Unpacking resilience in food webs: An emergent property or a sum of the parts?

Introduction Disturbance is a pervasive force in ecology. Understanding how disturbance influences natural systems is of growing importance as human impacts become increasingly widespread and drive changes in the magnitude and frequency of disturbance events (e.g., Archibald et al., 2012; Bellard et al., 2012). Critical to managing the effects of disturbance is gaining an understanding of what characteristics of natural communities allow them to persist in the face of disturbance. This endeavor has a long history. Early ecologists concluded that diversity begat stability based on observations of naturally fluctuating systems (e.g., Lindeman, 1942, see Rooney and McCann, 2012). These views were challenged by modeling work in the 1970s (e.g., May, 1972) which found that in highly simplified ecological models, diversity resulted in dynamic instability. Models in later years, which incorporated realistic distributions of link strengths and non-equilibrium dynamics, have yielded the hypothesis that diverse systems are stabilized by a complex net of weak interactions (e.g., Yodzis, 1981; McCann, 2000). Key to the concept of “stability” in ecological systems is the idea of dynamic responses that allow systems to “rebound” to their previous state after disturbance. Holling (1973) called this “ecological resilience.” Fundamental to the idea of resilience is that there are multiple stable states in which an ecosystem can exist, and which have characteristics that maintain those states (Walker et al., 2004). In food-web ecology, this is considered as the tendency of a food web to return to its original topology after a disturbance event (McCann, 2000). The mechanisms that underlie resilience in food webs can be divided into three groups (Figure 7.1). The first is that the individual nodes (populations of species) within the food web have resilient traits, such that when the populations are subject to disturbance, they are able to persist and recover. We term this “nodal resilience” (Figure 7.1a). Nodal resilience of a taxon is not affected by the impacts of disturbance on its resources or consumers, but rather is a direct consequence of the taxon's traits. Resilient traits of taxa can be extremely diverse but include the ability to seek refugia, resting stages that protect against disturbance, and life-history characteristics favoring fast reproduction (Townsend et al., 1997; Bolnick et al., 2011; De Lange et al., 2013).