Food Habits and Selective Foraging by the Texas River Cooter (Pseudemys texana) in Spring Lake, Hays County, Texas

Abstract

The fall-winter food habits of the Texas river cooter (Pseudemys texana) were investigated at Spring Lake, Hays County, Texas, by examination of stomach contents. Samples were collected from 33 live turtles by flushing stomach contents with water (Fields et al., 2000). Four aquatic macrophytes composed 91.6% of the diet. Aquatic macrophyte availability at Spring Lake was estimated using the Daubenmire technique (Daubenmire and Daubenmire, 1968) and compared to occurrence in the diet to assess selectivity in foraging. Cooters actively selected Carolina fanwort (Cabomba caroliniana), a plant low in fiber and high in total digestible nutrients. Hydrilla (Hydrilla verticilata), an invasive exotic aquatic plant, was an important food item due to the large amount present in the lake. The Texas river cooter, Pseudemys texana (Ward 1984), is a freshwater basking turtle of the family Emydidae. It is endemic to the Brazos, Colorado, Guadalupe, and San Antonio river basins of central and south Texas. Rivers, streams, and nearby ponds, especially those with abundant aquatic vegetation and basking sites, are preferred habitats ((??)Vermersch, 1992). It also is found in man-made impoundments, irrigation ditches, canals, and cattle tanks (Carr, 1952). Prior to Seidel and Smith (1986), some species of Pseudemys were included in the genus Chrysemys (Carr, 1952; McDowell, 1964; Zug, 1966; Weaver and Rose, 1967). In addition, P. texana was included within other species of Pseudemys, until it was recognized as a distinct species by Ward (1984). An early study of the food habits of P. texana (Strecker, 1927) found only mollusks (Sphaerium sp., Planorbis sp., and Lymnaea sp.) in the digestive tracts. Young P. texana have been observed to pursue insects, crayfish, and other invertebrates ((??)Vermersch, 1992). Other members of the genus are presumed to consume more invertebrates when young and become less carnivorous with maturation (Ernst and Barbour, 1989; Vermersch, 1992; Ernst et al., 1994). In general, P. texana is considered to be herbivorous when mature, although quantitative analysis of the diet is lacking. Analysis of stomach contents can provide informa- tion about which food items are palatable, nutritious, or abundant in the environment (Ward and Keith, 1962). Many herbivores have been shown to select plants based on their nutrient content (Gwynne and Bell, 1968). Consumption of a dietary component dispropor- tionate to its availability in the environment would suggest selectivity in foraging (Johnson, 1980). Al- though, the nutritional quality of plants is important to herbivores, important food plants also may be eaten in proportion to their availability without selection. Knowledge of food habits and foraging selectivity of P. texana is essential to the thorough understanding of how this species functions within its habitat and how it may influence the structure of the ecosystem. The objectives of this study were to describe the fall-winter food habits of P. texana at Spring Lake, Hays County, Texas; to determine if P. texana foraged selectively by comparing usage to availability in the environment; and to compare the nutritional quality of major components of the diet.