Population responses to harvest depend on harvest intensity, demographics, and mate replacement in sandhill cranes

Abstract

Recent examinations of hunting’s impact on populations of long-lived species illustrate the rarity of strong compensatory responses. The extent to which sandhill crane populations exhibit compensatory, partially compensatory, or additive responses to harvest mortality is poorly known. To examine how sandhill crane harvests might impact resident, breeding crane populations, we simulated harvests of a well-studied population of individually marked birds at varying harvest intensities and degrees of selectivity within the social structure of territorial and non-territorial cranes. Simulations occurred in a demographically-explicit stochastic population model developed from a previous long-term (2000–2014) study of a sandhill crane population at carrying capacity in southcentral Wisconsin, USA. Non-selective harvest models that did not account for reproductive lags after territorial birds replaced lost mates produced additive responses and resulted in declining populations when ≥ 7% of the population was harvested annually. When models of non-selective harvest incorporated reproductive lags, this threshold was reduced to ≥ 5%. Harvesting at or above 5% harvest intensities continuously decreased population size over a 100-year period. In all models of selective harvests, when only non-territorial birds were harvested, all populations returned to equilibrium. Population responses to harvest are sensitive to the number of territorial birds harvested, and this sensitivity increases when accounting for lags in reproduction stemming from mortality-induced mate replacement. If possible, limiting harvest of territorial birds from any one specific population would help maintain stability of populations when implementing a hunt. This study provides evidence for harvest impacts at a localized scale, and it represents an important first step toward better understanding the potential impacts of hunting on population responses at a regional scale, especially where resident, territorial birds might be harvested.