Restoring vertebrate predator populations can provide landscape-scale biological control of established invasive vertebrates: Insights from pine marten recovery in Europe

Abstract

Invasive species pose one of the greatest global threats to biodiversity. There has been a long history of importing coevolved natural enemies to act as biological control agents to try to suppress densities of invasive species, with historically limited success and frequent adverse impacts on native biodiversity. Our understanding of the processes and drivers of successful biological control has been focussed on invertebrates and is evidently limited and potentially ill-suited with respect to biological control of vertebrate populations. The restoration of native vertebrate predator populations provides a promising nature-based solution for slowing, halting, or even reversing the spread of some invasive vertebrates over spatial scales relevant to the management of wildlife populations. Here, we first review the growing literature and data from the pine marten-red and grey squirrel system in Europe. We synthesise a multi-decadal dataset to show that the recovery of a native predator has resulted in rapid, landscape-scale declines of an established invasive species. We then use the model system, predator-prey interaction theory, and examples from the literature to develop ecological theory relating to natural biological control in vertebrates and evolutionary processes in native-invasive predator-prey interactions. We find support for the hypotheses that evolutionary naivety of invasive species to native predators and lack of local refuges results in higher predation of naive compared to coevolved prey. We apply lessons learnt from the marten-squirrel model system to examine the plausibility of specific native predator solutions to some of the Earth's most devastating invasive vertebrates. Given the evidence, we conclude that depletion of vertebrate predator populations has increased ecosystem vulnerability to invasions and thus facilitated the spread of invasive species. Therefore, restoration of vertebrate predator populations is an underappreciated, fundamental, nature-based solution to the crisis of invasive species and should be a priority for vertebrate invasive species management globally.

Keywords: behavioural response; biological control; carnivore recovery; generalist predators; invasive naivety; invasive species; native predators; prey switching; refuge.

FIGURE 1

(a) Local scale impacts of pine marten recovery on invasive grey squirrels (left) and native red squirrels (right) in Scotland (Sheehy et al., 2018). (b) landscape‐scale impacts of pine marten recovery on invasive grey squirrels (left) and native red squirrels (right) in Northern Ireland (Twining et al., 2021). Lines are model predictions of the relationship between pine marten and squirrel occupancy.

FIGURE 2

Changes in number of 100 km² grid occupied by the pine marten, the red squirrel, and the grey squirrel over 12 years of monitoring in Ireland (Carey et al., ; Lawton et al., 2015, 2020). The blue dashed line is the pine marten, solid red is the red squirrel and dashed green is the grey squirrel.

FIGURE 3

Dynamics of predator recovery on native and invasive squirrels over 12 years in Ireland. Black squares show occurrence of (a) pine marten, (b) red squirrel and (c) grey squirrel in 2007 (Carey et al., 2007); (d) pine marten, (e) red squirrel and (f) grey squirrel in 2012 (Lawton et al., 2015); (g) pine marten, (h) red squirrel and (i) grey squirrel in 2019 (Lawton et al., 2020).

FIGURE 4

Diagrammatic illustration of the five case studies presented. (a) Marten–squirrel–pox system, (b) lynx–deer system, (c) panther–hog system, (d) otter–mink system, (e) wolf–lynx–raccoon dog–rabies system. Solid lines show direct effects, dashed lines show indirect effects. Grey boxes show native predators, orange boxes invasive species, blue boxes native prey species and gold boxes shows parasites and pathogens. Line width indicates strength of interactions.

FIGURE 5

A decision tree providing a general framework for identifying and assessing the potential of a native predator to provide control of an established invasive species should the species be restored or reintroduced.