MEASURING THE ALLEE EFFECT: POSITIVE DENSITY DEPENDENCE IN SMALL MAMMALS

Positive density dependence (the Allee effect) has numerous implications for the dynamics, distribution, and conservation of populations, but has been considered difficult to measure because it should occur only at small population sizes. Theories of density‐dependent habitat selection provide a simple solution. Allee effects produce erratic habitat selection where individuals that initially pack into the better habitat flow rapidly into the poorer one at a threshold density. The instability creates a hiatus or “hole” in abundance along habitat isodars (graphs of density assuming ideal habitat selection). The presence of an Allee effect can be detected easily by calculating the exact probabilities of the observed frequency of data points in the hiatus area with that expected if fitness is positively density dependent at low density. Tests of the theory using rodents occupying mesic and xeric habitats in Canada's Rocky Mountains reveal a clear example of the Allee effect in red‐backed voles, and hint at a similar effect in deer mice. The tests suggest that populations of small mammals may frequently reach low densities where positive density dependence in fitness may play a significant role in population persistence.