Western Painted Turtle (Chrysemys Picta Bellii)
- Assessment Summary
- Executive Summary
- COSEWIC History, Mandate, Membership and Definitions
- List of Figures
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status Designations
- Technical Summary
- Acknowledgements, Authorities Contacted, and Information Sources
- Biographical Summary of Report Writer and Collections Examined
Limiting Factors and Threats
Habitat loss, both of productive pond habitat and natural nesting habitat, is a primary threat to the Western Painted Turtle, at least in the British Columbia part of its range. Habitats that are not lost are often altered by habitat fragmentation, shoreline degradation, changes in hydrology, or water contamination, all of which will be discussed below. Habitat loss and degradation will rise in frequency and intensity as human populations continue to grow, and the link between habitat integrity and species’ persistence has been well documented (Gibbons et al. 2000).
Road mortality is suspected of contributing to widespread declines in turtles (10% annual mortality in adult turtles in eastern U.S.; Gibbs and Shriver 2002), particularly where roads are next to open water areas (Ashley and Robinson 1996). Gibbs and Shriver (2002) integrated road maps and traffic-volume data with simulated movements of small and large-bodied pond turtles and terrestrial turtles to model effects of roads on populations of pond turtles. Road density was strongly, linearly, positively correlated with road-crossing frequencies for all turtles. Roads bisecting movement routes to nesting sites threaten primarily females on nesting migrations and dispersing juveniles. In support of this, Steen and Gibbs (2004) observed male-biased sex ratios in wetlands associated with areas of high road density. In a radio-telemetry study by Baldwin et al. (2004), eight females made five road crossings in a single active season; two of these turtles (25%) were killed by vehicles. Road mortality has the potential to alter the structure of turtle populations because turtle life histories are characterized by low annual recruitment rates, high adult survival rates, and delayed sexual maturity (Steen and Gibbs 2004). Together, these features severely constrain the ability of turtle populations to cope with added adult mortality (e.g., Brooks et al. 1991; Congdon et al. 1993). Furthermore, roads have a negative impact on wetland quality and their influence can extend more than 100 m away (Forman and Deblinger 2000). Road construction can kill eggs and juveniles directly (Maltby 2000). Moreover, linear elements such as roads have a positive influence on the dispersal of many species, including mammalian predators of turtles and their nests, such as raccoons, skunks, coyotes and foxes, through the landscape (e.g., Merriam and Lanoue 1990; Spackman and Hughes 1995; Rosenberg 1998; Berggren et al. 2002). Roads surrounding wetlands may thus increase predation rates significantly, particularly for small slow-moving turtles and their relatively accessible eggs.
In the Canadian part of the Western Painted Turtle’s range, road density varies tremendously. Thus, roads are relatively sparse in northwestern Ontario and in parts of the species’ range in Saskatchewan and Manitoba. However in British Columbia, road densities can be quite high in the southern interior and the southwest mainland, where the best wetland habitats of this species occur.
In the absence of good nesting habitat connected to wetlands near human centres, grassy and sandy sites near buildings and fences can attract nesting turtles. Yet the nesting density and success at these disturbed sites can provide inferior growing conditions and even alter the sex ratio of populations (Kolbe and Janzen 2002a). Sandy and grassy patches in residential, commercial and industrial areas encourage this opportunistic nesting behaviour but can be population sinks (Kolbe and Janzen 2001). Roads also impinge on nesting habitat. Soils along their shoulders can be too compacted to allow nests to be successfully dug (Maltby 2000) and/or they can be overgrown with invasive plants possessing aggressive root systems that pierce the eggs and the bodies of hatchlings or physically impede their emergence in the spring (Maltby 2000; R. Clark pers. comm. 2004).
Disturbance of shallow wetlands by motorized vehicles and cattle can cause significant degradation of shoreline and riparian zones in open landscapes of the prairie, foothill and semi-desert rangelands. The removal of native plant cover, erosion of banks, soil compaction and water pollution (or eutrophication, if a wetland is overburdened with nitrogenous waste) that follow can all affect turtles, particularly hatchlings, juveniles and nesting females.
Habitat fragmentation by humans can cause rates of predation to rise (Marchand and Litvaitis 2004). Populations of some predators such as raccoons, coyotes, and foxes for example, increase as farms and suburbs replace forests (Oehler and Litvaitis 1996). Nests become more accessible along edges (Kolbe and Janzen 2002b).
PCBs, organochlorine pesticides, dioxins and furans can collect in the tissues of aquatic organisms like turtles. In the industrial region of southern Ontario, toxic residues in the liver and adipose tissue of snapping turtles (Chyledra serpentina) are commonly above human consumption guidelines (de Solla and Fernie 2004). These high levels bioaccumulate because snapping turtles live several decades, although there is some evidence that levels approach an asymptote when rate of accumulation equals rate of elimination (S. deSolla pers. comm. 2006). Given that Painted Turtles live at least as long as snapping turtles (Congdon et al. 2003; Samson 2003), we could expect adult Painted Turtles to also carry high levels of such residues where these chemicals are at high concentrations in the aquatic environment. High toxicity has the potential to influence individual as well as population fitness. In a study of Eastern Spiny Softshells (Apalone spinifera) in southern Ontario for example, de Solla et al. (2003) found that PCB and pesticide contaminant concentrations were positively correlated with egg viability, although there was no evidence for any negative effect of PCBs or pesticides upon hatching success. Crews et al. (1995) noted that PCBs can alter Painted Turtle population structure by reversing gonadal sex at otherwise male-producing temperatures. Although central and western Canada are not generally exposed to the same intensity of industrialization, pesticides, herbicides and chemical fertilizers are used throughout rangelands, large-scale farms, vineyards and orchards. Some wetlands may be more affected than others by toxic infusions depending on bathymetry, hydrology, and exposure to the chemicals (proximity and concentrations).
Humans can also affect Western Painted Turtles directly by harassing them at their basking or nesting sites, which is a common occurrence in popular recreation sites (Maltby 2000; R. Clark pers. comm. 2004), or by capturing turtles for pets (Orchard 1986; M. Machmer pers. comm. 2004). Sustainable use of long-lived turtles as pets can be problematic because their longevity is associated with low fecundity, delayed sexual maturity, and high adult survivorship: populations cannot remain stable (or grow) when adults and older juveniles are harvested at high rates (Congdon et al. 1993). The Eastern Box Turtle (Terrapene carolina) and Bog Turtle (Glyptemys muhlenbergii) for example, have declined significantly as a result of collection (Gibbons et al. 2000). Releasing pet turtles is equally harmful whether they are native species from a nearby pond or another location, or exotic species. Some introduced species, which can be common in urban areas, compete for food and space and are potential vectors for disease and parasites. Disease can also be a secondary expression of other environmental stressors associated with habitat degradation (Gibbons et al. 2000). Angling also contributes to turtle mortality; turtles have been found dead as a result of having the bottom of their jaws torn open by hooks, preventing them from feeding. (M. Sarell pers. comm. 2005).
Droughts and human-induced water-level fluctuations (reservoirs and dams) appear to cause high rates of mortality in turtles. In particular, shrinking wetlands have a reduced carrying capacity, and individuals seeking other wetlands are highly vulnerable to predation. For example, 18 dead Western Painted Turtles and 28 predator-destroyed nests were reported in a naturally dry channel bed of Kikomun Creek during a three-month study in 1986 (Macartney and Gregory 1986). Ray Buchner (pers. comm. 2004) found 50 to 60 Western Painted Turtle shells in Okanagan Mountain Park in 1985. Mike Sarrell (pers. comm. 2004) similarly found “lots of carcasses” at Post, Pillar and Island Lakes in the Okanagan in 1988, as well as some carcasses on Crooked Lake in 1987. Lindeman and Rabe (1990) reported a 70% decline in a C. p. bellii sub-population in Washington, at a small lake that dried up for two years from drought. There was evidence of emigration and mortality from increased overland migratory activity. The growth of individual turtles was markedly suppressed during the second year of drought. Although drought is an ephemeral phenomenon to an animal with a long lifespan, extended droughts can have significant impacts.
- Date Modified: