Using Predator-Prey Theory to Predict Outcomes of Broadscale Experiments to Reduce Apparent Competition

Abstract

Apparent competition is an important process influencing many ecological communities. We used predator-prey theory to predict outcomes of ecosystem experiments aimed at mitigating apparent competition by reducing primary prey. Simulations predicted declines in secondary prey following reductions in primary prey because predators consumed more secondary prey until predator numbers responded to reduced prey densities. Losses were exacerbated by a higher carrying capacity of primary prey and a longer lag time of the predator's numerical response, but a gradual reduction in primary prey was less detrimental to the secondary prey. We compared predictions against two field experiments where endangered woodland caribou (Rangifer tarandus caribou) were victims of apparent competition. First, when deer (Odocoileus sp.) declined suddenly following a severe winter, cougar (Puma concolor) declined with a 1-2-year lag, yet in the interim more caribou were killed by cougars, and caribou populations declined by 40%. Second, when moose (Alces alces) were gradually reduced using a management experiment, wolf (Canis lupus) populations declined but did not shift consumption to caribou, and the largest caribou subpopulation stabilized. The observed contrasting outcomes of sudden versus gradual declines in primary prey supported theoretical predictions. Combining theory with field studies clarified how to manage communities to mitigate endangerment caused by apparent competition that affects many taxa.

A spatial theory for characterizing predator–multiprey interactions in heterogeneous landscapes

“…Like similar tests of coarse spatial approximations (e.g. [38]), this boosts confidence that the model captured the key spatial processes needed to explain how spatial heterogeneity can fundamentally alter predator-mulitprey interactions, relative to aspatial conditions. Both caribou and moose had a relatively low probability of occurrence in cuts or on roads.…”

“…A mechanism generally put forward to explain the increased mortality is that timber harvest brings mature conifer forests back to an early-seral-stage that, once invaded by deciduous vegetation, attracts moose. The local increase in moose density then triggers a numerical response from wolf populations, which leads to more frequent interactions with caribou [38,45,46]. Our study outlines an additional mechanism that does not rely on a numerical response from moose or wolf.…”

Testing predator–prey theory using broad‐scale manipulations and independent validation

“…Yet, if depensatory predation is not accounted for while reducing the overabundant prey, these populations may decline faster than anticipated and exacerbate switching of predators to the rare prey (Serrouya et al . ). The outcome may be to unintentionally increase extinction risk to the rare prey.…”

Forest productivity mitigates human disturbance effects on late-seral prey exposed to apparent competitors and predators

“…The positive link between forest productivity and deciduous vegetation during early stages of forest succession (Figs 1 and 4) should therefore have a larger impact on food availability for moose than for caribou. Also, primary productivity influences interaction strengths in plant-prey-predator systems following a combination of non-linear inter-species relationships e.g., numerical and functional responses, and frequency-dependent prey selection by the predator 4, 5, 43 , and habitat-animal spatial relationships 16, 44, 45 . In this context, we can consider the typical situation where an increase in deciduous vegetation is followed by an increase in moose abundance that triggers a numerical response by the wolf population.…”