Global change alters the stability of food webs

Recent research has generally shown that a small change in the number of species in a food web can have consequences both for community structure and ecosystem processes. However 'change' is not limited to just the number of species in a community, but might include an alteration to such properties as precipitation, nutrient cycling and temperature. How such changes might affect species interactions is important, not just through the presence or absence of interactions, but also because the patterning of interaction strengths among species is intimately associated with community stability. Interaction strengths encompass such properties as feeding rates and assimilation efficiencies, and encapsulate functionally important information with regard to ecosystem processes. Interaction strengths represent the pathways and transfer of energy through an ecosystem. We review the best empirical data available detailing the frequency distribution of interaction strengths in communities. We present the underlying (but consistent) pattern of species interactions and discuss the implications of this patterning. We then examine how such a basic pattern might be affected given various scenarios of 'change' and discuss the consequences for community stability and ecosystem functioning.

Predator-prey body size, interaction strength and the stability of a real food web

1. We examined the empirical relationship between predator-prey body size ratio and interaction strength in the Ythan Estuary food web.

Interaction strengths in food webs: issues and opportunities

1. Recent efforts to understand how the patterning of interaction strength affects both structure and dynamics in food webs have highlighted several obstacles to productive synthesis. Issues arise with respect to goals and driving questions, methods and approaches, and placing results in the context of broader ecological theory.

Weak interactions, omnivory and emergent food-web properties

Empirical studies have shown that, in real ecosystems, species-interaction strengths are generally skewed in their distribution towards weak interactions. Some theoretical work also suggests that weak interactions, especially in omnivorous links, are important for the local stability of a community at equilibrium. However, the majority of theoretical studies use uniform distributions of interaction strengths to generate artificial communities for study. We investigate the effects of the underlying interaction-strength distribution upon the return time, permanence and feasibility of simple Lotka-Volterra equilibrium communities. We show that a skew towards weak interactions promotes local and global stability only when omnivory is present. It is found that skewed interaction strengths are an emergent property of stable omnivorous communities, and that this skew towards weak interactions creates a dynamic constraint maintaining omnivory. Omnivory is more likely to occur when omnivorous interactions are skewed towards weak interactions. However, a skew towards weak interactions increases the return time to equilibrium, delays the recovery of ecosystems and hence decreases the stability of a community. When no skew is imposed, the set of stable omnivorous communities shows an emergent distribution of skewed interaction strengths. Our results apply to both local and global concepts of stability and are robust to the definition of a feasible community. These results are discussed in the light of empirical data and other theoretical studies, in conjunction with their broader implications for community assembly.