The Ecology of a Robust Population of Actinemys marmorata in the San Joaquin Desert of California
The Western Pond Turtle (Actinemys marmorata), the only native freshwater turtle in California, occurs in a variety of habitats from sea level to about 2040 m elevation, from mesic forests to deserts. The San Joaquin Desert in California once supported large populations of this species in lakes, sloughs, and marshes fed by water from the mountains of the Sierra Nevada. Because of damming in the mountains and agriculture on the desert floor, much of the aquatic habitat is gone. Although some biologists proffered that only non-viable populations of Western Pond Turtles occurred in the San Joaquin Valley south of the delta, I found a surprisingly robust population of this species at Goose Lake, an ephemeral freshwater lake on the desert floor in northwestern Kern County. From 1995 to 2006, I marked 737 individuals. Growth rates and reproduction were fairly high compared to other populations of Western Pond Turtles in ponded waters. The average yearly population estimate was 597.4 turtles with annual survivorship estimates of 0.81 for adult males, 0.73 for adult females, 0.84 for juveniles 80–119 mm carapace length (CL), and 0.73 for juveniles <80 mm CL. The estimate of λ denoted a stable population. Although the population occurs in a habitat controlled by an agricultural water district, permanent water is always available and the site is secure from poaching. Despite severe decreases in numbers of turtles in the San Joaquin Desert over the past 100 y, based on this study and other recent studies, there are several populations of Western Pond Turtles in the area that appear to be large and stable.
Aerial view of Goose Lake and surrounding infrastructure of the Semitropic Water Storage District where I trapped Western Pond Turtles (Actinemys marmorata) from 1995 to 2006. Water from the California Aqueduct flows in from the west in the Main Inlet Canal to the trifurcation (southern tip of Main Trapping Area) and flows along canals to the north, east, and south to be pumped up to surrounding agricultural fields. (Image from Google Earth, 2006.)
Relationship between body mass and carapace length of male (circles) and female (squares) Western Pond Turtles (Actinemys marmorata) at Goose Lake, California.
Distribution of carapace lengths and estimated ages of first captures of Western Pond Turtles (Actinemys marmorata) from Goose Lake, California, 1995–2006 (n = 737). Open bars are juveniles, black bars are females, gray bars are males.
Somatic growth curves created using the Richards model of male (circles) and female (squares) Western Pond Turtles (Actinemys marmorata) at Goose Lake, California based on carapace lengths. Dashed lines are 95% confidence intervals.
Comparison of somatic growth curves of Western Pond Turtles (Actinemys marmorata) (sexes pooled) at Goose Lake, California to ponds at Gorman, Vandenberg Air Force Base, Hell-To-Find, Fresno wastewater treatment plant, and Hanford wastewater treatment plant in California, and Rawlins and Yoncalla in Oregon.
Percentage of female Western Pond Turtles (Actinemys marmorata) that were gravid by month, carapace length (mm), and age (years) at Goose Lake, California. The numbers above bars are the sample size. The abbreviation E is early in the month (day 1–15) and L is late in the month (16–30, 31).
Relationship of clutch size to carapace length of female Western Pond Turtles (Actinemys marmorata) at Goose Lake, California. Dashed lines are the 95% confidence interval.
Contributor Notes
Associate Editor: J. D. Litzgus.