Pacific pond turtle (Clemmys marmorata) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

From historic accounts of the Pacific pond turtle, it appears that the major cause of population decline was extensive commercial harvesting of these turtles for food. In 1879, Lockington wrote that the turtle had become scarce in the San Francisco area, despite being formerly abundant, because of high commercial demand (cited in Carr 1952). Another author (True 1884 in Carr 1952) noted that the turtle was “still almost constantly for sale in the markets of San Francisco” in the 1880s, with prices increasing to $3 - $6 per dozen turtles in the 1920s (Storer and Carl 1944 in Carr 1952). Even before European settlement in western North America, the Pacific pond turtle was harvested for food by the natives of California (Ernst et al. 1994).

More recently, habitat modification and destruction have caused a significant decrease in the Pacific pond turtle’s distribution along the west coast of North America. More than 90% of wetland habitat has been eliminated from California due to agricultural development (Lovich and Meyer In Press). Extensive damming, agricultural development and urban sprawl are cited as the causes of pond turtle decline in Oregon and, specifically, the Willamette Valley (USACE website). Damming alters flow rates and water temperature in the river and floods shoreline habitat, while development near waterways eliminates crucial terrestrial nesting and overwintering sites. These factors also fragment the turtle populations and habitat, and create impassable barriers between important habitat components (Reese and Welsh 1998a,b). Turtles have also been frequently seen crossing roads in agricultural areas of California (Reese and Welsh 1997), which puts them at risk of substantial road mortality.

Finally, in Washington state, disease and predation on juveniles by exotic species have severely limited recruitment in the two remaining populations. The large American bullfrog, an eastern North American native, was introduced to the west coast and has become one of the major predators of juvenile Pacific pond turtle (Cook 1984; WPTP website). As mentioned in the section on growth and survivorship, long-lived species with low fecundity and low nest survivorship must compensate by having high juvenile survivorship in order to maintain a stable population (Reese and Welsh 1998b; Congdon et al. 1993). This is particularly crucial in a small population trying to recover from increased mortality by stochastic events, such as the 1990 outbreak of respiratory disease in the Washington populations. That outbreak killed approximately 25% of 150-200 total Pacific pond turtles in the Columbia Gorge, and out of 40 sick turtles that were treated by the Woodland Park Zoo in Seattle, only 13 survived (WPTP website).

Turtles at the northern end of their range often show differences in life history traits that limit population growth rates (Litzgus and Brooks 1998). Lower overall temperatures, and particularly a shorter summer, lead to delayed sexual maturity, reduce the number of clutches a female can produce in a year and prevent many eggs from hatching. While all examples of limiting factors and threats are from American populations of the Pacific pond turtle, the pressures from harvesting, habitat modification, road mortality and predation by bullfrog can all be expected to have had an impact on the British Columbia population of the pond turtle. As the northern populations of this species were probably smaller and less able to rebound from population decline, these threats and limiting factors must have been at least partly responsible for the extirpation of this species from Canada.

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