The western pond turtle
Compiled by Ryan Peek/Stillwater Sciences. Introduction by W. Sears/Stillwater Sciences
Sandi Matsumoto of The Nature Conservancy relocates flooded out juvenile western pond turtles along the Santa Clara River. Photo: EJ Remson / The Nature Conservancy
In the news: " Shellshocked Turtles Go Home". Los Angeles Times (via The Nature Conservancy). August 9,2005
The floods of January and February 2005 were by all accounts destructive, leaving levees weakened, farmlands scoured and flooded, ripping tens of acres of mature willow riparian forests from the river bed. The floods also dislodged numerous western pond turtles (Clemmys marmorata) from their perches and winter hiding places, giving many free rides all the way to the Pacific Ocean. Of course, the Santa Clara River has witnessed major floods for milennia (the 2005 floods were the second largest in about 100 years of record keeping), and so too have the turtles, presumably. After the floods, biologists rounded up the beached turtles and relocated them in suitable habitats along the Santa Clara River (see sidebar at left).
In California, western pond turtle (WPT) populations have experienced extensive population declines as conversion of wetland and riparian habitats to urban and agricultural use has accelerated (Jennings and Hayes 1994, Germano and Bury 2001). The most extensive extirpation of western pond turtle populations may be taking place in southern California and the San Joaquin Valley (USFWS 1992, Germano and Bury 2001). Local population trends in the Santa Clara River watershed are currently unknown, however it is likely that most turtles currently observed in the Santa Clara River mainstem are primarily "fossil" populations of old animals and immigrants from tributaries such as Sespe Creek (pers. comm., Dr. Sam Sweet, March 28, 2006). It is unlikely that there has been much western pond turtle reproduction in the mainstem Santa Clara River in recent years, due to a lack of suitable habitat and high densities of raccoons, however, western pond turtle populations appear to be stable in lower Sespe Creek and in parts of the Piru Creek system (pers. comm., Dr. Sam Sweet, March 28, 2006). Several known western pond turtle populations occur in the upper Santa Clara River watershed near Santa Clarita and in the vicinity of Piru Creek.
Mainstem channels of large rivers such as the Santa Clara are not generally expected to provide optimal habitat for WPT, since they are not especially strong swimmers and can be easily displaced downstream by floods (Ashton et al. 1997, Reese 1998, Germano and Bury 2001, pers. comm., B. Bury, USGS, 3/22/05). Observations from the Trinity River and other large rivers in the Pacific Northwest appear to support the hypothesis that there is an inverse relationship between river size (measured by stream order) and density of western pond turtles in mainstem habitats (Reese and Welsh 1998). More suitable aquatic habitats generally have standing (lentic) and slow-moving (lotic) water, which typically occurs in off-channel areas, such as side channels and backwater areas.
Life History and Timing
Although primarily an aquatic reptile, the western pond turtle needs terrestrial habitat for basking, overwintering, nesting, and traveling between ephemeral sources of water (Reese 1996). Breeding activity peaks in June and July, but may occur throughout the year (Holland 1994, Reese 1996). WPT are philopatric (tendency of a migrating animal to return to a specific location in order to breed or feed). WPT lay 1-14 eggs per clutch (Holland 1994, Reese 1996 and Stebbins 2003). The incubation period for WPT eggs averages 80 days, but in some cases may exceed 100 days in California (Bettelheim 2005). Incubating eggs are extremely sensitive to increased soil moisture, which can cause high mortality (Bettelheim 2005, Shaffer 2005, Ashton et al. 1997). In wet conditions, eggs can literally explode from internal pressure caused by water absorption (Ashton et al 1997). In colder climates, hatchlings may often overwinter in their nests, emerging in the following spring (Bettelheim 2005). In warmer climates, in southern and central California, many hatchlings tend to emerge from the nest in the early fall (Bettelheim 2005). Hatchlings spend much of their time in shallow water, within dense vegetation of submergent or short emergent macrophytes (Holland, pers. comm., as cited in Jennings and Hayes 1994). Hatchling and juvenile survivorship is considered to be low because they are prone to predation (Holland 1994). WPT in California reach sexual maturity in 7 to 11 years. Survivorship for adults is thought to be high (Jennings et al. 1992). The WPT has a potentially long lifespan; one recaptured individual is known to have survived at least 42 years in Trinity County (Jennings and Hayes 1994), although 25 years is generally considered to be the rough upper limit on age for most adults in natural settings (Bury 2005).
Western pond turtles have a widely variable home range, and although they may disperse overland due to environmental stressors such as droughts or floods, most movement is associated with normal movement within a terrestrial home range (Holland 1994, Reese 1996, Bettleheim 2005). In southern California, linear aquatic home ranges can vary between 32 and 4,263 m and total aquatic home ranges can vary between 294 to 7,284 mÂ² (Goodman and Stewart 2000, Bettleheim 2005).
Habitat Requirements and Associated Vegetation
The western pond turtle inhabits a wide range of fresh or brackish water habitats including ponds, lakes, ditches, perennially filled pools of intermittent streams, and backwater and low-flow areas of perennial streams and rivers (Jennings and Hayes 1994). A key requirement is proximity to potential nesting sites. Although some general nesting habitat parameters have been quantified, data are sparse. Females build nests between 10 to 12 cm deep, in dry clayey, loamy, or silty soils (Bettelheim 2005, Ashton 1997, Reese 1996, Holland 1994), on gentle (
Canopy cover in both riverine and off-channel habitats is thought to provide western pond turtles with protection from avian predators. Documented canopy associations for juveniles and adults include immature riparian vegetation (i.e., early seral stage willow scrub) and canopy cover levels ranging from those of un-vegetated gravel bars to those of mature, late-seral stage riparian vegetation (Reese 1996, Reese and Welsh 1998b).
Whereas adults and older juveniles are considered aquatic habitat generalists, hatchlings and young juveniles require specialized habitat for survival through their first few years. For example, in addition to requiring low-flow and backwater areas of rivers, hatchlings need to spend much of their time feeding in shallow water amongst dense submergent and short emergent vegetation (D. Holland, pers. comm., as cited in Jennings and Hayes 1994). Young WPT growth rates are thought to be closely tied to the abundance of food, particularly the concentration of zooplankton fauna in the water column (Jennings and Hayes 1994, Holland 1994). Juveniles prefer habitats similar to adults, but generally with lower water flow (Bettleheim 2005). Often these low flow habitats are scarce, and may be especially sensitive to anthropogenic and natural disturbances (Jennings et al. 1992).
Ecological Interactions With Non-Native Species
The introduction of non-native species can be detrimental to native species assemblages. Of particular concern are non-native red-ear slider turtles (Trachemys scripta elegans), which have been introduced throughout California, largely as a result of escape or release from pet owners (Bettelheim 2005). Red-ear sliders may compete directly with native WPT for basking habitat, food, and nesting habitat (Spinks 2003, Reese 1996, and Holland 1994). Studies completed by Spinks et. al. observed significant reduction in WPT use of optimal habitat when red-ear sliders were present (Spinks 2003). Moreover, red-ear sliders are a vector for an unidentified upper respiratory disease (URD), which can be fatal for native WPT (Holland 1994). Both turtle species favor lentic waters and have similar diets, as well as sharing the need for aerial basking as a component of metabolism (Campbell 2005). Other species may have more indirect effects on WPT habitat and food resources. Introduced centrarchids may compete with hatchlings for zooplankton and other invertebrate prey items.
In general it is expected that WPT populations may be dispersed and distributed according pressure from predators, as well as physical habitat conditions. A number of species may prey on one or more WPT life stage. Raccoons can prey on WPT during all life stages, and have been observed in higher densities in areas WPT populations were found (Germano 2005). A case in point comes from studies in the San Simeon area of coastal California, in which fewer WPT were observed when raccoon numbers were high. Raccoons are an important predator of WPT and are known to prey on adults as well as juveniles (pers. comm., D. Germano, CSU Bakersfield, 2/16/05). Other possible predators include largemouth bass and bullfrogs, which would be expected to target hatchlings in particular, and are found in increasing numbers throughout California (Bettelheim 2005). Bullfrogs have been observed feeding on both hatchlings and juveniles (Holland 1994, Moyle 1973).
Sensitivity to Anthropogenic Watershed Disturbances
Reductions in the availability of quality nesting habitat due to channel alterations and urban or agricultural encroachment may have significant impacts on WPT populations along the Santa Clara River. WPT may resort to using semi-suitable nesting habitat in agricultural areas adjacent to the river corridor, however frequent re-working of agricultural fields can cause direct mortality to eggs, and there is an increased likelihood of egg mortality from inundation (via irrigation) or predation. Abnormally high raccoon populations (which have been linked with fragmentation of habitat, facultative supplemental feeding from garbage, and increased edge habitat) may severely limit WPT recruitment if they encounter WPT nesting areas (Holland 1994 and pers. comm., D. Holland 2005). Raccoons have been found to feed on all life stages of WPT, but nests and hatchlings are particularly vulnerable (Reese 1996, Holland 1994).
- What is the extent of WPT nesting along the Santa Clara River mainstem?
- How do non-native species affect the availability of nesting habitat, food, and basking habitat for WPT in the watershed?
- What are the dispersal dynamics for the WPT in the Santa Clara River system?
- What are the microhabitat preferences for nesting locations in the Santa Clara River system (e.g. vegetation affects on soil temperatures, protection from predation, soil water content)?
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