Genetic Dissection of Anthropogenic Impacts on a Declining Antarctic Fur Seal Population

Anthropogenic pressures such as climate change are reducing the population size of many species globally. Small populations are more vulnerable to extinction, not only because they are more susceptible to demographic and environmental stochasticity but also because the resulting decrease in genetic diversity can reduce their adaptive potential. However, many questions remain as to how demography and genetic diversity are linked and how genetic factors contribute to extinction. Another challenge that small populations face is that they are more susceptible to inbreeding which in turn can reduce fitness, a phenomenon known as inbreeding depression. Inbreeding depression affects numerous key fitness traits in captive populations. However, due to the difficulty of studying inbreeding in the wild, many questions remain about the relevance of inbreeding depression for species viability and the dependence of inbreeding depression on life-history and environmental factors. <br /><br /> Pinnipeds are well suited to exploring these questions. Many pinniped species were commercially exploited by 18th to early 20th century sealers, bringing some species to the brink of extinction, whereas others had barely any contact with humans. Although many populations recovered after sealing was banned in the last century, nowadays they are facing a new threat: global climate change. Pinnipeds are particularly vulnerable to environmental changes as they are long lived species that produce relatively few offspring, and populations of many pinniped species are currently declining as a result of climate change. One of these species is the Antarctic fur seal. The Antarctic fur seal was heavily hunted by sealers in the 18th and 19th century after which the species recovered from a handful of individuals to a global population of 3.5 million. Nowadays, the species is declining again due to a reduction in local food availability caused by climate change. <br /><br /> In this dissertation, I investigated how anthropogenic pressures such as commercial exploitation and climate change affect genetic diversity and what this means for individual fitness and species persistence in the Antarctic fur seal. I started by exploring the effect of contemporary bottlenecks caused by commercial exploitation on genetic diversity. Using both a comparative approach as well as a coalescent simulation method, I found that bottlenecks need to be very strong (effective population size below 50) and last for multiple generations (> 5-10 generations) to cause a severe reduction in genetic diversity. <br /><br /> Next, I elucidated how the Antarctic fur seal recovered from intense commercial sealing. Historical accounts suggest that only few individuals remained after sealing and that recolonization was driven by migrants from the largest colony at South Georgia. I used genetic samples from eight major breeding sites across the Antarctic fur seal's global distribution to elucidate population structure. I found evidence for four genetically distinct populations and one admixed population, suggesting that at least four remnant populations survived sealing. Furthermore, the presence of distinct genetic populations suggests that migration is limited. I confirmed this by analyzing the global occurrence of a polymorphism encoding for cream coat coloration, which was found to be limited to populations at South Georgia, the South Shetlands and Bouvetøya. This suggests that migration occurs locally but not on a larger scale, which contributes to the consensus that multiple small relict populations survived sealing.<br /><br /> I then investigated inbreeding depression in the Antarctic fur seal. The adult female population is declining and in parallel becoming more heterozygous, which suggests that selection against inbred individuals is increasing and occurs before recruitment. This leaves two, non-exclusive possibilities: selection acts during the pup life-history stage or the juvenile life-history stage. Studies of other pinnipeds have shown that the highest mortality occurs in the juvenile life-history stage after weaning, suggesting this is the most vulnerable period in many species and that selection may act more strongly in this life-history stage. Because Antarctic fur seal juveniles spend most of their time at sea, it is difficult to study this period. Pups, however, remain ashore until weaning and are therefore easier to study. I investigated whether inbreeding had an effect on three important early-life traits: pup birth mass, pup survival and pup growth. No effect was found of individual inbreeding on the three traits, suggesting that selection against inbred individuals does not play a major role in the pup life-history stage and hence that inbreeding depression mainly acts in the juvenile life-history stage.<br /><br /> Finally, I explored the effect of climate change on territorial male counts and male recruitment success. Although the breeding female population is declining as a result of climate change, it is unknown how the territorial male population is responding to environmental changes. I used genetic recaptures to reconstruct temporal changes in territorial male counts and found a temporal decline, similar to females. However, the territorial male count was not dependent on the current environmental conditions. Investigating the factors contributing to male recruitment success, I found that this is also declining over time and dependent on heterozygosity, birth mass and early-life environmental conditions. This suggests that the declining recruitment success in males is driving the decrease in territorial male counts and that male recruitment success is determined by deteriorating early-life environmental conditions. <br /><br /> In summary, I set out to dissect the genetic impact of historical and contemporary human pressures on the Antarctic fur seal. My results indicate that multiple populations survived historical sealing and genetic diversity loss was limited. Antarctic fur seals are now facing contemporary pressures in the form of climate change, which is causing increased selection in juveniles and affecting male recruitment success. My findings underline the importance of understanding the interactions between anthropogenic pressures, genetic diversity and population viability.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bielefeld

Year

2025

Author dc:creator

• Paijmans, Anneke

Repository record source_url

https://pub.uni-bielefeld.de/record/3005820

OAI identifier oai:identifier

oai:pub.uni-bielefeld.de:3005820