Seasonal Population Changes and Habitat Preferences in the Genus Laccophilus (Coleoptera: Dytiscidae)

The aquatic beetles of the genus Laccophi/uis represent a specialized and highly successful group widely distributed over the earth. The genus is found in nearly every tropical or temperate region, and in some tropical areas as many as a dozen species may be found in the same or similar situations. Various workers have concluded on the basis of morphological characters of the adults and larvae that Laccophilus represents a relatively new and expanding group of beetles. If this is so, the species have not yet had time to complete the adaptation to diverse habitats, that is, to undergo the adaptive radiation seen in many aquatic groups. This diversification of habitat often lessens the effects of interspecific competition by separating various closely related forms. Four species of Laccophil/us are found in Indiana (Blatchley 1910). Three of these, L. miaculosus Say, L. fasciatus Aube, and L. proximus Say, are widely distributed over much of the eastern United States. They are more or less ubiquitous in habitat selection and occur, as adults at least, in almost any kind of aquatic situation. The fourth species, L. iandatus Aube, has been reported sporadically in Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, and Indiana. The 4 Indiana species frequently occur together in the same plond and often within a few inches of one another, persisting together over much of the year. The proximity of the species is even more remarkable in that they are aggressive predators which often betray cannibalistic tendencies and, if in aquaria, need to be kept under special conditions to hold down mortality (Hodgson 1953). The assumption is, therefore, that these species compete with one another to some extent and affect each other's existence. Gause (1934) in presenting experimental sup-port for the theoretical conclusions of Volterra (1931) and Lotka (1932) concludes that species competing for a limited supply of food can not persist together in the same microcosm. Thomas Park (1955) restates the concept in this way: "Using the Tribolium model we have again confirmed an assumption that species of relatively similar ecology, when living together as interspersed populations, create a situation inimical for survival of both." The complexity of natural situations prevent application of the types of models employed by these workers. Crombie (1947), also working with gramnivorous insects states, "Species occupying different ecological niches survived in the same medium: while of those occupying the same niche, one always was exterminated." The situation in Lacco philus, however, could be similar to that found by Lack (1945) in the cormorant and shag (Phalacrocorax sp.). He demonstrated that what was apparently a case of 2 morphologically very similar species (P. carbo and P. aristotelis) occupying the same niche and habitat was not the case at all. After close analysis it was shown that not only the food, but also the feeding areas and nesting sites, were different. There is evidence that utilization of different prey is not the sole factor in permitting 2 species to persist in the same habitat. Lack (1946), from Uttendorfer's lists of prey species of German predatory birds, points out that, while no 2 congeneric species compete for the same food supply, there were 5 species of Falconiformes and 5 species of Strigiformes preying on a single species of often very abundant vole. Also, Crowell and Crowell (1946) reported the displacement of Common and Roseate terns (Sterna hirundo and S. dougalli) by Herring gulls (Larus argentatus) front the AVeepecket Islands off Massachusetts. These workers thought that, although some predation (which is not competition) by gulls on terns was present, it was inability of the terns to reproduce successfully in the presence of the gulls that was of greatest consequence. Andrewartha and Birch (1954, p. 404) state that competition is the ". . . relationship between species which utilize the same food and space." They also emphasize that competition exists only when food or space is limited in relation to the numbers of organisms utilizing it or when one species harms the other by fighting or in some other way. In this study competition is defined as the condition that exists when species of Lac' Contribution No. 662 from the Zoological Laboratories ,of Indiana University. 2 This work, part of a doctoral thesis, was done while I was a graduate student in the Zoology Department, Indiana University, under a National Science Foundation Fellowship for the academic year 1956-57.