Why are caribou on Pic Island?
Date
1982
Authors
Ferguson, Steven H. (Steven Harold)
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Abstract
The population dynamics of a small relic caribou herd on Pic Island, Ontario (10 km²) was studied (1977-81) to determine the factors regulating numbers and limiting population growth. Four alternative hypotheses were tested to determine why caribou have survived on Pic Island and disappeared on the adjacent mainland: ( 1) food resources were inadequate on the mainland, (2) the mainland caribou contacted disease which Pic Island caribou did not, (3) the mainland caribou were more susceptible to predators than Pic Island caribou, (4) the mainland population dispersed, whereas the Pic population was undisturbed.
The population on Pic Island was estimated at 42 in 1977, 49 in 1578, 54 in 1979, 39 in 1980 and 28 in 1981. Calf production was consistent at 2O.O percent of the population in 1978, 23.3 percent in 1579, 24.2 percent in 1980, 22.0 percent in 1981. At least 78 percent of the females, two years and older, gave birth. The sex ratio was equal. Winter calf mortality was the mechanism of decreased recruitment and relative population stability. A die-off during the 1976-77 winter, caused by nutritional stress and wolf predation, eliminated 47 percent of the fall population. Emigration was assumed to have occurred off the island during the 1979-80 and 1980-81 winters.
Seasonal feed preferences were determined and estimates of the availability of these plants were used to compare Pic Island against the adjacent mainland. The mainland was found to provide a greater abundance of preferred food plants. The bulk of the evidence, although mostly circumstantial and therefore not conclusive, suggests that predation has limited and eliminated the caribou which have resided on the adjacent mainland and it is the lack of predators and the presence of escape habitat that has allowed the caribou to survive on the island.