Webinar - Density-dependent Social Behaviour: From Individuals in Groups to Populations

Quinn Webber
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Density dependence is a key population ecology parameter that can influence variation in life-history, morphology, and behaviour. Caribou population density is known to fluctuate through space and time. In Newfoundland, caribou occupy approximately 14 distinct sub-populations, and since the late 1990s, nearly all of these sub-populations have experienced declines in population size. In this talk, we outline three potential behavioural outcomes of the drastic change in population density observed for Newfoundland caribou. First, based on a thirty-year dataset of caribou group size observations collected during aerial surveys, we found that groups varied in size both spatially and temporally. In contrast to our expectation, groups decreased in size as a function of increasing population density, while groups tended to be larger in winter compared to summer, presumably as a result of seasonal access to foraging opportunities. Second, we examined the role of the social environment to determine whether reproductive success varies for caribou that form calving aggregations during parturition compared to those that give birth solitarily. We found that approximately 80% of caribou in the Middle Ridge herd give birth within a social calving aggregation, but there was no difference in reproductive success between animals that gave birth on, or off, the calving ground. However, when only considering caribou on the calving ground, we found that animals living closer to the periphery of the social aggregation had lower reproductive success than those at the core of the social aggregation. Finally, we found that social network strength and habitat specialization were density-dependent, while more social individuals were habitat generalists. However, habitat specialization had a greater effect on fitness, where habitat specialists had higher fitness than habitat generalists, but only at high density. Our work on caribou in Newfoundland addresses questions about the density and context dependence of social behaviour and provides a theoretical framework for future studies to address similar questions. We broadly integrate aspects diverse ecological fields, including socioecology, spatial ecology, movement ecology, and conservation biology.