynamically.

Proceedings of the National Academy of Sciences, 2009

The pattern of predator-prey interactions is thought to be a key determinant of ecosystem processes and stability. Complex ecological networks are characterized by distributions of interaction strengths that are highly skewed, with many weak and few strong interactors present. Theory suggests that this pattern promotes stability as weak interactors dampen the destabilizing potential of strong interactors. Here, we present an experimental test of this hypothesis and provide empirical evidence that the loss of weak interactors can destabilize communities in nature. We ranked 10 marine consumer species by the strength of their trophic interactions. We removed the strongest and weakest of these interactors from experimental food webs containing >100 species. Extinction of strong interactors produced a dramatic trophic cascade and reduced the temporal stability of key ecosystem process rates, community diversity and resistance to changes in community composition. Loss of weak interactors also proved damaging for our experimental ecosystems, leading to reductions in the temporal and spatial stability of ecosystem process rates, community diversity, and resistance. These results highlight the importance of conserving species to maintain the stabilizing pattern of trophic interactions in nature, even if they are perceived to have weak effects in the system. biodiversity and ecosystem functioning ͉ dynamic index ͉ interaction strength ͉ predator-prey interactions ͉ temporal and spatial variability F or decades, scientists have argued over the natural phenomena that allow complex communities to persist in nature (1-3). Randomly assembled communities become less stable with increasing complexity (2, 4), but natural communities are finely structured (5, 6), displaying properties that promote stability despite complexity (7). Experiments (8-10) and theory based on empirical data (11, 12) have shown that real food webs are characterized by few strong interactions embedded in a majority of weak links. It is thought that this nonrandom arrangement of interaction strengths promotes community-level stability by generating negative covariances, which suppress the destabilizing effect of strong consumer-resource interactions . Theoretical studies provide overwhelming support for the idea that the pattern of strong and weak predator-prey interaction strengths confers stability to food webs (11-14); however, these predictions have never been tested experimentally in natural systems.

Journal of Theoretical Biology, 2002

Recent investigations on the structure of complex networks have provided interesting results for ecologists. Being inspired by these studies, we analyse a well-defined set of small model food webs. The extinction probability caused by internal Lotka-Volterra dynamics is compared to the position of species. Simulations have revealed that some global properties of these food webs (e.g. the homogeneity of connectedness) and the positions of species therein (e.g. interaction pattern) make them prone to modelled biotic extinction caused by population dynamical effects. We found that: (a) homogeneity in the connectedness structure increases the probability of extinction events; (b) in addition to the number of interactions, their orientations also influence the future of species in a web. Since species in characteristic network positions are prone to extinction, results could also be interpreted as describing the properties of preferred states of food webs during community assembly. Our results may contribute to understanding the intimate relationship between pattern and process in ecology.

Artificial Life and Robotics, 2009

Iraqi Journal of Science, 2021

This paper aims to study the role of a prey refuge that depends on both prey and predator species on the dynamics of a food web model. It is assumed that the food transfer among the web levels occurs according to Lotka-Volterra functional response. The solution properties, such as existence, uniqueness, and uniform boundedness, are discussed. The local, as well as the global, stabilities of the solution of the system are investigated. The persistence of the system is studied with the assistance of average Lyapunov function. The local bifurcation conditions that may occur near the equilibrium points are established. Finally, numerical simulation is used to confirm our obtained results. It is observed that the system has only one type of attractors that is a stable point, while periodic dynamics do not exist even on the boundary planes.

Mathematical Biosciences, 1983

The ecological assembly of food webs is considered as a process of predator colonizations and extinctions.

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.

2002

Most species live in species-rich food webs; yet, for a century, most mathematical models for population dynamics have included only one or two species. We ask whether such models are relevant to the real world. Two-species population models of an interacting consumer and resource collapse to one-species dynamics when recruitment to the resource population is unrelated to resource abundance, thereby weakening the coupling between consumer and resource. We predict that, in nature, generalist consumers that feed on many species should similarly show one-species dynamics. We test this prediction using cyclic populations, in which it is easier to infer underlying mechanisms, and which are widespread in nature. Here we show that one-species cycles can be distinguished from consumer–resource cycles by their periods. We then analyse a large number of time series from cyclic populations in nature and show that almost all cycling, generalist consumers examined have periods that are consistent with one-species dynamics. Thus generalist consumers indeed behave as if they were one-species populations, and a one-species model is a valid representation for generalist population dynamics in many-species food webs.

Ecological Modelling, 2003

Intraguild predation is a trophic interaction in which two consumers compete for one resource and where one of the consumer species may also feed on its competitor. The intraguild predator's diet follows from the relative strength of its interactions with its potential prey. Current view holds that weak interactions between species promote the stability of food webs. To the contrary, nutrient enrichment is predicted to destabilize ecosystems. We present a theoretical analysis of the interplay between intraguild predation and nutrient enrichment in a Marr-Pirt chemostat model of a microbial food web. We perform a two-dimensional bifurcation analysis along a gradient of allochtonous nutrient levels and a gradient of one out of two biologically plausible strategies to explore the spectrum of the intraguild predator's foraging interactions. Both strategies show that intraguild predation may • stabilize food chains;