The Fox and the Crow. A need to update pest control strategies

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The Fox and the Crow. A need to update pest control strategies

Frédéric Jiguet

To cite this version:

Frédéric Jiguet. The Fox and the Crow. A need to update pest control strategies. Biological Conser-

vation, Elsevier, 2020, 248, pp.108693. �10.1016/j.biocon.2020.108693�. �hal-02983580�

1 Perspective 1

The Fox and the Crow. A need to update pest control strategies 2

3

Frédéric Jiguet, UMR7204 Centre d’Ecologie et des Sciences de la Conservation, MNHN-CNRS-SU, 4

CP135, 43 Rue Buffon, 75005 Paris, France. frederic.jiguet@mnhn.fr 5

6

Abstract 7

The recent discovery that cats and mustelids can be infected by SARS-CoV-2 may raise the question of 8

monitoring domestic, feral and wild populations of such animals, as an adjunct to elimination of COVID- 9

19 in humans. Emergency solutions might consider large scale control of these animals in the wild.

10

However, looking at science recently published on native vertebrate pest control reveals first that usual 11

controls do not succeed in reducing animal numbers and associated damages, second that controlling 12

can be counter-productive in increasing the infectious risks for humans and livestock. The examples of 13

red fox and corvids are detailed in a European context, illustrating the urgent need for an ethical 14

evaluation of ecological and economic costs and benefits of pest control strategies. A complete 15

scientific evaluation process must be implemented and up-dated regularly, to be organized in four 16

major steps, once the aim of the control strategy has been defined: (1) evaluating damages/risks 17

caused by the animals, to be balanced with the ecosystem services they may provide, also in terms of 18

economic costs; (2) unravelling spatial and temporal population dynamics of target animals to identify, 19

if any, optimal control scenarios – which could be done within an adaptive management framework;

20

(3) estimating the economic costs of implementing those optimal control scenarios, to be compared 21

to the economic costs of damages/diseases; (4) finally evaluating how the control strategy reached its 22

aims. A modern fable of the Fox and the Crow should deliver a timely moral for an ethical, ecological 23

and economical appraisal of pest control strategies in Europe.

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2

Occurring during the ongoing biodiversity crisis, the current covid-19 pandemic will certainly question 25

the way growing human populations exploit wildlife, potentially increasing the exposition of humanity 26

to pathogens originally hosted by wild animals (Zhou, 2020). In this context, sustainable development, 27

including biodiversity conservation and climate mitigation, should definitely account for emerging 28

infectious diseases (Di Marco et al., 2020), acknowledging the synergies between human, animal and 29

environment health within the one health concept (Osterhaus et al., 2020).

30

Some native vertebrate predators are considered as pests because of their detrimental 31

impacts on human economic activities and human or livestock health. By controlling their numbers, 32

destruction campaigns aim at reducing damages to agriculture, forestry, livestock, and at decreasing 33

infectious risks due to their hosted parasites or pathogens. Behind these controls is the commonly 34

accepted evidence that reducing the numbers of pest animals would reduce the damages or risks. This 35

might be particularly sensitive in the near future, as post-covid strategies might consider an increased 36

control of those wild animals potentially responsible for infectious diseases, as an efficient tool to 37

reduce global sanitary risks for humans. Such a strategy would however clearly be challenged by 38

multiple recent scientific results, first reporting that controls do not necessarily succeed in reducing 39

animal numbers, second that controlling can be counter-productive and even increase the sanitary 40

risks for human populations. At play are complex interlinked processes in population dynamics, tightly 41

linked to disease prevalence and population size, such as compensatory breeding, higher survival of 42

survivors, enhanced dispersal and increased recruitment at looser densities following controls. Pest 43

control should probably not be questioned if it was able to target those individuals causing damages 44

or hosting the transmissible disease, though generally massive control occurs without discernment 45

between individuals. But predator controls should not be a shot in the dark (Treves et al. 2016).

46

In Europe, the Birds (79/409/EEC) and Habitat Directives (92/43/EEC) fix the list of protected 47

species, and state that hunting must be compatible with maintenance of populations at a satisfactory 48

level. The Bern Convention (82/72/CEE) further engages signatory parties to ensure the conservation 49

of wild fauna, including conditions for killing. Within the limits imposed by these directives and

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international conventions, member states are sovereign to draft their own regulations of hunting 51

activities, including vertebrate pest control. As an example, in France, a triennial ministerial decree 52

fixes the list of vertebrate pest species that can be shot, trapped, dug, based on the extend of reported 53

economic damages and sanitary risks as declared to administrative authorities – though with no 54

obligation to perform any evaluation of control efficiency. Reasons to declare a species as a pest are:

55

sanitary and security risks, flora and fauna protection, damages to agriculture/forestry/aquaculture, 56

and damages to any other form of human goods. Across Europe, large-scale culling is largely the rule 57

to control native vertebrate predators impacting human economic activities or representing potential 58

sanitary risks, without a systematic ethical evaluation of ecological and economic costs and benefits.

59

The few examples of cost-benefit economic or efficacy assessments can be counted on the fingers of 60

a single hand (see Jenkins et al. 2010).

61

A meta-analysis of targeted controls of predators revealed a global inefficiency to increase 62

breeding population sizes of predated birds of conservation concern (Côté and Sutherland, 1997).

63

However, some targeted predator control programmes are indeed successful. New Zealand holds an 64

acknowledgeable experience with 25 species of exotic feral mammals being actively managed as pests 65

to reduce their impacts on biodiversity and production value (Parkes and Murphy, 2003). Beyond the 66

classical eradications of mice, rat and/or rabbit from islands to restore seabird habitat and populations 67

(Towns et al., 2013), there is an abundant literature focusing on the necessary and successful control 68

of non-native vertebrates challenging the survival of native terrestrial fauna. Such programmes can 69

use a larger range of control tools with little non-target effects as most concerned island systems have 70

no native predators. As one charismatic example, the endemic and critically endangered Kakī 71

Himantopus novaezelandiae - the world’s rarest shorebird - is coming back from the brink of extinction 72

thanks to 40 years of active management. With only 23 adults in the wild in 1981, a comprehensive 73

framework of captive-breeding and non-native predator control started (Keedwell et al., 2002; van 74

Heezik et al., 2009). The Kakī population increased to 72 adult birds in 2006, 169 in 2020, especially 75

thanks to recent intensified control of ferrets and feral cats, with over 2000 new traps installed in 2019.

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Building on the experience of New Zealand in non-native predator control, Warburton and Anderson 77

(2018) proposed an interesting conceptual framework around sustainable or adaptive management, 78

based on the so-called three Es: Ecology, Economics and Ethics. Such approaches should be urgently 79

applied in Europe to the cases of native vertebrates challenging human economy and health.

80

Ethics in predator control is a recurrent question, and has been developed mainly from large 81

carnivore models such as bears, wolves and large felines (Littin et al., 2004; Bekoff 2010; Gamborg et 82

al., 2012; Vucetich and Nelson, 2017). From an ethical perspective, the aims or benefits and harms of 83

any control programme should be clear from the start, and whether or not a control programme 84

actually achieves those precise aims must be assessed (Littin et al., 2004). Indeed, the necessity of 85

intervention must be properly evaluated, as it involves killing animals. Justification for pest control is 86

only tenable if all negative impacts on people, animals and the environment are minimised and all 87

positive impacts are maximised (Littin et al., 2004), while their balance should be positive. Ecologically, 88

a pest control should prove efficient to durably regulate pest numbers, but most importantly decrease 89

associated damages or risks, while both are not necessarily linearly linked. Economically, the benefits 90

obtained in damage reduction should overpass the costs of pest controlling. While overall economic 91

costs of damages to e.g. crops or livestock are necessarily provided to justify the decision to control 92

pests, the balance with control costs is most often ignored.

93

Foxes and crows (Fig. 1) are native top predators inhabiting most habitats in the northern hemisphere, 94

and are appropriate examples of animals prosecuted across Europe in the need of a true ethical and 95

scientific reappraisal of control necessity, with sufficient recent research work challenging the 96

efficiency and even utility of massive culling.

97 98

The Fox 99

In Europe, the red fox Vulpes vulpes is a widespread predator that can locally cause damages to 100

industrial or private poultry. In the wider countryside, they can predate game species, though their 101

main diet is made of rodents (Jędrzejewski and Jędrzejewska, 1992) – hence limiting the potential

102

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damages caused by voles to agriculture. More worrying, foxes can carry rabies and echinococcosis (a 103

parasitic disease of tapeworms), both potentially transmissible to humans. As an example, France is 104

free from rabies since 1998, but faces on average 30 human cases of echinococcosis annually (mostly 105

farmers and hunters), causing one or two deaths a year. Humans become infected through the 106

accidental intake of parasitic eggs excreted in the faeces of definitive hosts (Otero-Abad and Torgerson, 107

2013), the commonest being foxes, dogs and cats (Karamon et al., 2019). Meanwhile, French hunters 108

kill 500.000 red foxes each year (Albaret et al., 2014; Aubry et al., 2016). In this context, some recent 109

researches challenge the efficiency of controlling foxes – though we can further acknowledge that 110

manipulating dead foxes could increase the risk of contracting echinococcosis for hunters.

111

Does fox hunting reduce fox numbers? Several studies reported it doesn’t. A nationwide one-year ban 112

on fox-hunting was imposed in the United Kingdom in 2001, during the outbreak of foot-and-mouth 113

disease. Previously, 400.000 foxes were hunted there annually, but the ban had no measurable impact 114

on fox numbers (Baker et al., 2002). This study failed to find a link between the reduction of hunting 115

pressure and fox density across 160 surveyed one-kilometre squares, and concluded that a permanent 116

ban on hunting is unlikely to result in a dramatic increase in fox numbers. An alternative explanation 117

is that the duration of the hunting ban was not long enough to allow populations to recover − if they 118

could do − as anti-predator interventions can remain effective for a time (Khorozyan and Waltert, 119

2019). A more convincing example comes from France, where four years of intense culling, with an 120

increase of fox bag by 35%, failed to reduce the size a regional fox population (Comte et al., 2017).

121

Meanwhile the prevalence of tapeworm Echinococcus multilocularis significantly increased in that fox 122

population from 40% to 55%, while it remains stable in an adjacent control area (Comte et al., 2017).

123

Increases in immigration and local recruitment is the best hypothesis for population size resilience, 124

while the ‘social fence’ hypothesis (Hestbeck, 1982) can explain the lower prevalence in control area.

125

The increase in prevalence is therefore considered to be linked to a higher rate of juvenile movement 126

within the culled area, an hypothesis supported by a negative link between density and natal dispersal 127

in mammals (Matthysen, 2005). The same trends in numbers and prevalence were observed in

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Luxembourg after the exclusion of the fox from the list of game species in 2015 – the number of foxes 129

did not increase, and infection rate of the fox tapeworm decreased from 40% to 25% since the hunting 130

ban. Furthermore, Hofmeester et al. (2017) empirically showed that foxes can lower the number of 131

ticks feeding on reservoir-competent hosts, namely their rodent preys, which implies that changes in 132

predator abundance have cascading effects on tick-borne disease risk. As a summary, uncontrolled fox 133

populations are good for human health with respect to tick-borne pathogens, lower prevalence of 134

Echinococcus, while foxes are predators of potential pests to agriculture. As a consequence, there is 135

an urgent need to evaluate the economic consequences of game and poultry predation by foxes, to be 136

balanced with the ecosystem services they provide, and to challenge the necessity to hunt or even 137

prosecute foxes, given the benefits of uncontrolled fox populations for human health. Meanwhile, 138

millions of foxes are killed annually across Europe.

139 140

The Crow 141

Corvids are avian predators and scavengers with an omnivorous diet, believed to limit other wildlife 142

(Heinemann et al., 2020). Despite this widely held belief, a comprehensive review failed to find 143

evidence for widespread effect of corvids on the demography of prey species (Madden et al., 2015).

144

Corvids also largely feed on seeds, providing ecosystem services as seed dispersal for wild flora 145

(Hougner et al., 2006; Czarnecka and Kitowski, 2010; Culliney et al., 2012; Martínez-Baroja et al., 2019;

146

see a review in Green et al., 2019), though are also widely recognized as causing damages to crops. A 147

consequence of local damages to agriculture is the implementation of culling in the purpose of limiting 148

these damages, either locally or globally. Across Europe, over 4 million corvids are killed annually, 149

including 1.150.000 crows Corvus corone/cornix, 1.145.000 jays Garrulus glandarius, 980.000 magpies 150

Pica pica, 600.000 rooks Corvus frugilegus, 250.000 jackdaws Corvus monedula (Hirschfeld and Heyd, 151

2005). A large part of these numbers are achieved in France, with mean estimates of 380.000 crows 152

and 230.000 rooks killed annually (Albaret et al., 2014; Aubry et al., 2016). Long-term repeated 153

trapping during the breeding season can durably reduce corvid densities at local (Díaz-Ruiz et al., 2010)

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or global (Chiron and Julliard, 2013) scales. However, recent investigations in corvid dispersal and 155

dynamics provide elements challenging the efficiency of any local control intended to reduce numbers 156

(Heinemann et al., 2020). Immense individual home ranges and fusion-fission group dynamics (Loretto 157

et al., 2017) have been documented in ravens Corvus corax and crows (Uhl, 2016) rendering any local 158

initiative to reduce numbers inefficient (Marchand et al., 2018). Immature ravens and crows gather in 159

large groups at sites where food is rich and predictable, while birds move regularly and individually 160

between such groups, functioning in metapopulations at huge spatial scales, up to 40,000km2 in a 161

French raven population (Marchand et al., 2018; see Fig. 2). Any local damages can not be solutioned 162

by culling locally, as the local turnover of individuals is high (0.68 in Marchand et al., 2018), and the 163

metapopulation is far larger than the local population. As a consequence, any efficient control could 164

only occur at those spatial scales able to alter the spatial dynamics of the species. Indeed, an attempt 165

to reduce damages to livestock by locally frightening ravens at dormitories, by relocating them at up 166

to 240km, by culling dozens, failed to reduce bird numbers and to stop the increase in sheep attacks 167

(Marchand et al., 2018). Preininger et al. (2019) also concluded that continuous hunting of carrion 168

crows over two decades did not reduce population size, and that a sustainable long-term stabilization 169

and reduction of generalist corvid species populations can only be achieved if anthropogenic food 170

resources are limited.

171

Measuring dispersal in spatially heterogeneous environments is essential for testing population- 172

dynamics models (Kareiva, 1990), but obtaining unbiased dispersal data is difficult, because previous 173

approaches mainly included analyses of ringing recoveries highly biased by heterogeneity in ringing 174

and recovery efforts (e.g. Holyoak, 1971), while long-distance dispersers often go undetected (Koenig 175

et al., 1996). Distance tracking using technological devices provides the clue. The dispersal of first-year 176

American crows was detected up to 28 km of their natal site using ragio-tags (Withey and Marzluff, 177

2005). An ongoing research on Carrion Crow in France reported the recovery of first-year individuals 178

up to 158 km from the ringing site (Fig. 2), while a starting GPS-tracking programme on Western 179

Jackdaw in Brittany already identified non-breeding individuals that dispersed up to 60 km from the

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tagging site within two weeks after tagging (Fig. 2). Finally, corvids can also host various viruses 181

associated with public-health risks, including West Nile virus (Reisen et al., 2006), avian influenza (Khan 182

et al., 2014) and Usutu flavivirus (Vazquez et al., 2011). The dispersal of such viruses has also to be 183

considered at the scale of the metapopulations, in order to evaluate the infectious risk for humans and 184

the potential necessity and estimated efficiency of any control strategy.

185 186

Lessons from the badger 187

Bovine tuberculosis (TB) is one of the most pressing cattle health problem in Europe, and Eurasian 188

Badger Meles meles acts as a vector and a reservoir of TB infection. As a consequence, TB eradication 189

or control programmes include badger controls. The aim of any badger control policy would be to 190

reduce confirmed cattle herd breakdowns. In the context of an increasing incidence of cattle TB since 191

the mid-1980s in the United Kingdom (Gilbert et al., 2005), Krebs et al. (1998) called the British 192

Government to adopt a more scientific approach to evaluate control strategies. This led to the 193

development of an appropriate ecological evaluation, by comparing culled and non-culled 194

experimental areas on a nationwide scale. It highlighted that localized badger culling not only failed to 195

control but also increased TB incidence in cattle (Donnelly et al., 2003). Indeed, the number of 196

subsequent infected herds in reactive-culling areas was 27% higher than in regions without culls (Giles, 197

2003). Furthermore, localised badger culling increased the risk of herd breakdown on nearby, not focal, 198

land (Jenkins et al., 2010; Vial and Donnelly, 2012; Bielby et al., 2016). As for foxes, demographic and 199

dispersal mechanisms and the ‘fence’ hypothesis probably explain why badger culling failed to reach 200

its objectives (Donnelly et al., 2006; Woodroffe et al., 2006). Replicated and controlled experiments 201

further revealed that badger culling was associated with increases in red fox densities, illustrating the 202

complex consequences of intervention in predator populations (Trewby et al., 2008). Badger control 203

in the United Kingdom also benefited from an economic evaluation. The economic comparison of 204

culling at appropriate spatial scale vs. sanitary management revealed that culling costs exceeded by a 205

factor 2 to 3.5 (Jenkins et al. 2010). Overall, badger culling is therefore considered as unlikely to

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contribute efficiently to the control of cattle TB. Finally, the badger lesson has also been touching the 207

ethical issue, by reporting that 6 to 19% of badgers are not recovered following a rifle shot, and are 208

therefore at risk of experiencing marked pain (Munro et al. 2014). In European countries other than 209

UK, evaluations if any have been restricted to the sanitary output (see e.g. ANSES 2016 for France).

210 211

The future of pest control strategies 212

Current control strategies of native predators need to be challenged, as they too often cruelly lack any 213

serious and scientific evaluation process, and as some even appear to increase the risks or damages 214

they are intended to reduce. A complete evaluation process must be implemented and up-dated 215

regularly, building on the best practices observed for badgers or large carnivores of high social value.

216

It could be organized in four major steps.

217

A first step should be to evaluate the extent of damages caused by the species of concern – ecologically 218

and economically - independently of other hazards, and needing to compare damaged to control areas, 219

or damages across gradients of pest densities. A second step is to understand how the animal 220

population functions, in terms of reproduction, survival, dispersal, recruitment, and spatial scale, 221

defining the functional metapopulaitons. Unravelling spatial and temporal population dynamics and 222

identifying the key driving demographic parameters should allow appraising potential control 223

scenarios and testing their efficiency to reduce animal numbers. A third step should be to estimate the 224

economic costs of implementing those optimal control scenarios, at appropriate spatial and temporal 225

scales, and to compare these costs to the financial costs of damages and zoonoses emerging from the 226

animals. If there is a potential economic benefit in reducing the number of pest animals, there is still 227

an uncertainty in the evidence that reducing the number of animals will reduce the extent of 228

detriments attributed to those animals. The last and fourth step should be to concretely evaluate how 229

the control reached its aims, in terms of decreases in damages or infectious risks. This necessitates to 230

compare control and control-free regions, in a before-after framework, and will provide the 231

opportunity to document how reducing animal numbers is affecting the targeted damage or risk. The

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need for experimental designs to unravel the ecological uncertainties around predator removal has 233

been highlighted by Treves et al. (2019), who argued that the field of predator control needs the “gold- 234

standard” of randomized, controlled experiment without biases. To date, these steps are not rigorously 235

conducted or are definitely lacking in almost all control programmes in Europe (but see Jenkins et al.

236

2010), while it would be the necessary argument to ethically justify the large scale killing of millions of 237

animals annually. It’s time for a scientific reappraisal of ongoing pest control strategies. A modern fable 238

of The Fox and the Crow should deliver a timely moral for an ethical, ecological and economical 239

appraisal of pest control implementation, and the development of common policy directives at 240

European level, informed by rational cost-benefit analyses, evidence-based approaches and basic 241

ecology and population dynamics.

242 243

Epilogue 244

Recent investigations revealed that ferrets and cats are susceptible to the SARS-Coronavirus-2 (Shi et 245

al. 2020, Kim et al. 2020), so that the surveillance for SARS-CoV-2 in such animals can be recommended 246

as an adjunct to elimination of COVID-19 in humans (Shi et al., 2020). Free-ranging cats are considered 247

as major predators of small wild vertebrates (Loss et al., 2015), while their potential as SARS-CoV-2 248

reservoir – together with mustelids - will soon raise sanitary questions. The dynamics of contamination 249

and prevalence of the virus in free-ranging domestic cats, in feral cat and wild mustelid populations 250

should first be carefully studied before deciding on whether such small carnivores are a definite risk 251

for human health. Laboratory experiments revealed the ease of transmission of SARS-CoV-2 between 252

domestic cats (Halfman et al., 2020), while an outbreak of influenza A(H7N2) virus in domestic cats in 253

New York resulted in zoonotic transmission (Marinova-Petkova et al., 2017). Humans recently lived a 254

worldwide experience of how limiting dispersal movements can reduce the spread of an infectious 255

disease. Stakeholders managing wildlife might now be more prone to understand that maintaining a 256

tight social structure in territorial pest animals should help to confine them within their territory and

257

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ensure large-scale social distancing – thus reducing the sanitary risk of infectious diseases, for them, 258

livestock, and humans. Pest animals might not all be sick of the plague.

259 260

Acknowledgements 261

Thanks are due to my colleagues Sébastien Dugravot, Christian Itty, Matthias Loretto and Thomas 262

Bugnyar for accepting to share their corvid tracking data, and to Conny Lundstrom and Remo Savisaar 263

for sharing their photos, and Romain Lorrillière for drawing the figure.

264 265

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Figure 1. A red fox Vulpes vulpes and ravens Corvus corax, Estonia (Remo Savisaar). Foxes and corvids 435

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Figure 2. Long-distance dispersal of immature corvids in France, as obtained from ringing or tagging 440

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monedula) during the first three weeks after their capture in May 2020 in Britany.

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