Physical Structures and the Social Factor as Determinants of Habitat Use by Graptemys geographica in Southwestern Quebec

Abstract

Used and unused basking sites, food resources and nest site qualities of the turtle (Graptemys geographica) were quantified and compared within occupied bays and between occupied and unoccupied bays. Although the mean characteristics of used sites differed markedly from unused sites in the same bays, there were few differences between site characteristics in occupied and unoccupied bays. Discriminant function analysis of sites also indicated many suitable but unused sites in both occupied and unoccupied bays. Some food resources were greater in unoccupied bays, and typical quality nesting habitat was available in other bays. We concluded that the population was too small to occupy all available quality sites and hypothesize that social function was responsible for the distribution of turtles on used basking and nesting sites. INTRODUCTION Habitat selection of Graptemys geographica has received only incidental discussion in accounts of its natural history (Penn, 1950; Webb, 1961; Vogt, 1981). However, the general importance of basking sites to emydid turtles is well-documented (Cagle, 1950; Boyer, 1965; Waters, 1974; Auth, 1975; Obbard and Brooks, 1979; Miller; 1979). Basking sites have been described by Boyer (1965) and Bury et al. (1979) as stationary, well-separated from the shore, near deep water, in direct sunlight for much of the day, high above water, and affording a good view of the surrounding area. Nest sites are also important. Shoreline characteristics associated with nest sites in bays occupied by turtles are reported to include lightly sloped terrain with low-growing, sparse vegetation (Petokas and Alexander, 1980), exposure to full sunlight (Ehrenfeld, 1979), few roots (Burger and Montevecchi, 1975) and few but small rocks (Gordon and MacCulloch, 1980). It is not surprising that Heatwole (1977) suggested that the physical structure of the habitat serves as a key stimulus for habitat selection in several reptiles. However, in a study of the movements and spatial distribution of a population of Graptemys at the northeastern limit of its distribution in the Province of Quebec, Canada, the physical structure of the habitat did not seem to be the key stimulus for habitat selection. In this case, not all potential basking and nesting sites were used equally and there was an apparent tendency for clumping of animals in bays of the lake. To demonstrate this, we determined the characteristics of used basking sites, determined whether or not there were differences in basking sites in occupied and unoccupied bays, and measured food resources to judge whether they might have limited the use of bays. Finally, we characterized nest sites and determined the difference in valkes to random sites from unoccupied bays. Since there was a definite tendency of animals to clump in bays, we hypothesize that social factors might be more important than physical structure when the habitat is not saturated.