Effects of population bottlenecks on the South Island robin, Petroica australis australis (1999)
AuthorsByrne, Amanda J.show all
In New Zealand, birds and other endangered animals are routinely transferred to offshore islands for conservation purposes. Such transfers typically involve only a small number of individuals and thus the effects of a population bottleneck on the 'fitness' of these organisms is of great interest to conservation biologists. In 1973, two populations of the South Island robin, Petroica australis australis, were established on Motuara and Allports Islands from outbred populations on Nukuwaiata Island and Kaikoura, respectively. The effect of these population bottlenecks on fitness related traits in the South Island robin is the subject of this thesis. Reproductive success is an important measure of individual fitness. I measured hatch rate, number of fledged young, and in some instances, number of young that reach independence. The inbred robin population on Motuara Island was found to have lower clutch size, fewer clutches per season, and lower hatching success compared to mainland outbred populations. The study of fluctuating asymmetry has generated increased enthusiasm over the last few decades as a potential tool for examining the overall stability of a population. Numerous studies have found that individuals exhibiting a higher level of heterozygosity usually have lower fluctuating asymmetry. The results of my study provide no evidence to support this association. Thus, the application of fluctuating asymmetry measurements as an easily interpretable conservation tool may not be useful in all situations. There are two main theories on how bird song might evolve in island populations. First, founder effects may constrain song structure and lead to a reduction in song variation. Second, weaker selective pressures for species-specific signals may lead to an increase in song variation. Song variation in the inbred robin population on Motuara Island seems to support the predictions of the second hypothesis. Since the initial transfer, robins on Motuara Island have increased their song variability. Overall, the low hatching success obtained from the South Island robin population on Motuara Island shows that population bottlenecks have a detrimental effect on individual fitness in the robin. However, fluctuating asymmetry measures, which might provide a quick estimate of the 'health' of a population, offered no evidence of detrimental effects. Consequently, there is a need to examine more than one population parameter when drawing conclusions about the effects of population bottlenecks and inbreeding.