DISTRIBUTION, COEXISTENCE, AND COMPETITION OF WHIRLIGIG BEETLES.

At present we know very little about the degree to which similar species, occurring together, may overlap in their utilization of a given array of resources. Larkin (1956) has suggested that this overlap may be great among freshwater organisms, particularly fish. MacArthur (1958) concluded that the opposite was true among birds of the north temperate zone. Restriction of ecological overlap would presumably arise from interspecific competition and it has been claimed that interspecific competition is a frequent consequence of the manner in which natural communities are organized (Hairston et al., 1960). Therefore, attempts to generalize about the diversity and relative abundance of species in a community or in a single trophic level might logically proceed from reasoning about how resources are apportioned through competition (MacArthur, 1957, 1960; Hairston, 1959). The effectiveness of this approach would be enhanced if definite statements about the relationship of interspecific competition and the division of resources among species could be derived from nature. The fact that some ecologists (Andrewartha and Birch, 1954; Thompson, 1956) have been unable to accept competition as an integral feature of community organization further indicates a need for specific field studies. The concept of interspecific competition has been derived in four, rather distinct ways: (1) theoretical investigations (Lotka, 1925, 1932; Volterra, 1926; Gause, 1934; Gause and Witt, 1935; Winsor, 1934; Hutchinson and Deevey, 1949); (2) laboratory experimentation (Gause, 1934; Gause et al., 1934; Park, 1948, 1954, 1957; Crombie, 1945, 1946; Birch, 1953; Utida, 1953; Frank, 1952, 1957; Mucibabic, 1957a, 1957b); (3) observation of local species distributions (Tansley, 1917; Broadhead, 1958; Williams, 1963); and (4) studies concerning the evolution of modern species (Lack, 1947; Bowman, 1961; Ross, 1962; Mayr, 1963). A clear understanding of what is meant by the term interspecific competition has been achieved as a result of the variety of endeavors just summarized, and Birch (1957) has composed a verbal definition. Birch correctly insisted on a strict meaning for the term and offered the following definition: "Competition between animals occurs when a number of animals (of the same or of different species) utilize common resources the supply of which is short. . . Interspecific competition then, refers to the interactions occurring between two or more species populations when they attempt to persist in the same place despite an overlap in their utilization of the same, limiting (insufficient) environmental resources. The ideas of "overlap" and "limiting" are crucial. However transient the periods of ecological overlap and resource limitation may be, they constitute competitive situations. A striking feature emerging from the whole gamut of studies involving interspecific competition is the elusiveness of this interaction in nature. It is very difficult to recognize and even more difficult to demonstrate competition. While theoretical 1 This paper is part of a thesis submitted to the University of Michigan for the degree of Doctor of Philosophy. The complete field and laboratory data are available in the thesis appendix or on microfilm from University Microfilms, Ann Arbor, Michigan. 2 Present address: Department of Biology, University of Rochester, Rochester, New York.