Catastrophic Mortality and Its Population Consequences

Abstract

ADVERSE environmental conditions may be expected to affect the survival of surplus individuals to a greater extent than the well-established segment of a population (see Brown, Wilson Bull., 81: 293-329, 1969). This assumption may apply especially to a species with a spatially closed territorial system that forces reserve individuals into peripheral habitats or otherwise into a subordinate position. Situations contrary to this may be treated as unplanned natural experiments without introduced arbitrariness and may provide valuable additional information population processes. In my discussion of the dynamics of an insular population of the Song Sparrow, Melospiza melodia (Acta Zool. Fennica, 109: 3-73, 1964), I referred to a late spring snowstorm that eliminated one-third of the adult population leaving the young surplus relatively intact and was the indirect cause of a substantial population increase one year later. This increase followed a period of relative stability at a high population level. In reference to this Lack (Population studies of birds, Oxford, Oxford Univ. Press, 1966, pp. 135-137) argues that on the view that territorial behaviour operates like a partly compressible rubber disc, one would have expected the upper limit of numbers Mandarte to be nearly constant. This appeared to be so in the first three years of Tompa's study, when the number of breeding pairs was respectively about 47, 47 and 44,. . .but in the final season 69 pairs bred, an increase of 50 per cent, and concludes that territorial behavior was ineffectual in regulating breeding density. To evaluate Lack's argument I subjected to analysis hitherto unpublished material relating to movements, status, and fate of individual birds before and after the storm. These results, presented here, clarify some of the previously published inferences and illustrate other interesting aspects of the functioning of social systems (see also Brown, ibid., pp. 306-308). For background information the reader is referred to the general discussion of this population (Tompa, ibid). I have tried to avoid unwarranted repetition of facts and arguments. Although the island, 6.26 ha in extent, lying off the Pacific Coast of southern British Columbia, is generally free of snow during the winter, a heavy snowfall in early March 1962 covered the most exposed parts of the island with a thick wet snow layer for 4 days, and particularly the northeast half of the island where the elongated shrubbery zone con-