UBC Theses and Dissertations
Habitat specificity among ground squirrel populations at multiple spatial scales Werner, Jeffery Ross
The arctic ground squirrel (Urocitellus parryii; AGS) comprised 17% of the herbivore biomass in the Yukon boreal forest during the summer months from 1987 to 1996 and accounted for 23% of the energy flow at the herbivore level. By 2000 these ground squirrel populations had collapsed to nearly zero and today they comprise ~1% of the herbivore biomass in this zone. Most forest populations (~95%) are extirpated, whereas only 65% of low-elevation meadow populations are extirpated. AGS remain abundant in alpine (93% occupancy) and in human altered habitats (97% occupancy). During spring, postpartum females in forests weighed less and were in poorer condition than females in meadows. However, by onset of hibernation, forest squirrels had reached parity with meadow squirrels in mass and condition. For squirrels in formerly occupied boreal forests a) poor spring body condition likely decreased reproductive success, and b) achieving compensatory growth, via increased foraging, may result in higher predation risk. These costs likely contributed to the recent local extinctions of AGS in boreal forest habitat. Population densities of AGS and snowshoe hares were estimated over a 25-year period encompassing two snowshoe hare cycles. A hyperbolic curve best describes the per capita rate of increase of ground squirrels relative to their population size, with a single stable equilibrium and a lower critical threshold below which populations drift to extinction. The crossing of this unstable boundary resulted in the subsequent uncoupling of ground squirrel and hare abundance following their mutual declines in 1998. Sustained top-down control of AGS, via a Type II predator response, probably led to their local extinction. Fine scale habitat features of 138 AGS colonies were measured over a 25,000 km² region of the southern Yukon, 2013-2015. Logistic regression models were constructed to determine which food, soil, and visibility characteristics best explain colony persistence. Habitat characteristics associated with food, predation risk, and overwinter survival jointly influence the distribution of colony extinctions over large spatial scales. Evidence for multi-factor control of the distribution of this important northern herbivore at the southern portion of its range indicate that numerical responses to environmental change will not be uniform.
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