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Biological Conservation
journal homepage: www.elsevier.com/locate/biocon
Review
Evaluating the efficacy of predator removal in a conflict-prone world
Robert J. Lennox
a,⁎
, Austin J. Gallagher
a,b,c
, Euan G. Ritchie
d
, Steven J. Cooke
a
a
Fish Ecology and Conservation Physiology Laboratory, Department of Biology, Carleton University, Ottawa, Ontario K1S 5B6, Canada
b
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA
c
Beneath the Waves, Inc., Miami, FL 33133, USA
d
Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Burwood, Victoria 3125 Australia
ARTICLE INFO
Keywords:
Conservation and wildlife management
Fisheries and agriculture
Human-wildlife conflict
Predator-prey interactions
Rewilding
Trophic cascade
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 ex-
panding 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 ex-
perimental 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 ab-
sence 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 re-
moval be further developed and researched. Ultimately, humans must coexist with predators and learning how
best to do so may resolve many conflicts.
1. 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 pre-
dators can therefore have profound ecological effects in certain con-
texts, 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 fol-
lowing 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.), verte-
brate 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 ha-
zardous 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
https://doi.org/10.1016/j.biocon.2018.05.003
Received 12 June 2017; Received in revised form 30 April 2018; Accepted 7 May 2018
⁎
Corresponding author.
E-mail address: robert.lennox@carleton.ca (R.J. Lennox).
Biological Conservation 224 (2018) 277–289
0006-3207/ © 2018 Elsevier Ltd. All rights reserved.
T
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 system-
atically 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, 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 lit-
erature 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) pre-
servation 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 pro-
blems 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 in-
corporated evidence from published and gray literature on a variety of
predatory taxa and from studies with varied predator removal moti-
vations.
2. Approach
Based on preliminary searches and our perceptions regarding the
quality of the evidence base (i.e., most studies had replication or in-
cluded 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, key-
words, and abstract of papers in the Scopus search engine: “predator
remov*”,“cull”, and “predator control”to identify relevant literature
(asterisks are wildcards in the Scopus search engine). Reference lists in
identified literature were consulted for additional resources and sear-
ches were repeated in Google Scholar. Articles were appraised at the
title and then abstract level for inclusion in a synthetic table. Referring
to our definition of success (see below), we sorted literature into suc-
cessful and failed applications of predator removal and by the objective
of the study in removing predators. All searches, filtering and analysis
were conducted by the same individual (RJL) following input from co-
authors. Bibliometric analyses were conducted in R (R Core Team,
2017). Figure plotted using the ggplot2 package (Wickham, 2009).
Included studies were stored in a table (Supplementary material) with
the predator species, motivation for removal, study duration, experi-
mental method, our evaluation of success or failure (or equivocality),
the removal method, a description of the study, and a citation (if not
included in main text).
2.1. Defining successful predator removal
Success is a difficult outcome to define in predator removal because
the motivations may be variable and idiosyncratic. Although we define
success in the context of ecological responses, we acknowledge that
successful predator removal must also consider the socioeconomic di-
mensions. For governments, the decision to implement predator re-
moval may be a balance between satisfying demands of constituents for
safety or prosperity against national or international agreements to
protect species and economic externalities associated with wildlife,
particularly ecological integrity. Nonetheless, we approach it from a
conservation perspective insofar as removal must not cause long-term
change or damage to the ecosystem while demonstrably benefiting the
prey species, be they domestic animals (e.g. reduced rates of depreda-
tion), species at risk (e.g. increased local abundance or population
growth rate) or of economic concern (e.g. increased harvest yield), or
humans (e.g. reduced conflict or fear from predators). From an ecolo-
gical and management perspective, we propose that successful predator
removal would reduce predator population to a size (or demographic state)
that would not negatively impact the persistence of that population or its
competitive status relative to mesopredators, but still provide demonstrable
benefits to the prey species following predator removal (Table 2).
Correlative methods used to evaluate success broadly match popu-
lation trends of predator and prey species and ascribe outcomes (in
terms of predator or prey densities) to the removal. Correlative ap-
proaches may lack the power to identify mechanisms (at least in the
short term) driving population dynamics (Grubbs et al., 2016;
Marcström et al., 1988) but can still provide insight into processes
underlying prey population dynamics, particularly where experiments
are infeasible. This can be observed in open marine systems where
marine mammal culling programs may be tested by measuring corre-
lations with fishery yields (Bax, 1998;Morissette et al., 2012). Short-
comings of retrospective analyses and correlational studies render it
difficult to identify evidence supporting any positive effects accrued
from predator removal, particularly in the context of different problems
that arise where predator removal is being considered as a management
strategy.
Experimental approaches to predator removal have more power to
detect main effects on livestock depredation or species recovery.
Controlled experiments using reference sites may be necessary but be-
fore-after-control-impact (BACI) studies can be useful to relate demo-
graphic trends to predator removal; however, BACI cannot account for
changes to the environment that occur over time (e.g. Hervieux et al.,
2014). Marcström et al. (1988) monitored grouse (Bonasa bonasia,La-
gopus lagopus,Tetrao tetri) and capercaillie (Tetrao urogallus) popula-
tions across eight years, the first four with fox (Vulpes vulpes) and
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
278
marten (Martes martes) removal followed by four years without killing.
Although removal improved nesting success and increased adult density
over time, the authors still cautioned that factors other than predator
removal could have stimulated the increases. Simulations can be useful,
such as Martin et al. (2010), in which the number of removed raccoons
(Procyon lotor) necessary to achieve oystercatcher productivity (Hae-
matopus palliatus) was simulated, suggesting that the specific number
targeted should depend on the density of raccoons. Ernest et al. (2002)
similarly used simulation to calculate the number of mountain lion
removals necessary to reduce extinction risk of bighorn sheep (Ovis
canadensis). Such frameworks are one solution for testing the efficacy of
predator removal programs prior to implementation.
Attributing predator removal to livestock depredation, species re-
covery, or direct conflict with humans is complicated when the mea-
surement of outcomes is restricted to relatively short intervals after
predator removal. The period immediately following the action of
predator removal is the period most likely to indicate a reduction in
predator density and conflict and an increase in prey density, but this
may decrease at longer post-treatment intervals (e.g. Engeman et al.,
2006;Magella and Brousseau, 2001;Sagør et al., 1997;van Eeden et al.,
2018). Short-term increases to nesting success or juvenile survival fail
to consider density-dependence that manifests in the longer-term and
cannot demonstrate success of predator removal when there is no de-
monstrated benefit to the population in subsequent years. Several stu-
dies observed increased nesting success of ducks following predator
removal (Garrettson and Rohwer, 2001;Pearse and Ratti, 2004;Pieron
and Rohwer, 2010), but a longer-term study conducted by Pieron et al.
(2013) found that benefits to nesting did not carry over to the breeding
population and therefore the latter studies provided no evidence to
support predator removal (see meta-analysis by Côté and Sutherland,
1997). Although removal must generally be sustained (e.g. seasonally
or annually) for benefits to be realized, success must be demonstrable
and persistent over time (Bergstrom et al., 2014;van Eeden et al.,
2018). Moreover, the benefits must outweigh the costs (Chessnes et al.,
1968). A lack of longer term monitoring to determine whether predator
removal was effective limits the power to interpret whether it was a
successful intervention (van Eeden et al., 2018).
3. 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), con-
trolled 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 populations, followed by studies de-
termined to evaluate impacts on domestic animals (N= 28) and direct
interactions with humans (N= 8).
We evaluated a large number of these studies (N= 37) to have
equivocal results, for example owing to a lack of statistical analysis,
poor control to detect main effects, or because the study did not include
sufficient information with which to make a determination about suc-
cess (Fig. 1). Frequently, this arose because predator removal resulted
in increased breeding success without evidence that this contributed to
subsequent increases in the population. Although the scope of a study
may have intentionally been focused on briefer time scales or questions,
for our purposes and based on our definition of success we could not
describe such results as indicative of success. Most studies we evaluated
we determined to have failed (N= 67) owing to direct evidence that
predator removal had either not succeeded in limiting the predator
population or had no statistical demonstration of success in reducing
livestock losses, increasing prey densities, or mitigating direct conflict
with humans (Fig. 1). Studies that were successful (N= 36) demon-
strated that predators were agents of additive mortality and that their
removal resulted in subsequent increases in prey.
An important caveat of this bibliometric approach is that studies
that were deemed to be successful or failed may have been so because
of some idiosyncrasy in the sampling protocol that could not be ex-
pected to be consistent among studies. When measurements were made
(e.g. when prey abundance was measured) and when interventions
were undertaken (e.g. what season the predator was removed in) could
influence the outcome and the determination of success or failure.
Successes or failures could also emerge consistently for similar taxa that
were overrepresented in the literature, a limitation of the vote-counting
approach that we used to present percentages.
The numbers presented in this bibliometric analysis are intended
only to represent relevant information and a summary of published
literature and are not intended to provide evidence for or against pre-
dator removal without further context. Below, we discuss factors as-
sociated with success and failure in predator removal with the objective
of introducing more context, nuance, and interpretation of the litera-
ture covered in our bibliometric review along with other research fo-
cused on the relationship between predators and their prey in an effort
to address the question of predator removal from a conservation per-
spective.
4. Factors contributing to success and failure
4.1. Resulting in success
A prevailing hypothesis is that predator removal can be im-
plemented to achieve wildlife management objectives. We predicted
that predator removal would be successful in some contexts, specifi-
cally, when implemented as a solution for short-term conservation
challenges in which the return or replacement of the predator popula-
tion in the long-term is not necessarily relevant to success (see Table 1).
Fig. 1. Bibliometric summary of studies reviewed in this paper based on the
three motivations for predator removal and the outcome. Studies are sum-
marized in Table S1. Success was evaluated based on the definition in Table 2.
Shading indicates our evaluation of the study as representing a success, failure,
or equivocal outcome. Equivocality was ascribed for studies with inconclusive
study design to determine success based on our definition.
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
279
Table 1
In our literature review we identified outcomes of experiments that yielded success or failure given three motivations for removing predators and based on our definition of success (see Table 2). Here we review 11 of the
common outcomes of predator removal, two of which we considered to be successful and nine of which we considered to result in failure. Rows are populated with examples of the predator removed and references to
literature demonstrating the given outcome for different motivations. References without species are those that removed multiple predators or predators were generalized (e.g. in a simulation). Refer to Table S1 for a
comprehensive review of the appraised literature.
Outcome Evaluation Protection of domestic animals Preservation of prey Mitigating risks of direct
conflict
Removal of “problem predators”known to instigate conflict reduces future
conflict
Success •Canis latrans:Blejwas et al., 2002,Till and
Knowlton, 1983
•Canis lupus:Blejwas et al., 2002
•Lynx lynx:Stahl et al., 2001
•Arctocephalus pusillus:Makhado et al., 2009
•Larus michahellis:Sanz-Aguilar et al., 2009
Measurable reduction in conflict or improvement in prey demographics
while maintaining predators in the ecosystem
Success •Canis lupus:Bradley et al., 2015
•Lynx lynx:Herfindal et al., 2005
•Pogonias cromis:George et al., 2008
•Arctocephalus pusillus:Weller et al., 2016
•Fletcher et al., 2010
•Canis latrans:Smith et al., 1986;Reynolds et al., 2010
•Canis lupus:Bjorge and Gunson, 1985,Boertje et al., 1996,
Gasaway et al., 1983,Hayes et al., 2003,Hervieux et al., 2014,
Keech et al., 2011,Potvin et al., 1992
Conspecifics immigrate, replace predators, and conflict persists Failure •Canis lupus (Wielgus and Peebles, 2014
[refuted by Poudyal et al., 2016],
Fernández-Gil et al., 2016
•Caracal caracal Bailey and Conradie,
2013;Conradie and Piesse, 2013)
•Puma concolor (Peebles et al., 2013), Canis
dingo (Allen, 2014, 2015)
•Greentree et al., 2000
Removal was equally or less effective than non-lethal alternatives Failure •McManus et al., 2015,Palmer et al., 2005
Predator population becomes imperilled by introgression with congeneric
species or suffers depensation due to superadditive mortality
Failure •Canis lupus:Chapron and Treves, 2016
•Puma concolor:Cooley et al., 2009
•Ursus arctos:Swenson et al., 1997
Unknown mechanisms of compensatory mortality reveal short-term
success (e.g. improved nesting or hatching rates) but no evidence of
increases in abundance or density in subsequent years
Failure •Amundson et al., 2013,Ellis-Felege et al., 2012,Littlefield and
Cornely, 1997
Conspecifics increase reproductive rate, predator population increases, and
conflict persists
Failure •Canis latrans:Knowlton, 1972
Density independent conflict yields no benefits of removing predators on
the incidence of conflict
Failure •Canis lupus:Eggemann, 2015
•Epinephelus itajara:Frias-Torres, 2013
•Larus cachinnans:Bosch, 1996
•Puma concolor:Peebles et al.,
2013,Teichman et al., 2016
•Ursus americanus:Obbard
et al., 2014
•Ursus arctos:Artelle et al.,
2016
•Ursus thibetanus;Huygens
et al., 2004
•Wetherbee et al., 1994
Disappearance of a predator releases a mesopredator from competition,
which maintains depredation
Failure •Canis dingo:Wallach et al., 2010
•Canis lupus:Rutledge et al., 2012
•Corvus brachyrhynchos:Clark et al., 1995
•Lynx lynx:Palomares et al., 1995;Bodey et al., 2011,Prugh and
Arthur, 2015
Loss of predators deregulates pathogens within populations, resulting in
increased disease-related mortality of prey
Failure •Packer et al., 2003
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
280
4.1.1. Protection of domestic animals
When predators encroach on property or property development
intersects with predator ranges, the presence of predators can become
problematic if they threaten production animals (e.g. farms, ranch land,
aquaculture facilities). Apex predators such as sharks, wolves (Canis
lupus), dingoes (Canis dingo), lions (Panthera leo), tigers (Panthera tigris),
cougars (Puma concolor), jaguars (Panthera onca), and leopards
(Panthera pardus), for example, can affect the livelihoods of pastoralists,
but so too can mesopredators (Davis et al., 2015) such as coyotes (Canis
latrans), jackals (Canis spp.), crows (Corvus corax) and red foxes (Vulpes
vulpes).
Predator removal can acutely reduce conflict when known predators
are dispatched, but removals must often be of sufficient frequency or
magnitude that they actually affect the population size or structure of
the predator such that immigration does not compensate for removal
(Bjorge and Gunson, 1985;Herfindal et al., 2005;Landa et al., 1999).
For example, Bradley et al. (2015) found that wolf removal was suc-
cessful at reducing livestock depredation if the entire pack was elimi-
nated. Wagner and Conover (1999) killed coyotes and found that pas-
tures with removal experienced slower rates of lamb depredation
following removal (but see Treves et al., 2016 Supplementary material).
In some cases, the success of predator removal is highly concentrated
and neighbouring areas will suffer increased pressure; this may be a
success on a small spatial or temporal scale but in general it would not
achieve the desired outcomes (Santiago-Avila et al., 2018). Whether
predators are actively targeting livestock or are encountering them
opportunistically can affect success of the removal program. Odden
et al. (2013) suggested that increased sheep production and decreased
roe deer (Capreolus capreolus) density triggered a shift by lynx (Lynx
lynx) towards sheep depredation, a type III functional response (i.e.
preferentially targeting abundant species) that supports either lynx
removal or roe deer conservation/supplementation. Moreover, there
are different patterns of depredation for male and female animals.
Males are generally more frequent livestock predators than females
among solitary species, requiring selective removal to be successful
(Felids: Odden et al., 2002; polar bear Ursus maritimus:Stenhouse et al.,
1988).
Individuals within a population can differ in their propensity to
depredate livestock for many reasons. Selective removal of individuals
known to depredate livestock could be most effective in reducing future
problems than haphazard culling (e.g. Woodroffe and Frank, 2005), the
challenge being to accurately identify the offending individuals (Stahl
et al., 2002;Swan et al., 2017). In our bibliometric review, we ascribed
success to 40% of selective removals (N= 10) and only 19% in which
predators were non-selectively removed by haphazard culling (N= 87)
or public hunts (N= 11). Blejwas et al. (2002) found that only selective
removal of coyotes following depredation events reduced subsequent
depredations and not pre-emptive or non-selective removal. Some
predators socially transmit knowledge that livestock are prey (e.g. to
offspring; Mondolfiand Hoogesteijn, 1986) and systematic removal of
known predators could instill wariness in predators by “hunting for
fear”(Cromsigt et al., 2013) or social transmission of risk (e.g. invasive
lionfish; Côté et al., 2014). In spite of a long history of predator per-
secution, we did not identify examples that support this, suggesting
more research is needed to address this question.
4.1.2. Preservation of prey species
When prey species or populations are declining in abundance, there
may be added pressure for managers to take remedial action (Lessard
et al., 2005;Reynolds and Tapper, 1996). This is particularly true of
economically important species that are hunted or fished or those that
are at risk of extinction. Most examples of success were from studies
aiming to preserve prey, although not on a relative basis as only 26%
were deemed to be successful.
Many of the most important terrestrial game species are herbivores
whose populations may be moderated by depredation. Removing pre-
dators can release prey species from predation and, so long as mortality
from those predators is additive and not compensatory, the prey species
could increase in following years and re-establish a higher abundance.
The most successful examples of preserving prey by removing predators
emerge from studies of predator removal in northern ecosystems with
fewer trophic linkages and more direct influences of predators. Jarnemo
and Liberg (2005) correlated roe deer (Capreolus capreolus) population
growth to a disease outbreak that reduced red fox density and released
the deer from predation. Moose and caribou (Rangifer tarandus caribou)
survival has also improved following removal of wolves as demon-
strated by several studies observing increases in prey abundance
(Boertje et al., 1996;Hayes et al., 2003;Gasaway et al., 1983;Keech
et al., 2011).
Prey species suffering from depensation may specifically benefit
from predator release (e.g. Liermann and Hilborn, 2001;Stephens and
Sutherland, 1999). For example, cormorant (Phalacrocorax auritus)
culling preceded yellow perch (Perca flavescens) abundance increases in
Lake Huron, suggesting that removing the predators assisted in rebound
of its prey. Although human intervention is generally the mechanism
for small population size of prey species, the added pressure of preda-
tion can still be linked to depensation (Gascoigne and Lipcius, 2004;
Kramer and Drake, 2010;Liermann and Hilborn, 2001). Juveniles of
Table 2
Proposed maxims of predator removal summarizing important findings about successful applications of predator removal for management.
1 Predator removal is an interdisciplinary topic necessitating consideration of ecological, economic, sociological, political, and other dimensions.
2 Failure to consider ecological issues when initiating predator removal can harm the ecosystem.
3 From an ecological perspective, successful predator removal would reduce predator population to a size (or demographic state) that would not negatively impact the
persistence of that population or its competitive status relative to mesopredators, but still provide demonstrable benefits to the prey species following predator removal.
4 The functional response of the predator is essential to consider because it influences the rate of depredation of prey species.
5 Targeted removal of problem individuals may be an effective application of predator removal (Swan et al., 2017), as opposed to indiscriminate or retaliatory killing, but it is
logistically difficult to confidently identify culprit predators (Stahl et al., 2002).
6 Killing predators seems to generally result in an increase of local depredation of livestock resulting from demographic compensation via increased birth rates of predators
(Knowlton, 1972), immigration (Sagør et al., 1997), or release of mesopredators/invasive species (Wallach et al., 2015).
7 Among humans, there are broad demographic differences in attitudes towards predators, with support for predator removal generally from older and more rural individuals
(Andersone and Ozolinš, 2004;Lüchtrath and Schraml, 2015).
8 Justifiable objectives for removal, especially the number to be removed, are necessary in planning predator removal rather than haphazard killing. Understanding the
demographics and population dynamics of the predator is therefore essential. Adaptive management approaches can be applied to attempt sustainable removal that does not
imperil the predator population (e.g. Martin et al., 2010).
9 Whether predator removal is actually effective at reducing conflicts or satisfying human attitudes towards predators is essential to its overall success as a management
practice but the evidence for it is either equivocal or deficient.
10 There are increasing examples of non-lethal alternatives to predator removal, although many require scientific validation (Ogada et al., 2003;Okemwa, 2015).
11 Evidence that conflicts are mechanistically linked to depredation is important before beginning predator removal, along with evidence that predator removal will resolve the
conflict, which can be tested via simulation (e.g. Morissette et al., 2012).
12 Coexistence with predators is possible and the most sensible way forward, but interdisciplinary research is necessary to continue to refine understanding of the human
dimensions of predator removal (Carter and Linnell, 2016; Johnson and Wallach, In Press; Woodroffe et al., 2005).
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
281
species at risk such as marine turtles (Gascoigne and Lipcius, 2004) and
salmon (Oncorhynchus sp.; Wood, 1987) that rely on safety in numbers
to saturate predators during migration can undergo rapid declines from
depredation (Hervieux et al., 2014;Liermann and Hilborn, 2001) and
predator removal may facilitate increased juvenile survival and re-
cruitment to such populations (Engeman et al., 2006;Pichegru, 2013;
Hervieux et al., 2014;Makhado et al., 2009). However, it is not uni-
versally effective and alternate actions may have higher success than
predator removal (Ratnaswamy et al., 1997). Improving juvenile sur-
vival may be a relevant management outcome for some species, but it
does not necessarily improve population growth rate or abundance
when there is density dependent or otherwise compensatory mortality
and therefore studies that only observed increased egg hatching or ju-
venile densities were evaluated as equivocal without longer-term in-
vestigation (see Pieron et al., 2013). Considering generalist predators
that consume fewer prey at smaller prey densities (type III functional
response characterized by a logistic-type relationship between prey
density and prey consumption, in which depredation is low until prey
achieve a relatively high density and predators begin targeting that
species), predator removal will not likely have a considerable effect
because they would more likely switch to alternative prey instead of
expending energy pursuing the rarer prey species (Murdoch, 1969; e.g.
Middlemas et al., 2006). Specialization may also occur within species,
in which cases the selective removal of specialized individuals can be an
effective application of predator removal to release prey from depre-
dation pressure (Sanz-Aguilar et al., 2009). Although predator removal
may be effective when problems arise because of specialization, re-
moval is not necessarily the most effective management option; alter-
natives such as exclosures may be more effective for reducing depre-
dation and recovery of species at risk and should be tested (Rimmer and
Deblinger, 1990;Reynolds and Tapper, 1996;Smith et al., 2011;
Stringham and Robinson, 2015). However, the logistics of fencing off
entire areas (e.g. breeding sites) to exclude predators are questionable
and the long-term consequences can also be destructive (Hayward and
Kerley, 2009).
4.1.3. Mitigating risks of direct human-wildlife conflict
Direct human-wildlife conflict has stimulated efforts to kill pre-
dators after attacks or a pre-emptive strike against future conflict
(Gallagher, 2016). Few examples in the literature were identified that
studied predator removal for relieving direct conflict between humans
and predators (N=8), with no examples of success. Fukuda et al.
(2014) was determined to provide equivocal evidence for predator re-
moval because it lacked proper control. However, they provide a salient
example for future research in which predators that attack humans may
learn to target them, in which case removing individual animals that
have attacked humans could reduce future conflict (e.g. saltwater cro-
codiles Crocodylus porosus). There is a threat of animals habituating to
humans, which may lead to more direct conflict in subsequent years
and require removal of problem individuals (Linnell and Alleau, 2016).
Predators infected with rabies or other diseases that increase conflict
may also require lethal control (Linnell and Alleau, 2016). However,
there is limited evidence that targeted killing of animals that have a
history of interacting with humans reduces future conflicts, probably
because such events are rare to observe, precluding experimentation or
analysis (Swan et al., 2017).
4.2. Resulting in failure
The prevailing alternate hypothesis that we tested in conducting this
literature review was that predator removal is not an effective tool for
conservation or management of ecosystems. We reviewed the literature
to identify research that described experiences or experiments with
predator removal that have yielded perverse impacts on the ecosystem,
or failure to achieve the desired objectives, which were different de-
pending on the motivation for predator removal. Thus, we have divided
this section into the familiar subheadings based on those motivations
(see Table 1).
4.2.1. Protection of domestic animals
Protecting domestic animals by removing predators should reduce
the rate of depredation on those domestic animals (Eklund et al., 2017).
However, predator removal efforts fail when depredation rates do not
respond to culling because the predator population compensates or is
replaced by another predator. When there are multiple predatory spe-
cies, Kissui (2008) found that pastoralists had difficulty identifying
which species was responsible for livestock depredation and that higher
visibility of lions during daytime caused them to be incorrectly accused.
Targeted killing of leopards and caracals (Caracal caracal;Bailey and
Conradie, 2013;Conradie and Piesse, 2013), cougars (Peebles et al.,
2013), dingoes (Allen, 2014, 2015), and wolves (Wielgus and Peebles,
2014 [refuted by Poudyal et al., 2016]; Fernández-Gil et al., 2016)
designed to reduce livestock depredation actually increased depreda-
tion in subsequent years (but see Bradley et al., 2015). Removal of
adults may have triggered compensation via rapid replacement by im-
migrants in open systems (e.g. Baker and Harris, 2006;Bjorge and
Gunson, 1985;Lieury et al., 2015;Sagør et al., 1997), enhanced local
juvenile survival (Kemp, 1976;Peebles et al., 2013), or increased re-
productive rates (Knowlton, 1972;Pitt et al., 2001). These demographic
responses maintain or increase the number of local predators, stabilize
the probability of further conflict, and represent distinct failures (Boyce
et al., 1999;Sacks et al., 1999). Demographic responses of predators to
culling may therefore render predator removal largely ineffective unless
removal is so extensive that it alters predator demography on a broad
scale, perhaps to impose an alternative stable state (Greentree et al.,
2000;Herfindal et al., 2005). Removal can imperil the predators by
accelerating their population declines if mortality is additive (or even
super-additive; Creel and Rotella, 2010), for example when it instigates
increased poaching (Chapron and Treves, 2016) or infanticidal beha-
viour (e.g. cougar: Cooley et al., 2009; grizzly bear Ursus arctos:
Swenson et al., 1997; lion: Packer et al., 2009). Removal can also isolate
remaining individuals, resulting in increased dependence on livestock
in the absence of a group that would otherwise target wild prey (Bjorge
and Gunson, 1985) or result in hybridization and degradation of genetic
integrity (Rutledge et al., 2012). Short of predator eradication, removal
generally does not protect domestic animals in the long-term.
Extensive removal of predators or eradication of top predators can
also release subordinate species from competition (i.e. mesopredator
release; Crooks and Soulé, 1999). Mesopredators can be of equal or
greater possible or perceived threat to livestock and may be invasive
species that become difficult to remove (Gross, 2008;Wallach et al.,
2010), with cascading changes at other trophic levels (Hebblewhite
et al., 2005;McPeek, 1998;McClanahan and Muthiga, 1988;Ritchie
and Johnson, 2009). Mesopredator release can undermine predator
removal and sustain depredation of domestic animals. In some cases,
multiple mesopredators replace one extirpated top predator, compli-
cating further control efforts.
4.2.2. Preservation of prey species
Removing predators theoretically reduces the extent to which prey
species are removed from a population (e.g. Weller et al., 2016) given
an assumption that predation contributes to additive and not compen-
satory mortality of the prey species, and therefore removal of the pre-
dators will directly contribute to an increase in prey (e.g. Flaaten,
1988). Evidently, this presupposes negligible effects of bottom-up
processes (see Grange and Duncan, 2006;Elmhagen and Rushton,
2007), that the prey would not be limited by density-dependent re-
source limitation, and that prey is limited by a specific predator (Frias-
Torres, 2013;Parker, 1984). However, most acknowledge both forms of
regulation are simultaneously important in ecosystems and the relative
importance of top down vs. bottom up control can shift in relation to
productivity (Oksanen et al., 1981). Despite repeated efforts to connect
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
282
predation to declines of economically important fishes, evidence for
such a relationship is tenuous (Anon., 1986;Trzcinski et al., 2006).
Eggemann (2015) also suggested that wolf depredation of moose (Alces
alces) is density independent, meaning that reduced pack size could not
succeed to increase moose escapement availability to hunters (also
Kauhala et al., 2000). Similarly, Serrouya et al. (2017) showed that
removing moose was effective for recovery of caribou in British Co-
lumbia because of apparent competition between wolves and caribou;
although moose removal was not compared to wolf removal, this shows
how predators can be incorrectly persecuted if alternative solutions to
maintaining prey densities are not explored. Using Ecopath with Ecosim
for mass balanced simulation based on foraging arena theory,
Morissette et al. (2012) tested whether marine mammal removal would
increase fishery yield and suggested that it would more likely lead to
reductions than increases because of limited actual competition be-
tween fisheries and whales (see also Gerber et al., 2009). Yodzis (1998)
also predicted a decline of fisheries yields during cape fur seal (Arcto-
cephalus pusillus) culling programs that were proposed to increase
yields. Lessard et al. (2005) simulated seal removal and predicted an
increase in Pacific salmon smolt survival but suggested that it might
increase predatory fish populations, which would replace the seals in
depredating the smolts; generalist predators such as seals often regulate
multiple populations within a community and removing these predators
can lead to disequilibrium in the ecosystem.
The trophic position of the predator contributes to its functional
response to changes in prey, an important factor when considering re-
moval (Bowen and Lidgard, 2013). Removing a mesopredator will most
likely yield compensatory depredation by other mesopredators (Clark
et al., 1995). Elimination of a top predator could release herbivores
from control, resulting in extensive damage to landscapes and changes
to habitat suitability that cause shifts in the community (Bertness et al.,
2014;Ripple and Beschta, 2006). Hunters can compensate for predation
mortality but will generally remove highly fit phenotypes (Allendorf
and Hard, 2009) whereas predators target weak or diseased prey
(Genovart et al., 2010;Krumm et al., 2010;Quinn and Cresswell,
2004); loss of predators can then proliferate disease within prey po-
pulations (Packer et al., 2003) and can spill over to infect domestic
animals (Cross et al., 2007). Even when removal is successful in the
short-term, compensatory processes may regulate predator populations
such that removal is ineffective in the long-term (e.g. Donehower et al.,
2007). Long-term studies or simulation models are necessary to detect
effects of predator removal on prey (see Costa et al., 2017).
4.2.3. Mitigating risks of direct human-wildlife conflict
Predatory animals are often perceived as threats to human safety in
spite of infrequent interactions and small odds of actual conflict relative
to many other habitual activities such as driving cars (Slovic, 1987).
According to the social amplification of risk framework, empirically
rare events contribute disproportionally to concern among the public
and lead to economically, socially, or ecologically illogical responses
(e.g. fear of flying; Kasperson et al., 1988). This framework could be
applicable to human-wildlife conflict if the perceived risk of direct at-
tack on humans is higher than the actual risk. Sharks are often victi-
mized by social amplification of risk, which has resulted in publicized
and prominent state-sponsored programs that aim to cull sharks near
beaches (e.g. Wetherbee et al., 1994;Gallagher, 2016). The major
failure of shark culling programs, however, has been exemplified by a
lack of evidence that it actually decreases attacks (Wetherbee et al.,
1994), arising in part because many large predatory shark species are
migratory and therefore there is a low probability that locally-based
actions will be effective once they cease and sharks from surrounding
and more distant areas move into these managed areas continually
(Holland et al., 1999). Gray and Gray (2017) found limited support
among patrons for lethal control of sharks. Correspondingly, we found
no research asking whether culling programs actually affected the
perception of risk by patrons; safety is difficult to guarantee, and a
perception of safety may encourage reckless behaviour (e.g. ignoring
key risk factors associated with shark attack) that increases the like-
lihood of negative encounters with sharks (e.g. swimming offshore).
The legacies of such efforts could instead just be negative public per-
ception of the animals, increased fear, and impoverished conservation
status of the targeted species. Perversely, Teichman et al. (2016) found
that human-cougar conflict was higher in areas of cougar trophy
hunting yet Gilbert et al. (2017) suggested that economic value of
cougar populations exceeds the costs because they control deer popu-
lations that cause costly collisions with vehicles. Skonhoft (2006) dis-
cussed this in terms of Scandinavian wolves, suggesting there is an
equilibrium possible between the economic losses of lucrative moose
depredated by wolves (Alces alces) and gains in terms of reduced ve-
hicle-moose collisions and damage to foliage caused by moose browsing
in the winter, emphasizing the value of maintaining predators and the
costs of predator removal.
There was no direct evidence that removing predators changes
outcomes for human-wildlife conflict. Obbard et al. (2014) found no
influence of black bear removal on future conflict with humans and
Artelle et al. (2016) perversely observed that removal of grizzly bears
was followed by no difference in future conflicts rather than a reduc-
tion. Although data in Artelle et al. (2016) do not suggest causality, it
does indicate that removal was not successful at mitigating conflicts.
Treves et al. (2010) further suggested that the number of black bears
(Ursus americanus) killed by hunters did not reduce, and was actually
correlated with increases in, reports of conflict in subsequent years
(although it is relevant to note that complaints were not necessarily
related to predatory activity of bears, but also property damage). Ap-
parently, overlap between humans and black bears increases during
poor years when urban resources aggregate the animals, meaning that
removal of predators in these years has disproportionately high impact
on the population (Baruch-Mordo et al., 2014).
5. 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 con-
text-specific (see Section 4.1). Removing predators presumes that eco-
system-level responses are predictable (Ramsey and Norbury, 2009),
yet theoretical and empirical evidence often suggests the contrary (Bax,
1998;Ruscoe et al., 2011;Yodzis, 2000). An exception may exist in
ecosystems where predators and prey are very closely linked (e.g.
northern terrestrial ecosystems) or the prey are suffering from de-
pensatory population declines associated with depredation by predators
with a type II functional response. Although predators can influence
ecosystems (Holt et al., 2008;Nelson et al., 2004), other factors can
make the ultimate response of an ecosystem unpredictable, even with
rigorous scientific evaluation. The full range of complexity at the eco-
system scale is poorly understood, especially as it pertains to processes
such as parasites in ecosystem dynamics (Roche et al., 2012). This was
observed consistently in study designs, which were often either short in
duration or lacking in control, rendering it difficult to avoid type I
error.
Many governments are responsible for establishing and maintaining
protected areas, zoning property (for agriculture or developing buffers),
and formulating wildlife management regulations (Rands et al., 2010;
Treves et al., 2017). Strong policy based on available evidence can
contribute to effective conservation of predators in many ways, in-
cluding the establishment of suitable regulations and protected areas
(Linnell et al., 2001). However, predators are important components of
the landscape not just in designated areas but also in areas of human
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
283
use (Dorresteijn et al., 2015;Gilbert et al., 2017;Kuijper et al., 2016;
López-Bao et al., 2017). When conflicts arise, retaliatory killing by local
stakeholders may be understandable but can undermine conservation
efforts for both predators and the broader ecosystem. It is important to
accurately document the movements and actions of depredating species
and maintain records of conflicts to determine the appropriate course of
action and to advance the science of predator conflict to develop re-
solutions. In its present form, our findings suggest that success in pre-
dator removal is highly contextual and should not be assumed by
management without rigorous testing.
5.1. Alternative actions for managing human-predator conflict
Human-predator conflict challenges managers because depredation
can be damaging to some livelihoods and traumatic for individuals (e.g.
pastoralists, aquaculturists, fishers; Butler, 2000;Graham et al., 2005;
Mishra, 1997;Patterson et al., 2004). Attitudes of retaliation (Holmern
et al., 2007;Kissui, 2008;Thorn et al., 2012) are understandable, even
though conflicts tend to be isolated incidents (Cozza et al., 1996;
Chavez and Gese, 2005). Economic losses to depredation are, however,
generally less than those attributable to other sources of mortality such
as disease (Breck and Meier, 2004;Frank, 1998;Mazzolli et al., 2002;
Mizutani, 1999;Kissui, 2008;Rasmussen, 1999). Livestock often com-
prises smaller components of the diet of predators than assumed by
some pastoralists (Allen, 2015;Boast et al., 2016;Davis et al., 2015).
Kaltenborn and Brainerd (2016) suggested that restoration of predators
to large population sizes and then opening recreational hunting seasons
could be a more effective alternative to balance socioeconomic objec-
tives. However, sustainable harvest limits are incalculable without de-
mographic data (Packer et al., 2009;Treves, 2009). Moreover, human
harvests tend not to be non-selective for problem predators (Sunde
et al., 1998) or can undermine conservation (Creel and Rotella, 2010).
Where livestock comprise a more important food source for predators,
conservation or restoration of native prey sources could mitigate losses
(Meriggi and Lovari, 1996;Odden et al., 2013). Husbandry practices
can alternatively reduce conflict with wildlife without ecological issues
or social controversy (e.g. Jackson and Wangchuk, 2004;Johnson and
Wallach, 2016). Fencing is already used by pastoralists (Hayward and
Kerley, 2009) with variable success (Eklund et al., 2017). Birthing of
calves during a short period may facilitate predator satiation, reducing
depredation on farms (Palmeira et al., 2008). Calves can also be kept
centralized and away from edges (Palmeira et al., 2008). Deterrent
devices (e.g. fladry) also hold promise for reducing depredation (Ogada
et al., 2003;Okemwa, 2015), evidenced by a 93–97% reduction in
depredation of aquaculture sites using a non-lethal deterrent by seals
(Götz and Janik, 2016). In scientific study, predator removal should be
tested against realistic alternatives because in some cases deterrents are
just as effective (Harper et al., 2008;Ratnaswamy et al., 1997) and may
be more economical (McManus et al., 2015). When conflicts do arise,
the costs can be offset with subsidies (Bulte and Rondeau, 2005;
Dickman et al., 2011;Mishra et al., 2003). Challengingly, some gov-
ernments do not have the resources to support conservation initiatives
or compensate farmers for losses and in others, the systems are not
developed to properly address the problems (Chen et al., 2015). In
developing countries, this leads to continued persecution of predators,
maybe out of bare necessity to maintain herds in some cases (Dar et al.,
2009), but improved education and validation of effective alternatives
hold promise for resolving conflict.
When prey species decline, hunters may support and lobby for
predator removal (Franzmann, 1993) and conservation movements
may support protection of species at risk by controlling their predators.
Species persistence is considered a priority of conservation science and
is often nested within the laws of regional and national management
plans. Predator removal may appear to be a logical solution for main-
taining adult populations and increasing juvenile survival during spe-
cies declines; however, our results clearly show that studies are needed
to demonstrate this (Oro and Martínez-Abraín, 2007). Deterrents or
barriers can reduce predator access to endangered species and may be
more effective and economical in many scenarios (Shivik, 2006;Smith
et al., 2011;Yurk and Trites, 2000). Emerging solutions that use sensory
modalities to mitigate predation can also yield promising results, for
example, Neves et al. (2006) tested taste aversion methods of reducing
nest predation of endangered roseate tern (Sterna dougallii). Guardian
animals have also shown promise for livestock (Meadows and
Knowlton, 2000;Smith et al., 2000;van Bommel and Johnson, 2012)
and species at risk (King et al., 2015).
The willingness to pay for hunting/fishing for large predators may
be high, species of recreational importance tend to have higher ac-
ceptability and be better conserved, and illegal hunting can undermine
ecological, economic, and sociological objectives of wildlife manage-
ment. Therefore, managed hunts or fisheries targeting predators have
been proposed as a solution to reduce poaching, maintain stable pre-
dator populations, fund conservation initiatives, and increase accept-
ability of some predators (Creel et al., 2016;Gallagher et al., 2016;
Kaltenborn and Brainerd, 2016;Lindsey et al., 2007). Sportspeople and
guides can keep watch for illegal activity, particularly in remote areas
and activity can also reduce predator activity (Harper et al., 2008). The
result of such managed hunts would, however, probably result in
random, rather than targeted, removal that would not likely have any
effect on the rate of predator conflict (Packer et al., 2009;Treves, 2009)
unless it can be confidently applied to maintain a smaller predator
population without resulting in depensation.
5.2. Social and economic costs of failure
Killing by people is the largest threat to the conservation of many
predators (Kissui, 2008;Ripple et al., 2016;Woodroffe and Ginsberg,
1998). In spite of the problems with implementing predator removal for
management, human-wildlife conflict persists (Treves and Karanth,
2003) and predator persecution and removal will likely continue, par-
ticularly when there is direct conflict between a human and a predator.
Research on human attitudes towards predators is plentiful, relating
demographics to perception of predators (e.g. nationality, gender, age;
Andersone and Ozolinš, 2004;Lüchtrath and Schraml, 2015). Attitudes
towards predators depend greatly on exposure and experience as well
as cultural values towards wildlife, for example, rural people tended to
favour control more than urban dwellers (Andersone and Ozolinš,
2004).
It should be possible to quantify the carrying capacities and de-
mographics of predators to maintain a smaller population of predators
to limit conflicts, although in general we found that this is likely only
possible via continued intervention (e.g. Landa et al., 1999). Careful
calculation and monitoring would be essential for this because of un-
anticipated changes in demography arising from human-induced mor-
tality and the potential for additive or super-additive (rather than
compensatory) mortality following intervention that imperils the pre-
dators (Creel and Rotella, 2010). Indeed, Bradley et al. (2015) found
that partial wolf pack removal was effective for mitigating livestock
depredation while maintaining wolves in the northern Rocky Moun-
tains; however, more rigorous methods can be implemented to calculate
removal targets. The justification for predator removal targets and how
they are defined is often weak and idiosyncratic. Strategies can include
controlled removal with a stated goal (e.g. 50% reduction), haphazard
culling (e.g. opportunistic removal), or selective removal (e.g. removing
problem individuals), with variation in the expected outcomes. In
Western Australia, the social licence and evidence for culling has re-
cently been questioned (Legge et al., 2017). Simulation to determine
the optimal number of predators to be removed to achieve conservation
objectives can assist with validating predator removal prior to im-
plementation (Ernest et al., 2002;Martin et al., 2010).
Modern management of predator conflicts must include stake-
holders (Breitenmoser, 1998) and consider predators in an ecosystem
R.J. Lennox et al. Biological Conservation 224 (2018) 277–289
284
context rather than as individual species in conflict with humans. There
is limited evidence that retaliation against a species or pre-emptive
culling decreases conflicts or generates a sense of security in landscapes
where predators exist. This points to a failure to consider the broader-
scale processes that regulate predator populations and ecosystems
(Berlow, 1999) as well as a lack of understanding of the human di-
mension of attitudes towards wildlife that promote negative percep-
tions. Moreover, it ignores the positive impacts of predators and intact
ecosystems by regulating herbivores, mesopredators, and disease. Pre-
dator removal can also disconnect public perceptions of nature by ac-
climating people to manipulated and arguably depauperate ecosystems
(Wallach et al., 2015), an outcome that can shift baselines and reduce
support for conservation initiatives (Chapron and Treves, 2016). This
problem is exemplified in coyote control, where Berger (2006) calcu-
lated a long-term expenditure of over a billion dollars for coyote re-
moval programs in the United States that were intended to improve the
sheep farming industry and wool production had no measurable ben-
efits across 78 years of data.
5.3. Study context and future research directions
Evaluating the contribution of predators and the success of predator
removal to conservation efforts has been attempted elsewhere in the
ecological literature. Meta-analysis is well suited to this problem be-
cause it reduces type II error (compared to vote-counting approaches)
and weights studies by their sample size; however, it can be overly
influenced by few studies with large sample sizes. Whereas meta-ana-
lysis is suited to analyzing studies with similar intervention, endpoint,
and subjects (Eysenck, 1994; see Smith et al., 2010, 2011 for effective
examples of this), it is constraining for broad topics such as predator
removal (Haddaway et al., 2015), which is conducted for many dif-
ferent reasons on a variety of taxa, making it difficult to generate re-
liable numerical assessments that could be considered relevant across
socioecological contexts. Instead, we opted for a qualitative review with
bibliometric analyses to reveal successes and failures with appropriate
consideration to context. There are lessons to be gained from viewing
many different, often disparate predator removal attempts through a
common lens and identifying how varying inputs (e.g. motivations,
taxa) contribute to outcomes to address future problems that arise.
Provided that future studies on this topic address some of the defi-
ciencies in experimental design noted here, there is potential to im-
prove the quality of the evidence base such that meta-analysis within
the context of a systematic review should be possible and will help to
ensure evidence-based environmental management in the future
(Sutherland et al., 2004).
6. Conclusion
Human-wildlife conflict will persist with direct impacts on ecosys-
tems 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 predator removal reduces direct conflict with humans or
human fear. However, there was some circumstantial support that re-
moving predators facilitated prey recovery and some evidence that it
assisted with protection of domestic animals. Nonetheless, a main ta-
keaway from this review is the inconsistencies and idiosyncrasies of
outcomes. Predator control should be pre-empted by research to justify
the action and set removal targets, with anticipated outcomes stated
and follow-ups planned to evaluate the action. Alternative actions may
be equally or more effective and should be studied in parallel when
possible. Some studies are not designed to detect main effects of pre-
dator removal and are instead retrospective and correlative because
predator removal may not always be motivated by conservation (Treves
et al., 2016). How the decision to remove predators is arrived at typi-
cally remains unclear. Although much can be learned from experi-
mental approaches (e.g. Lieury et al., 2015), they can be costly, ethi-
cally controversial, and require the removal of predators for didactic
purposes. Simulation approaches or predictive modelling have the po-
tential to become increasingly useful tools prior to implementing re-
moval in order to project whether the predator removal is likely to
achieve the desired outcomes (e.g. Martin et al., 2010;Morissette et al.,
2012;Yodzis, 1998). However, such efforts need to consider and ac-
count for many potentially confounding external variables such as food
availability and competition in order to conclude whether predator
removal is likely to be successful as well as the potential for immigra-
tion compared to compensation (Creel et al., 2015).
6.1. Promoting coexistence
Coexistence with predators is the desired way forward for many
(Bergstrom, 2017;Carter and Linnell, 2016;Johnson and Wallach,
2016;Woodroffe et al., 2005), and there are increasing examples that
predators can persist even among dense human populations (Chapron
et al., 2014;Elliot et al., 2016;Gilbert et al., 2017). Indeed, predators
play important ecological roles in rural areas and even in urban regions
(Gilbert et al., 2017). We propose that paradigms positing predator
persecution as a positive management intervention require reassess-
ment (see also Graham et al., 2005). However, interdisciplinary ap-
proaches that consider socio-ecological perspectives (e.g.; Bisi et al.,
2007; Elliot et al., In Press; Hill, 2015;Kaltenborn et al., 2006) will be
integral for determining how human perceptions, values, and attitudes
towards predators are shaped, and how they can be accounted for to
meet the needs of humans and predators and minimise conflict in an
increasingly crowded landscape.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.biocon.2018.05.003.
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