Vancouver Island COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

The major immediate threat to Vancouver Island marmots is predation, which is also believed to be the proximate cause of recent population declines (Bryant and Page 2005). Count and mark-recapture results suggested that annual survival rates have declined since the 1980s (Figure 7). Losses at individual colonies were often dramatic within single years, and colony-specific survival rates were spatially correlated (Bryant 2000.

Figure 7. Changes in marmot survival in the Nanaimo Lakes region over time. Data are means (SE) and reflect colonies in natural meadows only. Numbers of marmot-years for both tagged and untagged individuals are shown. Transplanted or captive-released marmots are excluded. (A. Bryant unpublished data)

Figure 7. Changes in marmot survival in the Nanaimo Lakes region over time. Data are means (SE) and reflect colonies in natural meadows only. Numbers of marmot-years for both tagged and untagged individuals are shown. Transplanted or captive-released marmots are excluded. (A. Bryant unpublished data)

Indices of cougar and wolf abundance on Vancouver Island have increased dramatically since the early 1980s (unpublished data, B.C. Ministry of Environment, Archibald et al. 1991), perhaps as a numerical response to deer populations (Bunnell 1990, Bryant and Page 2005). Increasing predation on marmots may also reflect a functional response by predators. Predators may have hunted more often, or more successfully, in natural habitats surrounded by clearcuts that were also occupied by marmots (Bryant 1998).

The impact of disease on marmots is poorly understood but potentially important. The loss of established colonies from North Hooper (1982-83) and Gemini Peak (1986-87), combined with observed population crashes at Haley Lake and Butler Peak suggests episodic mortality events. These events were consistent with a hypothesis of localized predation, but could also reflect disease (Bryant 2000).

Although Vancouver Island marmots harbour a variety of parasites including fleas, roundworms, tapeworms, ear-mites and coccidia, there is no evidence that parasites exert a significant population effect. Four of six animals relocated from different colonies to a vacant historic location in 1996 died during hibernation, possibly from bacterial infection (Bryant et al. 2002). Chronic mange has been diagnosed at Mt. Washington.

Inbreeding could depress reproductive or survival rates. It is unclear to what extent Vancouver Island marmots may be vulnerable to inbreeding depression or genetic drift. Because populations have presumably been low for many generations, deleterious genetic loads might have been purged (Janz et al. 2000). There is no evidence that inbreeding depression has reduced reproductive rates (Bryant 2005), although one pair of wild-caught captive marmots produced two pups from two separate litters with congenital defects (M. McAdie pers. com.).

Vancouver Island marmots are obligate hibernators and require specific microclimatic conditions. Climate change might influence hibernation timing (Inouye et al. 2000) and the survival of hibernating marmots might reflect snow conditions (Van Vuren and Armitage 1991, Barash 1989). The vulnerability of marmots to predation might increase if they remain active later in the fall or emerge earlier in the spring. Radio-telemetry data indicate no significant change in hibernation timing since the first records in 1992 (Bryant and McAdie 2003).

The extent and distribution of Vancouver Island marmot habitat is apparently limited by climatic conditions (Milko 1984, Milko and Bell 1986). Climate change could alter marmot habitat. The mountains of western North America were characterized by moving tree-lines over the past 10,000 years in response to climate (Rochefort et al. 1994). Recent analysis of pollen collected from natural sub-alpine marmot habitats is consistent with the hypothesis that marmot habitats were much more extensive in the recent prehistoric past (Hebda et al. 2005). These authors also conclude that global warming might increase the availability of natural habitat. Such long-term climatic changes presumably explain some of the palaeontological and archaeological records of marmots found well outside of their historical extent of occurrence (Calvert and Crockford 1983, Nagorsen et al. 1996).

The availability of marmot habitat may also have been influenced by naturally occurring vegetation changes over historical periods (i.e., 1900 to present). Within the past 100 years a warmer and drier climate has resulted in an invasion of subalpine meadows by trees in most of the western mountains including the Cascades, Olympics, and southern Coast Mountains. Using tree-ring measurements at historic and extant marmot colonies, Laroque (1998) found that at least two sites (Gemini Peak and Green Mountain) experienced tree invasions over a period spanning only a few decades. In contrast, in Strathcona Provincial Park, where marmots apparently disappeared 20-40 years ago, most trees above 1000 m elevation are more than 800 years old, providing little evidence of recent tree invasion despite close overlap of tree-ring sites with marmot records (e.g., Cruikshank Canyon, Circlet Lake, Greig Ridge, Philips Ridge: C. Laroque, University of Victoria, pers. comm.).

The role of fire in creating or maintaining marmot habitat is unclear but this process is likely important at some sites. Fire intervals are relatively short (<300 years) on southeastern Vancouver Island but substantially larger (700 - 3000 years) in western and central regions (Laroque pers. comm; Lertzman et al. 1998).

Post-logging succession in clearcuts has had a pronounced effect on marmot colonies. All 10 colonies in clearcuts described by Bryant (1996, 1998) became extinct by 2000. The longest colony persistence in a clearcut was 19 years (1981-2000; Road K44a). The median longevity of colonies in clearcuts was 10 years (range = 5-19 years), or 2-3 marmot generations. Bryant (1996, 1998) found some evidence that survival in man-made habitats was slightly lower than in natural sub-alpine meadow habitats. It seems likely that clearcuts were population "sinks" because forest succession quickly made habitat conditions unsuitable. Of the 10 clearcuts colonized by marmots, six populations went extinct after the last solitary marmot or few surviving marmots were "rescued" for transplant or captive-breeding purposes (Sherk Lake, Pat Lake, Mt. Franklin, Road D13e, K44a, Butler Peak "west roads").

The possible influence of endocrine-disrupting pollutants has been addressed (Lichota et al. 2004). Current exposure levels appear unlikely to pose a health risk. Published evidence (Bryant 1996, 1998, 2005) indicates that reproductive rates in Vancouever Island marmots have not declined over time, and are comparable to those of other alpine-dwelling marmot species. There is no evidence of diminished reproductive capacity. However there is strong evidence (Bryant 1998, 2000, Bryant and Page 2005) that survival rates declined from those observed during the 1980s.

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