Unequal density dependence between survival and recruitment affects harvesting effectivness

Abstract

ABSTRACTRecruitment and adult‐mortality may respond differently to increased population density. Such unequal density dependence (UDD) is evident for large mammals showing density dependence mainly in recruitment (reproduction rate or juvenile survival), whereas adult survival is high and varies little with density. Mechanistically, UDD is likely influenced by an unequal allocation of resources toward reproduction versus survival. Unequal density dependence is expected to affect compensation abilities, and hence may have serious implications with regard to population harvesting. We modeled UDD by linking population size to reproduction and survival through per capita resource availability and consumption. We integrated UDD into 2 population models (continuous vs. periodic recruitment) and investigated how it affected the compensatory dynamics of populations. For a population with continuous recruitment, our model predicted that although UDD stabilizes the population dynamics, it makes populations more vulnerable to harvesting as a consequence of lower compensatory ability, regardless of whether more resources are allocated to reproduction or to survival. For populations with periodic recruitment, on the other hand, an asymmetric effect of UDD is predicted. Strong density dependence in recruitment relative to adult survival yields a strong compensatory response, whereas equal or stronger density dependence in adult survival relative to recruitment yields weaker compensatory responses. Stronger density dependence in recruitment also causes the periodic fluctuations in population size to be amplified rather than dampened under harvesting. These results are robust to alternative model formulations (e.g., Ricker and Beverton‐Holt equations). Using an empirical example for coyotes (Canis latrans), wild boars (Sus scrofa), and white‐tailed deer (Odocoileus virginianus), we demonstrated the implications of our results for harvest management of medium‐size mammalian species that are periodic breeders and exhibit density‐dependent recruitment. Accounting for UDD in the population model of coyotes and wild boars, which are characterized by large litters, predicts compensation that is stronger than the response obtained without UDD. For white‐tailed deer, which are characterized by substantially smaller litters, accounting for UDD does not change the outcome. Harvest‐based control attempts of medium‐size mammals may hence be much less effective than predicted when the population models used to estimate them ignore UDD, suggesting the necessity of integrating UDD into these models. © 2018 The Wildlife Society.