The effects of predator recovery and climate change on the long-term demography of a flagship herbivore

Abstract

In the western United States, ecosystems are being reshaped from both bottom-up and top-down processes. Widespread carnivore recolonizations after 20th century extirpations have returned top-down pressures to ecosystems, and climate change is reshaping bottom-up forces through shifts in the timing and length of growing seasons, resulting in reduced forage quality and quantity. Large herbivores, such as elk (Cervus canadensis), function as flagship species for conservation and have potential for both top-down and bottom-up forces to shape demography as ecosystems change. To test the roles of top-down (puma density (Puma concolor)) and bottom-up (drought severity and elk density) processes in shaping large herbivore demography, and to tie those effects to population trajectories, we built an integrated population model (IPM) using 36 years of elk data spanning the recolonization of pumas and long-term climate change in eastern Oregon, USA. We tested the effects of top-down and bottom-up forces on elk recruitment, calf survival, and population growth. We also tested effects of bottom-up forces on elk pregnancy rates. Puma recolonization corresponded with declines in mean recruitment from 0.44 to 0.32 calves per female and in calf survival from 0.92 to 0.69, corresponding with a 4% drop in population growth. Drought severity increased over our study period, explaining a decline in mean recruitment from 0.39 to 0.32 calves per female, which corresponds to a 3% reduction in population growth. Drought severity likely acted on recruitment through reduced pregnancy rates. Lactating females showed lower pregnancy rates in drought conditions, while drought from fall breeding periods to the following fall had no effect on recruitment. While both top-down and bottom-up forces explained variation in elk demography over the course of our study, bottom-up forces are likely to be more influential on large herbivores moving forward. Predator populations stabilized following recovery, limiting the top-down contribution to annual variation in vital rates. Climate change-induced drought patterns, however, are accelerating during summer and fall throughout large areas of the interior western U.S. Thus, our results indicate the potential for increasingly strong bottom-up negative effects on herbivores whose performance depends on a narrow period of summer nutrition. Open Research StatementData are not yet provided. Many of the data from Starkey Experimental Forest and Range are already publicly available, and we are currently working to post data for this manuscript on the Forest Service Research Data Archive (FSRDA) linked to the rest of the data from Starkey. Upon acceptance, all data and the relevant code will be archived the FSRDA. Currently, the code for the paper is publicly available on Github at https://github.com/keloonam/starkey_ipm