THE NATURE OF CYCLES IN POPULATIONS OF CANADIAN MAMMALS

Abstract

The number of pelts of the various fur animals collected each year since 1915, are examined critically to determine whether the peak or high collections are cyclic, or whether they could arise by chance in a series of random numbers as suggested by Cole. Cole's criterion of a peak is rejected as impractical and misleading. By splitting the country into 53 sections, it is shown that peaks do not occur at random as would be expected if these are chance occurrences. Nor do the peaks occur in the same year all over the country, but rather they are spread over three to four 'good' years which are followed by five or six 'bad' years, when no peaks occur in any section. Peak collections for the whole of Canada are the resultant of simultaneous high collections in most parts of the country. Rarely are all the sections of the country in phase; whenever a high-producing section gets out of phase it may produce a secondary peak, or even a split peak in the total for the whole country. It is shown that there is a relationship between the number of sections experiencing a peak, and the peak for the whole country. In general, the peak collection of most furs is reached first in the central provinces, then in the western, and finally in the eastern provinces. Data from annual questionnaires confirm this sequence. They show that the increase phase for snowshoe rabbit, mink, and fox begins as an island of increase in the prairie provinces surrounded by an area of decrease or no change. The reports of increase on subsequent years can be plotted by isophasal lines which spread out like waves from the original area of increase. The easiest interpretatation for these waves is migration from the areas where increase began into the more sparsely populated surrounding sections. These migrations would have a synchronizing effect on the population changes over the whole area. They would also give hybrid vigor to the local population by mixing gene pools which have been isolated for several generations. It is suggested that the causal agent of these cycles is the accumulation of favorable factors in good years, acting over most of the country on populations of different initial densities. Migration from the more favored areas will cause an upswing of the cycle over most of the range. Decrease, being largely density-dependent, will follow across the country in the same order.