Population Persistence Time: Estimates, Models, and Mechanisms

The conservation and management of populations require some insight into mechanisms of population regulation, yet unambiguous identification of density dependence in vital rates is notoriously difficult. Through the reanalysis of data published by Stacey and Taper for a semi-isolated sub-population of the Acorn Woodpecker we demonstrate the utility of an approach in which a single data set is “attacked” using a suite of models with different assumptions. Tests for density dependence of the reproductive rate and of juvenile survival are inconclusive, but there is evidence for density-dependent adult survival. Since “survival” rates estimated from banded birds confound the effects of mortality and immigration/emigration, we construct and analyze three models: (1) a closed population with density-independent vital rates and an upper limit to population size, (2) a regulated closed population, (3) a regulated sub-population with immigration and emigration. Very small interchanges with other sub-populations can have a very large effect on population persistence; thus our results highlight the importance of distinguishing the effects of interchange from density dependencies in vital rates.