Most previous analyses of demographic compensation have been phenomenological rather than mechanistic. Classic studies of North American mallards (Anas platyrhynchos) revealed that survival did not decline in the face of increased hunting mortality, but investigators were unable to demonstrate an ecological mechanism for compensation, and simulation studies suggest that density-dependent variation in natural survival cannot act quickly enough to achieve compensation. This has led to speculation that compensation might be achieved through density-dependent fecundity or individual heterogeneity in survival, but empirical studies have been lacking.
We developed an integrated population model (IPM) for American black ducks (Anas rubripes) based on harvest surveys and band-recovery data. Black ducks were banded extensively during late summer (Aug-Sep) and late winter (Jan-Feb), which allowed us to estimate fecundity, sex- and stage-specific population sizes, fecundity, and survival rates during breeding versus wintering/hunting periods using harvest-based Lincoln estimators and two-season Brownie recovery models. Most notably, the IPM framework allowed us to assess annual variation in each vital rate (fecundity, juvenile survival, adult breeding season survival, and adult wintering survival) in relation to harvest rate and population size at the beginning of each season.
Adult survival during the wintering period (Sep – Jan) was high (range: 0.91 – 0.96) and mortality was comprised almost entirely of hunting, but winter survival of adults was uncorrelated with harvest rate or population size. By contrast, fledged juveniles had much lower survival during winter (range: 0.42 – 0.63) and survival was negatively correlated with harvest rate (juvenile males) and population density (both sexes). Adults had lower survival during the breeding season (Feb – Aug; Ad F: 0.57 – 0.64, Ad M: 0.60 – 0.74), and breeding season survival of adult females was negatively correlated with breeding population density. Annual population growth was most strongly correlated with adult female survival during the breeding season, with overwinter survival of juvenile females playing a secondary role. We suggest that predation on breeding females may be the most important regulator of population dynamics in American black ducks. IPMs provide a powerful tool for assessing multiple and diverse responses to anthropogenic and ecological impacts, including harvest mortality and density dependence.