Seed-cache exchange promotes coexistence and coupled consumer oscillations: a model of desert rodents as resource processors

Most models of resource competition assume that coexistence of consumers depends on tradeoffs in their abilities to exploit shared resources along dimensions of environmental heterogeneity generated by factors external to the consumers. However, consumers may create heterogeneity themselves by modifying resources that they do not immediately consume; such resource processing is predicted to allow coexistence if consumers vary in use of resources in primary vs. modified form. To explore whether external food storage (caching) represents a form of resource processing that contributes to observed patterns of species coexistence, we developed a biologically explicit simulation model of competition for a well-studied system, seed-eating desert heteromyid rodents. Here we present the model, compare competitive outcomes with and without inter-specific exchange of cached food, and describe population dynamics of coexisting competitors. The model predicts stable coexistence only when there is exchange of cached seeds via scavenging of caches left undefended by mortality or by pilferage of defended caches. Net interactions between coexisting consumers ranged from competition (10% of cases) to host-parasite (77%), commensalism (12%), and mutualism (1%). Population dynamics of coexisting consumers often showed strong periodicity and coupled synchronous or slightly lagged cycles, a possibility not previously anticipated for desert rodents occupying constant environments. Our model confirms that caching does represent a form of resource processing likely to play a significant role in the dynamics and diversity of communities of desert rodents and other caching animals. (C) 2003 Elsevier Science Ltd. All rights reserved.