III.7 The Structure and Stability of Food Webs
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III.7 The Structure and Stability of Food Webs Kevin McCann OUTLINE 1. Introduction 2. Diversity and stability: The early years 3. Robert May and the limits to diversity 4. Diversity and food web structure 5. Structure, variability, and stability 6. Future directions: Food webs across space and time The role of diversity and structural complexity in the dynamics and stability of ecosystems is a longstanding and unresolved issue in ecology. Here, I review the history of this major ecological problem and highlight three relatively distinct historical periods in thought. The first period was one of mostly intuitive belief that suggests nature’s diversity gives rise to stability. This period was followed by a second that arose with the rigorous application of mathematics and dynamic systems theory that, more or less, puts this intuitive belief to the test. This theoretical result ultimately pushed ecologists to look beyond diversity to understand the dynamics of these complex natural entities. In response to this theory, a group of intrepid empirical ecologists began to map real food webs and so begin the search for patterns in food web structure. More recently, conceptual developments in ecology have begun to consider how specific food web modules (i.e., common natural food web structures) and variability in space and time govern the stability of ecological systems. The emerging answer appears to suggest that the variability itself may ultimately be responsible for the persistence of these enormously complex entities. GLOSSARY food web compartment/channel. A highly and strongly connected set of species (i.e., subweb) that connect with much lower frequency and much lower strength to other species in the larger web. food web connectance (C). Given S species in a food web, then connectance is the number of actual links or interactions (L) divided by the maximum possible links (S2 ), so C¼L/S2 . food web modules/motifs. All possible topologies of sub–food webs of n-species; thus, a specific module or motif consists of a given two-species interaction (e.g., predator–prey, mutualism), three-species interaction (food chain, omnivory, etc.). food web or ecological network. A set of species that are connected to one another via trophic interactions (i.e., fluxes of matter and energy). food web pathways. A directed set of interactions from any one species to another (e.g., a resource to consumer to a predator of the consumer). food web structure. At its most general level, nonrandom patterns in the food web topology, interaction strengths, densities, and other ecological traits (e.g., age structure). As one example, some authors have argued that omnivory is ubiquitous and so is found in real food webs more than expected in randomly constructed food web networks. Network analysts use ‘‘motifs’’ to ask if there is a specific topology that is significantly overrepresented relative to random networks. interaction strength (IS). The dynamic influence of one species on another. This is measured in a variety of metrics, but some standard measures have emerged. (1) Direct metrics: these measures estimate the direct influence of one species on another. Energy or biomass flux has been frequently employed (e.g., the IS of predator on prey is equivalent to the amount of biomass consumed by the predator). Another similar measure is the elements of the Jacobian matrix that assesses the instantaneous rate of change of one species with respect to a very small change in density of another species. (2) Indirect metrics: often employed in the field, these metrics assess the change of one species with respect to the change in density of another species (often the complete exclusion of the species). These are not instantaneous measures and so include both direct (one species immediate effect on the other species) and indirect consequences (e.g., the excluded species has effect on other species, which in turn changes the focal species density). stability. There exist several common dynamic measures of stability that generally assess the food web’s rate at which it returns to some defined aspect of its food web structure or a food web’s ability to retain some defined aspect of its structure. (1) Resilience: dynamic response to a temporary perturbation (e.g., equilibrium or nonequilibrium stability; effectively tracks how fast densities return to their original values). (2) Variability: the variability in population dynamics of individual species or groups of species. Large variability is assumed to mean that there is significant chance of losing food web structure in a...