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UBC Theses and Dissertations
A Study of factors influencing a non-cyclic, island population of snowshoe hares Zimmerling, Todd
In this study I examined potential causes of the observed demographic differences between Jacquot Island and mainland snowshoe hare (Lepus americanus) populations, in the southwestern Yukon. The population dynamics of snowshoe hares on Jacquot Island have been observed to differ from the dynamics of a population undergoing a "10-year cycle," such as that on the adjacent mainland. Hare densities, predation rates, reproduction and juvenile survival were all monitored on both Jacquot Island and mainland study areas. Habitat on the study sites was also examined for cover and browse availability. Jacquot Island juveniles had survival rates 3-15 fold higher than mainland juveniles. The difference in leveret survival was attributed to the lack of small mammalian predators on the island. The Jacquot Island study grids had habitat with more dense under story than that of mainland areas. The density of cover at 10 cm above the ground was positively associated with juvenile survival, and may have also contributed to the observed differences in juvenile survival rates. Differences in predation pressure on adult hares were observed between the island and mainland. I examined the influence of stochastic predation on the Jacquot Island hare population through the use of simulation models. I found that stochastic predation alone could not account for the observed dynamics of the Jacquot Island population. Through the use of a simulation model I was also able to show that the Jacquot Island hare population would cycle in a manner similar to the mainland if it were exposed to predation which followed a Type II functional response and a 1-year-delayed density-dependent numerical response. This model produced cycles with a period of 9 years and a maximum hare density of 3.15 hares/ha. I concluded from this study that the observed differences in population dynamics between the mainland and Jacquot Island hare populations are caused in part by: 1) Differences in predation pressure between the mainland and Jacquot Island, where Jacquot Island experiences stochastic variation in predation pressure from year to year, and the mainland experiences delayed density-dependent predation pressure. 2) Differences in the numbers of predators of juvenile hares, where Jacquot Island has only red squirrels as a small mammalian predator, while the mainland has red squirrels, Arctic ground squirrels and weasels. 3) Differences in habitat, resulting in Jacquot Island having more dense "refuge habitat" than mainland areas.
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