Density dependence, individual heterogeneity, and demographic compensation: New approaches to age-old questions

Dr. Todd Arnold

Professor, University of Minnesota

Abstract

Understanding the impacts of human harvest on population dynamics was a key motivation behind the development of modern mark-recapture-recovery models, and the general framework can be adapted to consider any source of natural or anthropogenic mortality of interest. Mortality sources are considered additive if they lead to a net reduction in annual survival that cannot be ameliorated over the course of the annual cycle, or fully compensatory if the reduction is completely ameliorated through increases in natural survival. Compensation can also occur if harvest mortality falls disproportionately upon frail individuals that were less likely to survive anyway. Additive and compensatory mortality are reference points along a continuum, and methods to estimate partial compensation are sorely needed. If harvest or other anthropogenic mortality sources are seasonally restricted, then models that allow annual mortality to be partitioned into seasonal hazard rates (rather than annual survival probabilities) further facilitate assessment of compensatory mechanisms, as do integrated population models (IPMs) that allow compensation to occur via other vital rates such as fecundity or immigration. IPMs also facilitate direct assessment of density-dependent relationships that might promote demographic compensation. In this seminar, I review existing approaches to discriminate between additive and compensatory mortality, and propose new methods that can better resolve this age-old quandary.