Evol Ecol Res 11: 517-540 (2009) Full PDF if your library subscribes.
The implications of using multiple resources for consumer density dependence
Peter A. Abrams
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada
Correspondence: P.A. Abrams, Department of Ecology and Evolutionary Biology, University of Toronto, 25 Harbord St., Toronto, Ontario M5S 3G5, Canada.
Questions: What is the relationship between the population size and per capita growth for a consumer species experiencing resource limitation? How is this relationship influenced by the presence of multiple resources that differ in their vulnerability to the consumer? How well do traditional models of density dependence represent these relationships?
Methods: Simple differential equation models of one-consumer/multiple-resource systems are used to determine the relationship between the per capita mortality of the consumer and the equilibrium (or average) consumer density, as well as the associated relationship between per capita growth and population size. Most models have either two or very many resources.
Key assumptions: Resources are nutritionally substitutable and different resources do not interact with each other.
Predictions: If total mortality is low, use of multiple resources is likely to produce a decelerating decrease in consumer population size with an increase in imposed mortality. Deceleration is caused by relaxed apparent competition between resources as consumer mortality increases. This represents a relaxation of the conditions for persistence in a game played between the strategies represented by different prey species. Because most functional responses saturate at high resource densities, the density–mortality relationship becomes accelerating at high mortalities. As a consequence, generalist consumers often have different curvature of density dependence at high and low population sizes (unlike the widely used ‘theta-logistic’ model of density dependence).
Keywords: apparent competition, consumer–resource interaction, density dependence, exclusion, growth–defence trade-offs, harvesting populations, logistic model.
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