Searching for genetic evidence of demographic decline in an arctic seabird: beware of overlapping generations

Genetic data are useful for detecting sudden population declines in
species that are difficult to study in the field. Yet this indirect
approach has its own drawbacks, including population structure, mutation
patterns, and generation overlap. The ivory gull (Pagophila eburnea), a
long-lived Arctic seabird, is currently suffering from rapid alteration of
its primary habitat (i.e., sea ice), and dramatic climatic events
affecting reproduction and recruitment. However, ivory gulls live in
remote areas, and it is difficult to assess the population trend of the
species across its distribution. Here we present complementary
microsatellite- and SNP-based genetic analyses to test a recent bottleneck
genetic signal in ivory gulls over a large portion of their distribution.
With attention to the potential effects of population structure, mutation
patterns, and sample size, we found no significant signatures of
population decline worldwide. At a finer scale, we found a significant
bottleneck signal at one location in Canada. These results were compared
with predictions from simulations showing how generation time and
generation overlap can delay and reduce the bottleneck microsatellite
heterozygosity excess signal. The consistency of the results obtained with
independent methods strongly indicates that the species shows no genetic
evidence of an overall decline in population size. However, drawing
conclusions related to the species' population trends will require a
better understanding of the effect of age structure in long-lived species.
In addition, estimates of the effective global population size of ivory
gulls were surprisingly low (approximately 1000 ind.), suggesting that the
evolutionary potential of the species is not assured.