1. Harvest of wild organisms is an important component of human culture, economy, and recreation, but can also put species at risk of extinction. Decisions that guide successful management actions therefore rely on the ability of researchers to link changes in demographic processes to the anthropogenic actions or environmental changes that underlie variation in demographic parameters. 

2. Ecologists often use population models or maximum sustained yield curves to estimate the impacts of harvest on wildlife and fish populations. Applications of these models usually focus exclusively on the impact of harvest and often fail to consider adequately other potential, often collinear, mechanistic drivers of the observed relationships between harvest and demographic rates. In this study, we used an integrated population model and long-term data (1973-2016) to examine the relationships among hunting and natural mortality, the number of hunters, habitat conditions, and population size of blue-winged teal (Spatula discors), an abundant North American dabbling duck with a  relatively fast-paced life history strategy.

3. Over the last two and a half decades of the study, teal abundance tripled, hunting mortality probability increased slightly (< 0.02), and natural mortality probability increased substantially (> 0.1) at greater population densities. We demonstrate strong density-dependent effects on natural mortality and fecundity as population density increased, indicative of compensatory harvest mortality and compensatory natality. Critically, an analysis that only assessed the relationship between survival and hunting mortality would spuriously indicate depensatory hunting mortality due to multicollinearity between abundance, natural mortality, and hunting mortality. 

4. Our findings demonstrate that models that only consider the direct effect of hunting on survival or natural mortality can fail to accurately assess the mechanistic impact of hunting on population dynamics due to multicollinearity among demographic drivers. This multicollinearity limits inference and may have strong impacts on applied management actions globally.

Adult female blue-winged teal (n = 112,639) were captured in traps and nets prior to the hunting season (July-September) in the prairie potholes and aspen parklands of the North American midcontinent from 1973 to 2016 (Figure 1). Teal were ringed with uniquely engraved metal markers, and some marked individuals were killed by hunters. A portion of these markers were retrieved and reported to the USGS Bird Banding Lab (n = 2,518; USGS Patuxent Wildlife Research Center). From 1974-2016, waterfowl breeding population and habitat surveys were flown at the beginning of the breeding season over the same area by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service to estimate the total number of breeding pairs of teal (y_n,t) and other ducks, and the number of ponds (y_p,t), a landscape scale measure of habitat suitability for breeding waterfowl (Walker et al. 2013, U.S. Fish & Wildlife Service 2018). We downloaded the ringing and recovery data from the GameBirds Database CD (Bird Banding Lab, USGS Patuxent Wildlife Research Center), and the Waterfowl Breeding Population and Habitat Survey data from the USFWS Migratory Birds Data Center. We retained females marked in Canada and the United States in Waterfowl Breeding Population and Habitat Survey strata 20-49 (U.S. Fish & Wildlife Service 2018), and we restricted re-encounters to harvested individuals recovered and reported by hunters in the United States and Canada from September through early February, with half of all reported hunting mortality occurring in September. We excluded recoveries in Mexico, Central and South America, and the Carribean (n = 316) due to the inclusion of band reporting probabilities (r = r_1973, ... , r_2016) in our analyses, which were not available for Latin America. Mark-recovery data were downloaded from the USGS Bird Banding Lab Celis-Murillo et al. 2020. We accessed estimates of teal abundance and pond abundance from the Waterfowl Breeding Population and Habitat Survey (U.S. Fish & Wildlife Service 2018), as well as data on federal duck stamp sales, which are required to hunt for waterfowl in the United States. Third party data were used for this study, collection of which followed appropriate ethical guidelines. No additional ethical approval was required from our respective insitutions. We formatted the capture-recovery data into a multinomial array to reduce computational requirements.

Please contact the authors for additional information about data processing.

The open-source programs R and JAGS are required to run the integrated population model described in this manuscript.