Elsevier

Biological Conservation

Volume 224, August 2018, Pages 277-289
Biological Conservation

Review
Evaluating the efficacy of predator removal in a conflict-prone world

https://doi.org/10.1016/j.biocon.2018.05.003Get rights and content

Highlights

  • Predators are essential components of ecosystems but can threaten humans.

  • Economics, conservation, or security may inspire culls but success is poorly defined.

  • Across environments and taxa, we identified infrequent success of predator removal.

  • Consideration of demographics, sustainable yield, and problem individuals are relevant.

  • Co-existence is possible but typically requires research to inform co-existence strategies.

Abstract

Predators shape ecosystem structure and function through their direct and indirect effects on prey, which permeate through ecological communities. Predators are often perceived as competitors or threats to human values or well-being. This conflict has persisted for centuries, often resulting in predator removal (i.e. killing) via targeted culling, trapping, poisoning, and/or public hunts. Predator removal persists as a management strategy but requires scientific evaluation to assess the impacts of these actions, and to develop a way forward in a world where human-predator conflict may intensify due to predator reintroduction and rewilding, alongside an expanding human population. We reviewed literature investigating predator removal and focused on identifying instances of successes and failures. We found that predator removal was generally intended to protect domestic animals from depredation, to preserve prey species, or to mitigate risks of direct human conflict, corresponding to being conducted in farmland, wild land, or urban areas. Because of the different motivations for predator removal, there was no consistent definition of what success entailed so we developed one with which to assess studies we reviewed. Research tended to be retrospective and correlative and there were few controlled experimental approaches that evaluated whether predator removal met our definition of success, making formal meta-analysis impossible. Predator removal appeared to only be effective for the short-term, failing in the absence of sustained predator suppression. This means predator removal was typically an ineffective and costly approach to conflicts between humans and predators. Management must consider the role of the predator within the ecosystem and the potential consequences of removal on competitors and prey. Simulations or models can be generated to predict responses prior to removing predators. We also suggest that alternatives to predator removal be further developed and researched. Ultimately, humans must coexist with predators and learning how best to do so may resolve many conflicts.

Introduction

Predators can influence ecosystems through top-down control of the distribution and abundance of other species (Estes et al., 2011; Mills et al., 1993; Newsome et al., 2017; Pace et al., 1999). The loss of predators can therefore have profound ecological effects in certain contexts, including disease outbreaks, biodiversity loss, and ecosystem state changes (Myers et al., 2007; Ripple et al., 2014). There is evidence to suggest that ecological communities can exhibit dramatic shifts following the loss of predators (Crooks and Soulé, 1999; Pech et al., 1992; Ritchie and Johnson, 2009; Wallach et al., 2010), including changes at other trophic levels (Anthony et al., 2008; Atwood et al., 2015; Suraci et al., 2016). Although predators occur among diverse animal taxa (e.g., arthropods, molluscs, teleosts, raptors, canids, mustelids, etc.), vertebrate predators frequently conflict with humans, and many species are threatened (Ripple et al., 2014); they are therefore the focus of this paper.

Many predatory vertebrates are vulnerable to disturbances because they generally have slower life histories, higher investment in parental care, lower abundances, and patchy distributions (Purvis et al., 2000). Yet, predators are challenged by a perception of being a threat to human interests or safety. Indeed, predators can be considered hazardous to domesticated animals (Gusset et al., 2009; Mishra, 1997; Oli et al., 1994), prey species of economic importance (Dalla Rosa and Secchi, 2007; Henschel et al., 2011; Weise and Harvey, 2005), or human safety via direct conflict (Dickman, 2010; Gore et al., 2005; Löe and Röskaft, 2004; Penteriani et al., 2016). Consequently, predators are often negatively perceived and persecution of vertebrate predators has a long a history (Bergstrom et al., 2014; Kruuk, 2002; Reynolds and Tapper, 1996; Treves and Naughton-Treves, 1999). Competition with predators yielded many institutionalized campaigns against them dating back to ancient Greece and Rome, a trend that pervaded through medieval Europe and was exported to North America with emigrants in the 1700s (Reynolds and Tapper, 1996 and references therein). Today, state, regional, and agency-led programs to systematically control predator populations exist. Predator removal is carried out systematically via a number of methods and across various geographic scales (Bergstrom et al., 2014; Reynolds and Tapper, 1996), including poison baiting, trapping, hunting, and culling or via bounty or reward systems in public hunting or fishing events, but may also be more haphazard as retaliation for encroachment or interaction with humans or their property (e.g., farmer killing a wolf encroaching on their herds; e.g. Bergstrom et al., 2014; Treves and Karanth, 2003).

The significance of predators in ecosystems is well established yet their removal remains a component of the management toolbox. Owing to a lack of clarity pertaining to how and when removal can be expected to be successful, it may be difficult for management agencies to decide whether to proceed with predator removal when confronted with a problem. Furthermore, there is mounting opposition from advocacy groups (especially animal rights) and conservation-aware citizens that provide social inertia and pressure on animal control (van Eeden et al., 2017), which may complicate and influence decision-making (see Wallach et al., 2015). The science of predator removal therefore could benefit from an objective evaluation to identify successes and failures to both inform decision-making and identify lingering research gaps across multiple taxa (Treves et al., 2016; Eklund et al., 2017). Syntheses of this topic have focused on using meta-analysis, particularly for nesting birds (Côté and Sutherland, 1997; Smith et al., 2010, Smith et al., 2011), but it is challenging to apply such an approach across taxa and research paradigms (i.e., motivations). In this review, we evaluated these two competing hypotheses by considering of the available evidence for predator removal to determine whether predator removal is successful for wildlife conservation and management. We reviewed relevant literature and evaluated outcomes. In doing so, we propose a definition of success that can be applied to predator removal programs and we provide examples of success and failure in predator removals based on the following motivations 1) protection of domestic species, 2) preservation of prey species (e.g. economically important species or species at risk), and 3) mitigating risks of direct human-wildlife conflict. We conclude by considering evidence for the costs of failure in predator removal and a discussion of alternatives to predator removal. Although there are social and economic motivations associated with predator removal (Reynolds and Tapper, 1996; Engeman et al., 2002; Eklund et al., 2017; Swan et al., 2017), we focus on the ecological motivations aiming to synthesize perspectives on this practice. In this context, we refer to removal interchangeably with killing or lethal control. Removal may also refer to translocation, however, translocating predators has generally been demonstrated as ineffective for reducing conflicts (Athreya et al., 2011; Linnell et al., 1997; but see Hazin and Afonso, 2014). We focus on examples of aquatic and terrestrial vertebrate predators and ecosystems that include urban and rural areas. Moreover, we restrict the scope of this review to native predators. Invasive species are a global threat to biodiversity (Doherty et al., 2016) and the problems associated with biological invasions, although not necessarily unique or distinct from the problems that create nuisance predator conflict, are sufficiently different from a conservation and management perspective (see Doherty and Ritchie, 2016). Specifically, we incorporated evidence from published and gray literature on a variety of predatory taxa and from studies with varied predator removal motivations.

Section snippets

Approach

Based on preliminary searches and our perceptions regarding the quality of the evidence base (i.e., most studies had replication or included appropriate controls) we opted to conduct a qualitative literature review rather than a systematic review. Because the scope of our paper was broad, we used general search terms of the title, keywords, and abstract of papers in the Scopus search engine: “predator remov*”, “cull”, and “predator control” to identify relevant literature (asterisks are

Synthesis

Our searches identified 141 empirical studies in which predator removal was studied by haphazardly culling predators with traps, guns, or poisons (N = 87), selectively removing predators (N = 10), controlled removal (i.e. a pre-specified number; N = 21), observing a natural decrease (N = 1), or in a simulation (N = 10). Studies were conducted on data from 1 to 78 years (mean ± SD = 9 ± 12 years). Most studies (N = 104) were conducted to evaluate whether predator removal could improve prey

Resulting in success

A prevailing hypothesis is that predator removal can be implemented to achieve wildlife management objectives. We predicted that predator removal would be successful in some contexts, specifically, when implemented as a solution for short-term conservation challenges in which the return or replacement of the predator population in the long-term is not necessarily relevant to success (see Table 1).

Discussion

We evaluated the two opposing hypotheses considering the (a) success or (b) failure of predator removal as in the conservation and management of ecosystems. We selected a qualitative approach to testing these hypotheses by searching for published evidence of success and failure. We identified examples of success but ultimately found much more consistent evidence for failure (Table 1). Evidence that removing predators achieved conservation-sound outcomes was context-specific (see Section 4.1).

Conclusion

Human-wildlife conflict will persist with direct impacts on ecosystems globally. Desire to manage predator populations will therefore continue in spite of growing conservation concern for many predators (and in some cases, recovery of their populations; Curtis et al., 2014).

Our review suggests that the success of predator removal depends on the motivation and design of the effort because of the variability in success identified across studies. More research is needed to determine whether

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