Reintroducing endangered Vancouver Island marmots: Survival and cause-specific mortality rates of captive-born versus wild-born individuals
Introduction
Understanding causes of population decline is an important first step towards recovery of endangered species (Caughley, 1994, Caughley and Gunn, 1996). Change in population size is the consequence of changes in one or more vital demographic rates (Caswell, 2001, Mills, 2007, Oli and Armitage, 2004); effects of the environment and management actions on the dynamics of populations are mediated through their influences on demographic variables. Furthermore, reduction in survival can lead to decline of wildlife populations whose growth rate is very sensitive to changes in survival rates (Heppell et al., 2000, Oli and Dobson, 2003, Stahl and Oli, 2006). Therefore, rigorous estimates of survival and knowledge about how various factors influence this key demographic rate are necessary for formulation of recovery plans and conservation of species at risk (Le Gouar et al., 2008).
The Vancouver Island marmot (Marmota vancouverensis) is endemic to Vancouver Island, British Columbia, Canada. Concern brought on by the restricted geographic distribution and low numbers, led to M. vancouverensis being listed as endangered in 1978 (Bryant and Page, 2005, Shank, 1999). Systematic field surveys suggest that the marmot population increased during the early 1980s to a peak of 300–350 individuals during the mid 1980s (Bryant and Janz, 1996). However, by 2004 the population had declined to approximately 35 individuals in the wild (Bryant, 2005). Landscape changes due to clearcut logging and increases in predation-related mortality are thought to have contributed to the population decline (Bryant and Page, 2005).
Significant conservation accomplishments towards recovery of M. vancouverensis have been made through planned captive-breeding and release programs (Janz et al., 2000). Captive-breeding facilities were established both on and off the island beginning in 1997, and captive-bred marmots have been released into their natural habitat since 2003 (Bryant, 2007). There have been both reintroductions of marmots into empty but historically occupied sites and re-enforcement of existing colonies with additional marmots. During 2003–2007, 96 marmots were released and monitored using radio-telemetry. Arguably, the most important conservation measure currently in place is the release of captive-bred marmots into their natural habitat. Although re-introduction programs have been successful in some cases, limitations of captive-breeding and re-introduction programs for species conservation are well documented (e.g., Caughley and Gunn, 1996, Snyder et al., 1996, Moorhouse et al., 2009). Success of such programs depends to a large extent on the ability of released animals to survive and reproduce in their natural habitats.
Often, released animals have a lower probability of survival in the wild than their wild-born counterparts, reflecting some cost associated with being raised or held in captivity (Beck et al., 1994, Mathews et al., 2005, Sarrazin and Legendre, 2000, Snyder et al., 1996). Any additional mortality may only apply for a period immediately post-release after which survival of released individuals increases to a rate near that of wild-born individuals (Bar-David et al., 2005, Sarrazin et al., 1994, Maran et al., 2009). Age at which animals are released can also influence their subsequent survival and the success of a reintroduction program (Green et al., 2005, Le Gouar et al., 2008, Sarrazin and Legendre, 2000). Understanding differences between captive-born and wild-born marmots in survival and causes of death can provide information to potentially improve success of the recovery program. However, whether and to what extent survival and causes of mortality of captive-born marmots differs from those of wild-born marmots, or if age at release influences survival of captive-born marmots, remained unknown until this study.
To provide demographic information needed for effective recovery of M. vancouverensis, Bryant and Page (2005) analyzed radio-telemetry data collected through 2004. These authors provided estimates of survival and also examined causes of mortality and seasonal variations in survival. Field study has continued since this work was completed, and many more marmots have been monitored via radio-telemetry. Consequently, sample sizes and our ability to test hypotheses regarding factors influencing survival of marmots have increased substantially in recent years.
Our goals were to build upon previous work, provide more rigorous estimates of survival, and test hypotheses about factors influencing survival of marmots using radio-telemetry data. Using a subset of radio-telemetry data collected since the first releases of captive-born marmots in 2003, we also tested the hypotheses that wild-born marmots would have a higher probability of survival than their captive-born counterparts, and that captive-born marmots released at an older age would survive better than those released as yearlings.
Predation is the most important cause of mortality of wild Vancouver Island marmots (Bryant and Page, 2005). Given that captive-born marmots are not exposed to predators during early, possibly important developmental stages as wild-born marmots are, their behavioral responses to the risk of predation may differ (Blumstein et al., 2001). Alternatively, their ability to escape various predators may differ due to disparities in experience or body condition. Thus, we also estimated cause-specific mortality rates, and tested for differences in mortality rates between captive-born and wild-born marmots for each cause.
Pups were seldom implanted with radio-transmitters. Consequently, radio-telemetry data were sparse and it was not possible to estimate survival rates for this age-class. We combined the radio-telemetry and long-term capture-mark-resighting (CMR) data and analyzed the combined dataset within a multi-state CMR modeling framework (Williams et al., 2002). Combining the data from two sources allowed us to estimate age-specific survival rates, and to test hypotheses regarding the influence of age, sex, habitat type, and geographic location on survival of marmots.
Section snippets
Study species
The distribution of M. vancouverensis is restricted to the interior mountainous zones of Vancouver Island (Nagorsen, 1987). Marmots live in open, subalpine meadows characterized by colluvial soils, diverse vegetation, and lookout spots (Heard, 1977, Milko and Bell, 1986). These small patches of suitable habitat were historically interspersed within much larger areas of dense old-growth forest, creating a pronounced metapopulation structure (Bryant, 1998).
Vancouver Island marmots are generalist
Analysis of radio-telemetry data
One hundred and thirty-two marmots were radio-tracked during 1992–2007 for a total of 367 marmot-years. The number of marmots radio-tracked varied over the study period at each site (Fig. 2). There were 92 marmot-years of radio-tracking recorded for yearlings, 89 marmot-years for 2-year-olds, and 186 marmot-years for adults (Table 1).
Although annual survival of females (S = 0.750; 95% CI = 0.654–0.826) was slightly higher than that of males (S = 0.675; 95% CI = 0.584–0.754), there was no evidence of a
Discussion
Low numbers, restricted and sparse habitat, small geographic distribution, and anthropogenic influences on marmot habitat continue to cause concern regarding the persistence of Vancouver Island marmots. Given the extremely small population size, the marmot population is at substantial risk of extinction due to demographic stochasticity alone (Caswell, 2001, Caughley and Gunn, 1996, Morris et al., 2002). This risk is exacerbated by the harsh and dynamic environment M. vancouverensis inhabits,
Acknowledgements
Project funding was provided in various years by the BC Habitat Conservation Trust Fund, BC Ministry of Environment, World Wildlife Fund (Canada), Forest Alliance of BC, Forest Renewal BC, Cowichan Valley Field Naturalists Society, Nanaimo Field Naturalists, BC Wildlife Federation, BC Hydro Bridge Coastal Fish and Wildlife Restoration Program, TimberWest Forests, MacMillan Bloedel Limited, Environment Canada (administered through the University of British Columbia), and the Marmot Recovery
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