Mechanisms of population regulation in wolves and their community consequences
Understanding the mechanisms of population regulation requires knowledge about the processes causing density-dependent population growth. Despite persistent controversy about the role of density-dependent processes in regulating populations, empirical understanding of how density dependence is manifested in natural populations remains limited. Moreover, little is known about the effects of different regulatory mechanisms on the dynamics of natural communities. We evaluated two classic, yet largely untested hypotheses: (1) the primary mechanism regulating vertebrate populations (density-dependent food availability) is overridden by a secondary mechanism (density-dependent social aggression) in food-rich environments; (2) consumer-resource dynamics are more stable when consumers are regulated by density-dependent factors besides resources. To address hypothesis (1), we used linear regression to compare time series data on population size, food availability, and intergroup aggression among wolves inhabiting a prey-rich environment (Yellowstone National Park; YNP) to those inhabiting a comparatively prey-poor environment (Isle Royale National Park; IR). We tested hypothesis (2) with a consumer-resource model parameterized for Yellowstone wolves and their primary prey (Cervus elaphus).
Average per capita food availability among YNP wolves was twice that of IR wolves and average wolf densities in YNP were 41% greater than on IR. Increasing wolf density was associated with decreased per capita food availability in both populations and increased intergroup aggression in YNP wolves. The density-dependent increase in aggression among YNP wolves was unrelated to food availability, but rather an independent effect of density related to overcrowding. In IR wolves, 67% of density-dependent population growth was attributable to food availability. By contrast, none of the density-dependence in YNP wolf population growth rate was due to food availability. Rather, intergroup aggression explained 96% of the density-dependence in YNP wolf population growth. That the source of density-dependence is food in IR and aggression in YNP supports the hypothesis that vertebrates in food-scarce systems are regulated by food availability whereas vertebrates in food-rich systems are regulated by social aggression. Finally, adding density-dependence to empirically-based functions of wolf survival and recruitment in a model of wolf-elk dynamics dampened oscillations in both wolf and elk numbers, suggesting that density-dependence related to predator territorial aggression is an important stabilizing influence on the dynamics of large vertebrate predator-prey systems.