Using predator-prey theory to predict outcomes of a broad-scale test of apparent competition

Background/Question/Methods

Large-scale ecosystem experiments can provide considerable payoff for scientific inquiry, but also come with substantial risks because unintended outcomes can be costly and in some cases virtually impossible to reverse. To mitigate these risks, we used basic predator-prey theory to predict outcomes of an ecosystem manipulation intended to recover an endangered species affected by apparent competition. Apparent competition predicts that prey with a lower intrinsic growth rate or greater vulnerability to predation can be driven to extinction when faced with novel prey.  Intuition suggests that removing novel prey would reduce apparent competition on native prey, so our simulations focused on varying the intensity and rate (sudden vs. gradual) of removing novel prey.  Finally, we confront predictions with data from two case studies (one natural experiment, one ecosystem manipulation) with endangered mountain caribou (Rangifer tarandus) as the victim of apparent competition.

Results/Conclusions

During the transient phase following the reduction of novel prey, simulations predicted increased losses to the native prey; losses were exacerbated by a higher carrying capacity of novel prey and a longer numerical lag of the predator. Importantly, the greater the reduction of novel prey, the greater the loss to the native prey. These results occurred because the ratio of predator to native prey was increased, at least temporarily. A gradual reduction of novel prey was less impactful to the native prey. Our field experiments provided an opportunity to validate these predictions, particularly the gradual vs. sudden removal of novel prey. The first experiment was observational, where increasing deer (Odocoileus sp.) populations declined suddenly following a severe winter. After the deer crash, cougar (Puma concolor) declined with a 1-2 year lag, more caribou were eaten by cougars, and caribou populations declined by 40%. The second study was a manipulation, where abundant moose (Alces alces) were gradually reduced to help recover caribou. Wolf (Canis lupus) populations declined in the treatment area and did not shift consumption to caribou based on scat and kill-site investigations. Ten years post-treatment, two caribou populations continued to decline, but were very small at the outset (<50) and subjected to Allee effects. The third, larger subpopulation stabilized and possibly increased.  The contrast of sudden vs. gradual decline of novel prey supported the theoretical predictions.  Our approach highlights a fruitful integration of validating theory to answer new questions, but also informing an applied problem of how to resolve species endangerment from apparent competition that now affects many taxa.