Rats dying for mice: Modelling the competitor release effect

Abstract

Introduced vertebrate predators are one of the most important threats to endemic species throughout a range of ecosystems, in particular on islands in biodiversity hot spots. Consequently, the reduction of predator numbers is considered a key conservation action in the management of many native vertebrates vulnerable to predators. It is now established that control attempts may affect non-target species through trophic interactions, but little is known concerning their consequences on competitive relationships. We study a mathematical model mimicking the effects of controlling introduced species in the presence of their competitors.We used two competing rodents to illustrate our study: black rats, Rattus rattus, and mice, Mus musculus. Analyses of the model show that control of only one introduced species logically results in the dramatic increase of the overlooked competitor. We present empirical data that confirm our theoretical predictions. Less intuitively, this process, which we term 'the competitor release effect', may also occur when both introduced competitors are simultaneously controlled. In our setting, controlling both predators can promote their coexistence. This occurs as soon as the inferior competitor benefits from the differential effect of the simultaneous control of both competitors, that is, when the indirect positive effect of control (the removal of their competitors) exceeds its direct negative effect (their own removal). Both control levels and target specificity have a direct influence on the extent of this process: counter-intuitively, the stronger and more specific the control, the greater the effect. The theoretical validation of the competitor release effect has important implications in conservation, especially for control management.

The invasion ecology of mammals: a global perspective

“…For example, 'mesopredator release' (Courchamp et al 1999) may become evident if a top predator is removed whilst leaving other predators over which this predator may have exerted some previous control. 'Competition release' may similarly occur when an introduced competitor is removed from a system (Caut et al 2007). More research is needed to understand such potential effects.…”

Differential response of a carnivore community to predator control: a spatio-temporal observational study

“…In general, the effect of predator control varies depending on the selectivity, intensity, scope and duration of the programme, and the biology of the species involved (Reynolds et al 1993;Harding et al 2001;Baker & Harris 2006;Duarte et al 2012). When control is maintained over time, the interactions that occur can be explained by the mechanisms of the competitor release effect (Caut et al 2007;Casanovas et al 2012). When the control is not maintained over time, or only covers small areas, the results are usually very limited (Reynolds et al 1993;Harding et al 2001;McDonald & Harris 2002).…”

The successful eradication of introduced roof rats (Rattus rattus) from Buck Island using diphacinone, followed by an irruption of house mice (Mus musculus)

“…According to previous sightings, this species may have been present all along, but its population and activity was perhaps greatly suppressed by the rats. Removal of the rats allowed the house mouse population to flourish in what can be described as a 'competitor release' effect (Caut et al 2007). Introduced house mice are common on many of the islands and cays of the Caribbean (Dammann and Nellis 1992).…”