Demographic rescue and maintenance of population stability in grouse - beyond metapopulations
Martin, K., Stacey, P.B. & Braun, C.E. 1997: Demographic rescue and maintenance of population stability in grouse - beyond metapopulations. - Wildl. Biol. 3: 295.
Population managers usually focus on recruitment and survival of individuals within populations, with relatively little emphasis on demographic contributions from other populations. However, periodic 'rescue' of populations that are declining with recruitment from productive populations may be an important feature of population biology for many species. Four populations of white-tailed ptarmigan Lagopus leucurus were studied breeding in highly fragmented alpine habitats, with corresponding small population sizes, in the Rocky Mountains of North America from 1987 to 1996. The populations showed dramatic variation in young production and local survival (return) of adults over years and sites. Variation in reproductive success and survival were unlinked over sites and appear to be driven mainly by internal ecological processes such as depredation of eggs and young. Computer simulation models of individual populations predicted that all populations should become extinct within 2-10 years, assuming no linkage between them. Treating the four populations as different fragments of a single large closed population resulted in predicted persistence times of about nine years. Yet all populations have persisted for at least 30 years and likely substantially longer. In one site, the population of breeding pairs remained stable despite zero production the previous year and no return of females. Local populations of ptarmigan were able to avoid extinction due to external recruitment. However, almost all of the recruitment (particularly females) was from outside the population and possibly from outside nearby populations. Sites may vary temporally in predation risk as ptarmigan have generalist predators which live for several years and whose hunting skills are likely to improve with age. Excellent dispersal abilities allowed recruitment from populations during a productive episode to 'rescue' populations at risk of collapse if they were dependent solely on internal recruitment. Experiments on mate and territory replacement showed no apparent reproductive costs to females switching mates or territories, suggesting that dispersal to unfamiliar sites may not be costly. Ptarmigan have adapted to breeding in highly fragmented and stochastic alpine habitats by existing in a rescue effect metapopulation system functioning at a landscape scale. This pattern is well developed for ptarmigan that have low populations sizes in naturally fragmented habitats, but the pattern of demographic rescue may well apply to several other grouse species and, in fact, may apply generally to other taxonomic groups.
Key words: alpine habitat, demographic rescue, external recruitment, Lagopus leucurus, simulation model, white-tailed ptarmigan
Kathy Martin, Canadian Wildlife Service, RR 1, Delta, British Colombia, V4K 3N2, Canada
Peter B. Stacey, Program in Ecology, Evolution and Conservation Biology, University of Nevada, Reno, Nevada 89512, USA
Clait E. Braun, Colorado Division of Wildlife, Fort Collins, Colorado 80452, USA