Predation and reduced grazing interact to reduce recruitment and population growth in Black Brant

Dynamics of avian populations may be governed by a complex interaction between immediate effects of predation and longer-term trophic feedbacks between individuals and their foods. We used a long-term study of uniquely marked Black Brant geese (Branta bernicla nigricans) to estimate recruitment into the breeding population. We related recruitment to nest success, which directly affects recruitment 2 to 3 years later. We also assessed similarly time-lagged relationships between recruitment and number of nests in the colony, a measure of local density, and pre-fledging and first-year survival, the latter of which is strongly influenced by growth conditions and food availability for young of the year. We assessed relationships between number of recruits and explanatory variables in 2 sets of models, which either included or excluded a linear trend across years in numbers of recruits. The best-performing models in each model set explained a substantial proportion of the variation in numbers of recruits 2 to 3 years later (85 and 78% in the 2 model sets, respectively). First-year survival was an important predictor of number of recruits in both sets of models, whereas nest success was important only in models lacking a trend across years. Number of nests in the colony had a relatively weak negative association with number of recruits. First-year survival reflects a lagged response to earlier nest-predation events. Reduced grazing during predation events can result in loss of grazing lawns and thus reduced growth rates of future cohorts of goslings, which are regulated by food abundance and density of broods. Our finding that recruitment is insufficient to maintain the breeding population results from both direct effect of predation on nests, primarily by Arctic foxes (Alopex lagopus), and longer-term indirect effects of such predation on first-year survival of Black Brant.