Identifying mesopredator release in multi-predator systems: a review of evidence from North America

The mesopredator release hypothesis, defined as the change in distribution, abundance or behaviour of a middle-ranking predator in response to a decrease in density or distribution of an apex predator, is an increasingly popular topic in ecology. Terrestrial mesopredators have been reported as being released in multiple systems globally, particularly in North America, over the past century. We reviewed 2687 scientific articles, of which we determined that 38 met our criteria for investigating mesopredator release (MR) in terrestrial North American mammalian predators. We observed no support or mixed support for MR in 46% of all relevant studies, including conflicting evidence between measures (mesopredator distribution, abundance or behaviour) within a given study and among studies of the same community in different settings. To advance the study of MR, we provide a conceptual framework that 1) highlights the multiple spatial, temporal and ecological scales at which mesopredator responses can occur; 2) suggests the relative weight of evidence for MR that is provided by measures of mesopredator responses at each scale; and 3) clearly defines the threshold for determining when MR is occurring. In increasingly reshuffled predator communities with declining apex predators, there is a need for future studies to assess in more detail the contexts in which mesopredator behavioural responses scale up to the population-level processes and species-level distribution changes needed to identify these responses as MR.