Kevin McCann

Weak trophic interactions and the balance of nature

Ecological models show that complexity usually destabilizes food webs,, predicting that food webs should not amass the large numbers of interacting species that are in fact found in nature. Here, using nonlinear models, we study the influence of interaction strength (likelihood of consumption of one species by another) on food-web dynamics away from equilibrium. Consistent with previous suggestions,, our results show that weak to intermediate strength links are important in promoting community persistence and stability. Weak links act to dampen oscillations between consumers and resources. This tends to maintain population densities further away from zero, decreasing the statistical chance that a population will become extinct (lower population densities are more prone to such chances). Data on interaction strengths in natural food webs indicate that food-web interaction strengths are indeed characterized by many weak interactions and a few strong interactions.

Effects of partitioning allochthonous and autochthonous resources on food web stability

The flux of energetic and nutrient resources across habitat boundaries can exert major impacts on the dynamics of the recipient food web. Competition for these resources can be a key factor structuring many ecological communities. Competition theory suggests that competing species should exhibit some partitioning to minimize competitive interactions. Species should partition both in situ (autochthonous) resources and (allochthonous) resources that enter the food web from outside sources. Allochthonous resources are important sources of energy and nutrients in many low productivity systems and can significantly influence community structure. The focus of this paper is on: (i) the influence of resource partitioning on food web stability, but concurrently we examine the compound effects of; (ii) the trophic level(s) that has access to allochthonous resources; (iii) the amount of allochthonous resource input; and (iv) the strength of the consumer–resource interactions. We start with a three trophic level food chain model (resource–consumer–predator) and separate the higher two trophic levels into two trophospecies. In the model, allochthonous resources are either one type available to both consumers and predators or two distinct types, one for consumers and one for predators. The feeding preferences of the consumer and predator trophospecies were varied so that they could either be generalists or specialists on allochthonous and/or autochthonous resources. The degree of specialization influenced system persistence by altering the structure and, therefore, the indirect effects of the food web. With regard to the trophic level(s) that has access to allochthonous resources, we found that a single allochthonous resource available to both consumers and predators is more unstable than two allochthonous resources. The results demonstrate that species populating food webs that experience low to moderate allochthonous resources are more persistent. The results also support the notion that strong links destabilize food web dynamics, but that weak to moderate strength links stabilize food web dynamics. These results are consistent with the idea that the particular structure, resource availability, and relative strength of links of food webs (such as degree of specialization) can influence the stability of communities. Given that allochthonous resources are important resources in many ecosystems, we argue that the influence of such resources on species’ and community persistence needs to be examined more thoroughly to provide a clearer understanding of food web dynamics.

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.

DENSITY-DEPENDENT COEXISTENCE IN FISH COMMUNITIES

A set of stage-structured competition models is considered. The models are parameterized using allometric relationships specific to four fish life history strategies, constraining the model to biologically plausible regions in parameter space. Using a coupled consumer–resource competition model, I get the paradoxical result that two populations can coexist in equilibrium on an identical resource base, without recourse to temporal or spatial partitioning. In contrast to much theory on limiting similarity, the model also predicts that fish populations of similar life history strategies can coexist in equilibrium with complete resource overlap. This form of coexistence (called density-dependent coexistence) requires that life history strategies differ such that an advantage at one stage of the life cycle implies a disadvantage at another stage in the life cycle. These exact life history trade-offs appear in small-bodied fish. Furthermore, it is postulated that this result may be applicable to other taxa.

Empirical Methods of Identifying and Quantifying Trophic Interactions for Constructing Soil Food-Web Models

Introduction Food-web models, which depict the trophic relationships between organisms within a community, form a powerful and versatile approach to study the relationships between community structure and ecosystem functioning. Although food-web models have recently been applied to a wide range of ecological studies (Memmott, 2009; Sanders et al., 2014), such approaches can be greatly improved by introducing high-resolution trophic information from empirical studies and experiments that realistically describe topological structure and energy flows (de Ruiter et al., 2005). Over the last decades major technological advances have been made in empirically characterizing trophic networks by describing, in detail, the connectedness and flows in food webs. Existing empirical techniques, such as stable isotope probing (SIP) (Layman et al., 2012), have been refined and new approaches have been created by combining methods, e.g., combining Raman spectroscopy or fatty acid analysis with SIP (Ruess et al., 2005a; Li et al., 2013). These empirical methods can provide insight into different aspects of food webs and together form an extensive toolbox to investigate trophic interactions. It is crucial to recognize the potential and limitations of a range of empirical approaches in order to choose the right method in the design of empirically based food-web studies. Empirically based food webs are generally classified according to the type of input information that is required. In the following lines we will provide an overview of four types of food-web model: connectedness webs, semi-quantitative webs, energy-flow webs, and functional webs. Paine (1980) introduced three of those webs, which are widely accepted and applied in food-web studies across ecosystems. We propose to add a fourth type of empirically based food web, the semi-quantitative web. All of these food webs have the same basic structure, but the conceptual webs differ in the type of trophic information they describe and represent (Figure 16.1). Connectedness webs (Figure 16.1a) define the basic structure of a food web by describing the food-web connections per se.