Demographic changes in a Scottish red deer population (Cervus elaphus L.) in response to sustained and heavy culling: an analysis of trends in deer populations of Creag Meagaidh National Nature Reserve 1986-2001.

摘要

From the time of its designation as a National Nature Reserve in 1986, declared policy at Creag Meagaidh NNR has been to reduce populations of sheep and native red deer to levels compatible with achieving the active regeneration of native birch woodland. Sheep grazing has been removed from the Reserve area, and deer populations subject to reduction through culling and live sale. Regular monitoring of both vegetation and deer populations has been maintained since the Reserve was established, with detailed records kept of number of deers (and sheep) recorded on the Reserve in regular counts (monthly since 1992). Records are also maintained of the number of deer culled, together with details of age, sex, weights and reproductive status. These data are examined in order to explore the effects of a sustained reduction of overall density on the demography and performance of a Highland red deer population. It is clear that despite a heavy and sustained culling pressure on red deer populations of Creag Meagaidh, the Reserve area still supports a significant population of animals. Although numbers on the Reserve initially showed a significant decline until the mid-1990s, observations of animals using the Reserve area have subsequently increased once more to levels approaching those of the mid-1980s. There is however marked (diurnal and) seasonal variation in number of animals recorded on the Reserve and it would appear that current usage is largely by transient individuals or relatively recent immigrants. While significant differences in carcase weight were observed between male and female calves, no differences across years were determined for calves, adult or yearling stags. A significant change in weights of adult hinds was however apparent through 1992 and 1993 resulting in a stepwise increase between weights of the periods 1988-1992 and 1994-2000. Consistent data on lactational status and pregnancy of hinds culled are available only from 1992. The overall proportions of milk and yeld hinds in the culled population has shown no significant change between 1994 and 2000. Although there is some suggestion of an increase in the overall proportion of milk hinds between 1992 and 1993 and the years after 1994 (milk hinds as proportion of all hinds shot) data are insufficient to test whether this is a statistically significant trend, or merely year-to-year variation. There is further no evidence of a consistent difference in rates of pregnancy between animals classified as lactating or yeld at time of death. Overall some 77.9% of yeld animals and 74.4% of milk animals were recorded with foetuses. Finally, over the period for which data are available (1992-2000) there is no evidence of a consistent change in time in the proportion of mature animals found pregnant and recruitment rates (as number of surviving calves counted in late winter per 100 hinds) have averaged at approximately 0.39 or 39 calves per 100 hinds. Given the efforts made to reduce population size and maintain deer at low density, such a lack of response in recorded body weights or fecundity is perhaps unexpected. However, accepting the composition of the Reserve's deer present population (reduced to a resident herd of minimal size and composed primarily of transients which are in practice spending only a relatively small part of their time visiting the Reserve), perhaps the lack of clarity in demographic traits is not unduly surprising. We are not documenting in practice changes in any residual, original population following some reduction of density, but rather in recent or even temporary immigrants. The implications of these conclusions for population management, and the effects of the sustained culling operations at Creag Meagaidh on the deer populations within neighbouring Estates are explored; some comparisons are made with preliminary results emerging from a similar exercise in reducing deer population density and browsing pressure at the RSPB's Abernethy Reserve.