Fuzzy structure and spatial dynamics of a declining woodland caribou population

Abstract

Examining both spatial and temporal variation can provide insights into population limiting factors. We investigated the relative spatial and temporal changes in range use and mortality within the Red Wine Mountains caribou herd, a population that declined by approximately 75% from the 1980s to the 1990s. To extract the spatial structure of the population, we applied fuzzy cluster analysis, a method which assigns graded group membership, to space use of radio-tracked adult females, and compared these results to a hard classification based on sums-of-squares agglomerative clustering. Both approaches revealed four subpopulations. Based on the subpopulation assignments, we apportioned the number of animals, radio-days, calving events and mortalities across subpopulations before and after the decline. The results indicated that, as the herd declined, subpopulations were disproportionately affected. In general, subpopulations with the greatest range overlap with migratory caribou from the George River herd experienced comparative reductions in activity and increased mortality. The subpopulation with the least overlap exhibited the converse pattern. The infra-population imbalances were more pronounced when hard clustering was employed. Our results reiterate that refugia from other ungulates may be important in the persistence of taiga-dwelling caribou. We propose that changes across time and space are valuable assays of localised demographic change, especially where individuals exhibit spatial hyperdispersion and site fidelity.