MORPHOEDAPHIC AND BIOGEOGRAPHIC ANALYSIS OF CRAYFISH DISTRIBUTION IN NORTHERN WISCONSIN

Abstract

We surveyed crayfish distribution and abundance among 67 lakes in the Vilas County area of northern Wisconsin, and then used multiple regression to analyze the relationship between our results and various physical, chemical, and biological variables. The region is dominated by Orconectes rusticus, 0. propinquus, and 0. virilis, but Cambarus diogenes, Procambarus acutus, and 0. immunis are also present; only 0. virilis was previously documented from the area. No single variable or combination of variables fully explained variations in crayfish distributions. However, substrate, calcium level, geographic isolation as related to colonization opportunity, and lake size appear to be of general importance. Continuing introductions and competitive exclusion are also probably significant, resulting in a dynamic situation involving an interaction of multiple controlling factors. Although crayfish (Decapoda, Cambaridae) are a common component of lake and stream communities throughout most of the United States, relatively little is known about the mechanisms that determine species composition and abundance. In many areas species composition has changed significantly during the past few decades (Penn and Fitzpatrick, 1963; Schwartz et al., 1963; Hobbs and Walton, 1966; Crocker and Barr, 1968; Capelli, 1982). Physical-chemical factors, competition for food, interspecific aggressive interactions, and predation have been suggested as important (Bovbjerg, 1952, 1970; Eberly, 1960; Penn and Fitzpatrick, 1963; Stein, 1976; Berrill, 1978). Introductions by fishermen using crayfish as bait may be responsible for initiating many such changes (Crocker and Barr, 1968; Berrill, 1978). The general question of what determines species composition (both numbers and kind) in a community is central to ecology. During the past 20 years much of the theoretical work on determinants of species composition has been developed from the study of islands (MacArthur and Wilson, 1963, 1967; Rusterholtz and Howe, 1979). Barbour and Brown (1974) and Magnuson (1976) suggested that island biogeographic theories may apply in some degree to lakes, which like islands tend to be small, geologically recent, and isolated from other similar habitats. In this study we examined the relationship of crayfish species composition and abundance to physical and chemical factors, abundance of other crayfish species, and biogeographic factors, such as lake size, geographic isolation, and human use, in 67 lakes in the Vilas County area of northern Wisconsin. These waters exhibit a great range of physical, chemical, and biological characteristics, and are located in three major drainage systems: the Lake Superior system flowing north, the Flambeau-Chippewa River system flowing southwest to the Mississippi River, and the Wisconsin River system flowing south, also to the Mississippi (Fig. 1). Because the glacial drift of the area is low in calcium and other soluble materials (Black et al., 1963), lakes generally have low alkalinity and low to moderate productivity. The majority are small seepage lakes of kettle origin, with no inlet or outlet, but numerous drainage lakes also occur. Drainage lakes tend to have higher levels of calcium and other dissolved materials, as well as higher overall productivity. Drainage lakes also provide better colonization opportunities. The major crayfish species in the area are restricted continuously to water and have no obvious natural way of colonizing a seepage lake.