vol. 155, no. 2 the american naturalist february 2000 Grazers and Diggers: Exploitation Competition and Coexistence among Foragers with Different Feeding Strategies on a Single Resource Shane A. Richards, 1,* Roger M. Nisbet, 2 William G. Wilson, 1 and Hugh P. Possingham 3 1. Department of Zoology, Duke University, Durham, North Carolina 27708-0325; 2. Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106; 3. Department of Applied and Molecular Ecology, University of Adelaide, Waite Campus, PB 1 Glen Osmond, South Australia 5064, Australia Submitted January 25, 1999; Accepted August 23, 1999 abstract: A mathematical model is presented that describes a sys- tem where two consumer species compete exploitatively for a single renewable resource. The resource is distributed in a patchy but homo- geneous environment; that is, all patches are intrinsically identical. The two consumer species are referred to as diggers and grazers, where diggers deplete the resource within a patch to lower densities than grazers. We show that the two distinct feeding strategies can produce a heterogeneous resource distribution that enables their co- existence. Coexistence requires that grazers must either move faster than diggers between patches or convert the resources to population growth much more efficiently than diggers. The model shows that the functional form of resource renewal within a patch is also im- portant for coexistence. These results contrast with theory that con- siders exploitation competition for a single resource when the re- source is assumed to be well mixed throughout the system. Keywords: exploitation competition, foraging, coexistence, invasion analysis. Understanding the mechanisms that allow species co- existence remains a key topic in community ecology. The mathematical model of Volterra (1926) was the first to suggest that the indefinite coexistence of more than one species on the same resource was impossible. This result * Present address: Population Biology Section, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The Netherlands; e-mail: sarichar@ duke.edu. Am. Nat. 2000. Vol. 155, pp. 266–279.  2000 by The University of Chicago. 0003-0147/2000/15502-0009$03.00. All rights reserved. was then expanded, and the competitive exclusion prin- ciple (Hardin 1960) was proposed, which stated that n species could not coexist on !n resources. In order to apply this principle, a clear way in which to identify distinct resources and consumers is required (Haigh and Maynard Smith 1972; Schoener 1974; Levins 1979), as well as a clear definition of coexistence (Koch 1974; Hsu et al. 1978; Armstrong and McGehee 1980; Smith and Waltman 1995). There have been numerous theoretical studies where mod- els have apparently violated the competitive exclusion principle. Competing species can coexist if mechanisms are present that effectively increase the number of re- sources present and/or allow populations to exhibit stable cyclic behavior. Examples include resource partitioning of the same prey item (Haigh and Maynard Smith 1972; Schoener 1974), interspecific variation during the re- source’s life cycle (Briggs 1993; Briggs et al. 1993), life- history variation among the consumers (McCann 1998), temporal fluctuations in environmental conditions (Koch 1974; Levins 1979; Turelli 1981; Abrams 1984; Chesson 1990), disturbance (Hastings 1980), interference compe- tition (Vance 1985; Fishman 1997), and spatial structuring of the habitat (Tilman 1994). In this article we investigate whether two species (or phenotypes) that differ in their strategies for resource ex- ploitation can coexist in a system where they both utilize the same resource. We assume the resource is distributed in a patchy but homogeneous environment; that is, all patches are intrinsically identical. The two species differ in the degree to which they deplete resources within a patch and hence differ in their feeding strategy. The species that depletes the resource to lower levels is termed the “digger” species; the other is termed the “grazer” species. An important assumption we make is that the two species interact only through exploitation competition (Milinski and Parker 1991). There is no direct interference between individuals, all interactions occur through each species’ influence on a shared food resource. Schmitt (1996) has studied an example of such a system, where two species