Population Structure and Growth of the Turtle Actinemys marmorata from the Klamath–Siskiyou Ecoregion: Age, Not Size, Matters

Determining the demographic structure of turtles is important to understanding their population status and conservation needs. Concern has been raised for the long-term persistence of the Western Pond Turtle (Actinemys marmorata) based on demographic analyses using size while ignoring age. Here, we compare the size versus age structures, and examine growth curves, for turtle populations from four sites in the Klamath–Siskiyou ecoregion of northern California and southern Oregon. We show that age structure does not correspond to size structure for two populations. Also, the most abundant of these populations had relatively few small turtles, which suggests inability of previous researchers to locate small turtles. Growth rates and adult size differed among populations, with turtles from two sites in the Coast Range significantly smaller and slower growing than turtles from either a reservoir on the eastern lower slopes of the Coast Range or the Klamath Basin east of the Cascade Mountains. Neither air temperature nor elevation explained the differences in size and growth rates. We hypothesize that larger body size and faster growth rates for some populations of A. marmorata may be due to high local productivity. We show that use of size alone gave an erroneous interpretation of population structure. Finding a few small-sized turtles in populations may not represent a lack of recruitment but, instead, a need to determine the proportion of young turtles based on their actual ages.

Figure 1

Location of study sites for Actinemys marmorata in the Coast Range of northern California and in the Klamath Basin along the California–Oregon border.

Figure 2

Comparison of mean monthly air temperatures for four sites in northern California and southernmost Oregon where Actinemys marmorata were captured. Upper: maximum daily values summed by month; Lower: minimum daily values summed by month. (Data taken from http://www.wrcc.dri.edu/summary/Climsmnca.html.)

Figure 3

Frequency distribution of carapace lengths (left) and ages (right) of Actinemys marmorata captured at four sites in northern California and southernmost Oregon. Black bars are males, striped bars are females, and open bars are turtles for which sex could not be determined. The number of old turtles (20+ yr) for Hell-to-Find Lake, Hayfork Creek, and Whiskeytown NRA were truncated to improve visibility of other ages; their numbers are shown to the side of the bars.

Figure 4

Growth curves (carapace lengths on ages) of Actinemys marmorata captured at four sites in northern California and southernmost Oregon, using the Richards (1959) growth model.

Figure 5

Mean (±95% confidence intervals) maximum and minimum air temperatures for the active season (May–September) of Actinemys marmorata at four sites where turtles were captured.

Figure 6

Relationship between primary growth (G, in years) of Actinemys marmorata and elevation (top) and mean minimum temperatures (bottom) for three sites in northern California (W = Whiskeytown, Ha = Hayfork, He = Hell-to-Find) and one in southernmost Oregon (K = Klamath).

Contributor Notes

¹ U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, Oregon 97331, e-mail: (RBB) buryb@usgs.gov. Send reprint requests to RBB.

² Department of Biology, California State University, Bakersfield, California 93311, e-mail: dgermano@csub.edu

Associate Editor: T. W. Reeder.

Received: 27 May 2008

Accepted: 10 Feb 2010