Habitat suitability and demography of muskrats inhabiting a heterogeneous landscape

Abstract

Recent ecological theory predicts that change in population size is linked to the spatial and temporal variation in demographic performance of individuals. In particular, directional dispersal of individuals from prime to marginal habitats can regulate abundance in prime habitat, and influence local population size. However, few empirical studies have adequately tested all predictions of this spatially-dependent population model. The focus of my research was to measure the spatial and temporal variation in demography of a local muskrat population inhabiting a landscape with unequal suitability among habitats. First, I tested the hypothesis that water level limits population size and distribution of muskrats during the breeding period (May-October). Next, I measured nine nutrient-dependent, phenotypic variables of muskrats and one index of forage quality during the ice-free period. Data were used to test the hypothesis that differences in food resources limits the demographic performance of muskrats in marginal versus prime habitats (food limitation hypothesis, FLH). Lastly, I examined the spatial and temporal variation in demography among habitats during the breeding (May-October) and non-breeding (October-May) period. Specifically, I tested several predictions of the source-sink model. Water level strongly limits population size of muskrats by influencing adult and juvenile survival during the breeding and non-breeding period. Consequently, water level plays a major role in determining the abundance and distribution of muskrats among habitats. Differences in habitat suitability were primarily related to the survival rate of juveniles during the breeding period, and all individuals overwinter. Variation in demographic parameters indicated that residents of island shorelines (prime habitat) were less affected by the density-independent effects of water level than individuals occupying emergent vegetation (more marginal habitats). Since food resources could not be linked to habitat suitability, I hypothesize that spatial variation in predation risk is a key factor influencing habitat suitability in muskrats. Spacing behaviour plays an integral part in regulating the breeding density of muskrats in prime habitat. At the local level, population dynamics appears to result from the interaction of spacing behaviour, predation, and water level. (Abstract shortened by UMI.)