A new study by Saoirse Leonard and co-authors from the Institute of Zoology, London and the University of Liverpool model the potential survival of brown bears in an Irish glacial refugium. The study has just been published in Biology Letters.
The study examines the presence of the now extinct brown bear (Ursus arctos) in Ireland during the Last Glacial Maximum (between 15,000 and 24,000 years ago) and aims to address the possibility that bears survived in Ireland during this period. The question has arisen because there is still lengthy debate as to whether Ireland remained partially ice free during the last glaciation, and if so, how many species, if any, survived in this ice free refugium?
Recent work suggests that brown bears in Ireland hybridised at some point with polar bears (Ursus maritimus), and dating this potential period of overlap between the species is of interest. Although there has been suggestion that brown bears survived in Ireland up to about 3000 years ago, it is not known what the potential for overlap was to allow the meeting of both polar and brown bears in Ireland. See Edwards et al. (2011).To help address this, Leonard et al. conducted population variability analysis to ascertain the possibility of the brown bear surviving in Ireland during the last glaciation. The programme is designed to work with wildlife populations that have long life spans and low reproductive rates, making it suitable for use with the brown bear. The study gathered data from modern day brown bear populations including population statistics, the rate of reproduction and incorporated habitat variables. The authors built a model based on the assumption that during the last glaciation, an ice free habitat in Ireland would have been similar to Thundra regions today, and hypothesised a glacial refugium of approximately 29,000 square kilometers assuming 14,000 km ice free landmass on the present day Ireland, with the remainder landmass off shore and now submerged under water due to the subsequent rise in sea-levels since that period. The model assumed that Ireland had a maximum population of just over 500 individuals and assumed that Ireland was a barren landscape, with a median home range size of modern bears.
The results showed that brown bears in Ireland were highly unlikely to have survived the last ice age. The models predicted that extinction occurred very quickly during the last glaciation with the likelihood that the bears died out some time between 150 – 1500 years during the last ice age period. The authors suggest that based on their model, that brown and polar bears must have hybridised before the last glaciation (around 25,000 years ago), or after the ice-sheet retreated and a chance opportunity arose for the two species to meet (around 10,000 years ago). Although the authors also admit that brown bear behaviour changed during the last glaciation, with the possibility that brown bears semi-adapted to ice sheet habitats and opportunistically mated with polar bears during transition periods prior to extinction. The authors believe that even if ice free areas did occur within Ireland, they would have been too small to support a brown bear population for any lengthy period, and the bears would have simply died out.
Interestingly, there is suggestion in the paper that present day brown bear populations are at a similar risk of extinction due to the isolation of populations caused by habitat fragmentation and human prosecution. Models such as the one used in this study have the potential to be used to predict the long-term survival ability of such populations and could be used to help manage these threatened populations before they end up in the history books like the Irish brown bear.