Ecology of the western pond turtle in the Mojave River

The western pond turtle Clemmys marmorata is protected by the State of California and recent surveys have confirmed that many populations have been extirpated or are declining, particularly in southern California. Threats to the continued survival of the species include habitat modification/destruction, grazing, commercial harvest, vehicle-related mortalities, and competition from exotic species. Over 90 percent of the wetland habitats within the historic range of the species in California have been eliminated due to agricultural development, flood control and water diversion projects, and urbanization.

Figure 1. The western pond turtle occurs along the west coast of North America from southern Washington, USA to northern Baja California, Mexico. Many populations have been extirpated and others continue to decline throughout the range, especially in southern California.

Little is known of the ecology of C. marmorata relative to other species of North American turtles, and nothing has been published on the ecology of relict, morphologically and genetically distinct populations in the Mojave River of California. A significant portion of the historical range of the turtle in the Mojave River occurs at the Afton Canyon Area of Critical Environmental Concern (ACEC) administered by the Bureau of Land Management (BLM). Populations in this area and nearby Camp Cady, administered by the California Department of Fish and Game, are small and tenuous. Much of the Mojave River flood plain has been infested with the exotic pest plant saltcedar (Tamarix ramosissima), replacing an estimated 70 percent of the native riparian vegetation in the ACEC. The changes in channel morphology and hydrology associated with saltcedar invasion in the Mojave River have degraded what little western pond turtle habitat exists in this arid region.

Figure 2. Jeff examines a western pond turtle after placing a radio transmitter on the animal.

	The BLM is the lead agency in the preparation of the West Mojave Coordinated Management Plan, an ambitious effort aimed at protecting biodiversity in the increasingly urban western Mojave Desert. The western pond turtle is one of many species for which data are critically needed to ensure that they are adequately protected under the developing Plan.
Figure 3. There is very little suitable habitat for an aquatic turtle in the Mojave Desert. Only a few places along the Mojave River support populations of the western pond turtle.

The objectives of our study are as follows: 1) to determine the distribution and status of populations in the West Mojave Planning Area (WMPA); 2) to elucidate the demographic structure (adult sex ratios, size- age-class distributions, and population density/biomass) of C. marmorata in the WMPA; 3) to determine clutch size and clutch frequency of females; and 4) to examine seasonal changes in habitat use in this uncharacteristically harsh portion of the western pond turtle's range.

Figure 4. Kathie removes a couple of western pond turtles from a trap.

Preliminary data show males have an average carapace length of 13.1 cm, while females are slightly larger at 14.5 cm. Average weight of adult males and females is 400 and 525 g, respectively. The adult sex ratio at two study sites (n=34) is biased at 1.3 males per female. Egg laying occurs in late May and early June. Clutch size for 5 gravid females ranged from 4-6 eggs with an average of 4.8. Several animals were equipped with radio transmitters to monitor movements. Small movements between adjacent ponds and wetlands have been observed on several occasions. Support for this research has been generously provided by the California Desert District and the Barstow Resource Area of the BLM, and the California Department of Fish and Game.

PRODUCTS Lovich, J. and K. Meyer. 2002. The western pond turtle (Clemmys marmorata) in the Mojave River, California, USA: highly adapted survivor or tenuous relict? Journal of Zoology, London 256: 537-545.