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The effects of hunting and seral succession upon Vancouver Island black-tailed deer Smith, Ian Donaldson

Abstract

The role of seral succession and hunting in the regulation of Vancouver Island black-tailed deer populations (Odocoileus hemionus columbianus, Richardson) was studied from comparison of observed changes in the Northwest Bay herd over the period 1954-1966 with those predicted from a computer simulation of population processes of such a herd. Northwest Bay was chosen because there were accurate logging records since the first cut was made in 1939, and accurate kill records since hunters were first allowed into the area in appreciable numbers in 1954. Previous studies had shown that deer populations increase rapidly after fire and logging, but tend to return to former levels 15 to 20 years later. Indices of range condition were calculated by multiplying the number of square miles of land in certain seral stages by the number of deer per square mile expected to occupy each stage (as determined by previous studies). Over the period 1956-1966 the Northwest Bay area was in decline as deer habitat, while hunting pressure increased. Two independent indices suggested that deer numbers declined over this period, in association with a decline in condition (measured by weight in fall) of the younger age-classes of males. Average weights of the lower age-classes of females followed similar patterns, but the decreases in weight were not significant—perhaps because of inadequate samples. These changes supported the hypothesis that seral succession is the most important long-range determinant of deer numbers, but one expected change (the development of an older average age) was not found. The proportion of 1.5-year-old animals increased and decreased in cyclic fashion over the period 1954-1966, but no consistent trend towards an older or younger age-class structure was observed when the period 1954-1959 was compared with the period 1960-1966. The absence of such a change was attributed to the effects of hunting, which would be expected to produce a younger average age if significant numbers of animals were harvested, and thus counteract the effects of a deteriorating habitat. In the years of heaviest hunting mortality (1962 and 1963 ) hunters probably accounted for 15 per cent of the fall population as a whole, and over 20 per cent of the fall male population. These levels of exploitation apparently resulted in significant declines in numbers of animals after both of these years, which supported the hypothesis that hunting can serve as a regulatory mechanism to maintain numbers within the limits determined by seral succession. Further support for this viewpoint came from the fact that year-to-year fluctuations in numbers, which were apparently great in the period 1954-1962, appeared to be dampened over the last four years of the study—a result which was predicted by computer simulation. A second major direct cause of mortality over the period 1954-1966 was believed to be winter weather. Significant declines in both numbers of animals and condition of males after the 1955-56 winter, which according to temperature and snowfall records was more severe than normal. There is reason to believe that this winter also resulted in disproportionately-high mortality among female fawns and 1.5-year-olds, but sample sizes were too small to permit adequate assessment of this point. A second winter, 1964-65, which was believed to have affected many Vancouver Island deer herds adversely and which was more severe than normal according to temperature and snowfall indices, had negligible effects upon the Northwest Bay deer. This may have been because, by this time, numbers had been reduced by hunting to levels that the area could effectively support even in a severe winter. One paradox was found, however. Hunting patterns resulted in males being shot in proportionately greater numbers than females, and in latter years this difference was apparently great enough to reduce the proportion of adult males. However, no corresponding significant change in the pattern of differences in male and female age-class structure was observed. Computer simulation of Northwest Bay population processes (not including fawns during the first two months of life) indicated that natural mortality from accident, predation, disease and other miscellaneous causes excluding winter loss associated with malnutrition was approximately 10 per cent of the herd every six months. It was concluded that seral succession had been the indirect cause of the decline in numbers of Northwest Bay deer over the period 1954-1966, but that numbers of animals in any year during the period 1954-1961 were dependent upon the severity of the winter, while following this time year-to-year population levels were dependent primarily upon the effects of hunting.

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