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Keywords:

  • effectiveness;
  • predation;
  • proactive measure;
  • quantitative;
  • subsidies;
  • wildlife conflict

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

1. Predation on livestock is one of the main reasons for low tolerance against large carnivores in many parts of the world. Measures to reduce the conflicts have been developed, but resources for using them are often scarce. If wildlife managers as well as farmers learn more about when the risk of predation on livestock is higher, they will be able to make more effective use of resources for reducing predation.

2. In this study, we tested the hypothesis that the risk of predation on livestock immediately after an attack is higher on the affected farm compared with other farms in the same area. Data on sheep predation by brown bear Ursus arctos, lynx Lynx lynx and wolf Canis lupus in Sweden 1998–2006 were used in the analysis.

3. On depredated farms there was approximately a 55 times higher risk for a repeat predation event within 12 months compared to any other farm in the same area. During the first 5 weeks, 63%, 60% and 50% of the repeat attacks had occurred.

4. We suggest that the main mechanism behind repeat attacks on livestock is that carnivores return to the kill site to feed on carrion. Where livestock are still present and unprotected at the kill site when the carnivore returns, the farms will suffer a higher likelihood of a further attack compared to livestock on other farms. This study uses data from Sweden but we argue that the pattern will be the same in any part of the world where the ranges of livestock and large carnivores overlap.

4.Synthesis and applications. As the risk of an attack is higher directly after an initial attack, it will be more cost-effective to implement measures designed to reduce livestock predation by large carnivores at that time, i.e. within the following 5 weeks. Temporary proactive measures are usually simpler and cheaper than permanent deterrents and we recommend their use wherever resources are limited.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The conflict between humans and wildlife is a widespread issue of growing concern for conservationists and wildlife managers, especially in relation to large carnivores that kill both livestock and humans (Woodroffe, Thirgood & Rabinowitz 2005). Livestock depredation occurs in all areas where livestock and large carnivores co-exist to various degrees. Partly because of this, human behaviour, legal or illegal, is the main factor affecting distribution and numbers of large carnivores in many parts of the world today (Thiel 1985; Mech et al. 1988; Mladenoff et al. 1995; Mace & Waller 1996; Sunde, Snorre & Kvam 1998; Treves & Karanth 2003; Andrén et al. 2006). In recent years, several research projects have tried to identify those factors influencing livestock depredation in order to find solutions to the problems (Linnell et al. 1999; Stahl et al. 2002; Wydeven et al. 2004; Bagchi & Mishra 2006; Goldstein et al. 2006; Van Bommel et al. 2007).

Historically, the most common way to reduce depredation has been to reduce overall carnivore population density (Stahl et al. 2001; Woodroffe et al. 2005). Public support for lethal control has, however, decreased over time (Treves & Naughton-Treves 2005). Lethal techniques may also be used in more sophisticated ways than earlier and directed towards specific areas or carnivore individuals. The effectiveness of removal programmes is affected by a range of factors: see Treves & Naughton-Treves (2005) for a review. Lethal control may, in some areas, be a threat to entire carnivore populations (Breitenmoser et al. 2005) and, in western countries, it can be highly controversial.

Non-lethal techniques may also be effective in reducing the risk for depredation, without the threat to carnivore populations or the controversy. For example electric strained-wire fences with short distances between strands, or woven wire fences supplemented with live strands, have been used successfully to protect livestock from coyotes Canis latrans (de Calesta & Cropsey 1978; Dorrance & Bourne 1980), cheetah Actinonyx jubata and caracal Caracal caracal (Bowland, Mills & Dawson 1993) and electric fencing has reduced sheep Ovis aries losses to wolves Canis lupus (Mertens, Promberger & Gheorge 2002). Apiaries have been used to provide protection from black bear Ursus americanus (Storer, Vansell & Moses 1983) and brown bear Ursus arctos (Svensson 1998).

Subsidies for proactive measures to reduce the risk of depredation from large carnivores are available in several European countries (e.g. Sweden, Norway, Finland) and North American states (e.g. Montana, Wyoming, Idaho and Minnesota). These include funding for purchasing and installing electric fences and dogs to guard livestock (Shivik 2006). Similar subsidies are not usually paid in Asia or Africa but this does not mean that proactive deterrents are less applicable, less used or less needed there.

Proactive deterrents can either be permanent or temporary and applied directly after a predation event. Temporary measures are often less expensive to use but may not be effective against all species of large carnivores (Shivik, Treves & Callahan 2003b). Permanent measures are more expensive but are judged to be more effective in the long term. Decisions are usually made on the basis of resource availability at a given point in time.

The objective of this study was to test the hypothesis that the risk of an attack on livestock by a large carnivore is higher just after a first attack compared to any other time for the respective farms and to provide recommendations for the efficient use of mitigation measures or deterrents to reduce livestock predation.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Large carnivore predation on sheep occurs all over Sweden, although attacks are more common in south-central Sweden because this area has the highest density of sheep farms within the carnivore distribution range (Statistics Sweden 2006). Our study area is therefore centred between latitude 54°–75°N and longitude 27°–14°E. The area has an altitude range from 50 to 1000 m a.s.l. The climate is continental with average temperatures of 15 °C in July and about −7 °C in January (Swedish Meteorological and Hydrological Institute, 2006). From December to March, the ground is generally covered with snow of varying depth (20–50 cm).

Most of the study area is covered by boreal coniferous forests. The most common tree species are Norway spruce Picea abies and Scots pine Pinus sylvestris, mixed with birch Betula pendula and Betula pubescens, aspen Populus tremula and alder (Alnus incana and Alnus glutinosa). The area is characterized by intensive forestry with clear-cuts, areas of young forest and a high density of forest roads. Human population density varies greatly, but averages <1 person km−2 (Wabakken et al. 2001). Potential prey species in the area are moose Alces alces, roe deer Capreolus capreolus, red deer Cervus elaphus, badger Meles meles, beaver Castor fiber, mountain hare Lepus timidus, capercaillie Tetrao urogallus and black grouse Tetrao tetrix (Olsson et al. 1997). Large carnivores present in the area are lynx, brown bear and wolf. The distribution of wolverine Gulo gulo does not overlap with the range of sheep farming in Sweden; both farm and wolverine density are very low and no wolverine attacks on sheep have occurred during the last 10 years.

In Sweden, wolves attack on average 100–200 sheep, lynx 100–200 sheep and brown bears 50–100 sheep each year (Swedish Wildlife Damage Center 2008). Livestock killed, injured or missing after an attack by large carnivores are compensated at a rate slightly higher than the market value. The number of killed, injured or missing sheep not reported is expected to be small, because (by law) sheep are grazed in small fenced pastures and there are a relatively small number of sheep on each farm. Ninety-two per cent of the Swedish sheep farms have fewer than 50 ewes (Statistics Sweden 2006). Furthermore, animal welfare law compels sheep owners to check and count their sheep at least once a day. All farms reporting suspected depredation are visited by government personnel trained to examine carcasses and to determine the true cause of death or injury. All carcasses are skinned and examined. No compensation is paid unless these experts attribute death or injury to large carnivores, which further motivates farmers to report suspected depredation.

A repeated attack was defined as the occurrence of two or more predation events within a year on the land of the same farm within 1 km of each other. The distance criterion was used to separate different farms with the same name. A Kaplan–Meier procedure was then used to analyse how risk for a subsequent predation event varied with the number of weeks after the initial depredation event.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Between 1998 and 2006, a total of 509 attacks on sheep occurred on farms in Sweden resulting in c. 2000 dead sheep. In this study, we only used data from 485 attacks by wolves, lynx and brown bear (24 attacks were by golden eagle Aquila crysaëtos). The majority (56%) of these predation events could be attributed to lynx. More recently, wolves have increased in number leading to an increase in the proportion of predation events that can be attributed to wolves (24%).

Of the 485 sheep farms that suffered attacks by large carnivores, 81 (17%) suffered repeated attacks within 1 year of the first attack (Table 1). There was no difference between the carnivore species in the proportion of attacks that were followed by a second predation event (wolf vs. brown bear: χ2 = 0·005, = 0·95; wolf vs. lynx: χ2 = 1·71, = 0·19; brown bear vs. lynx: χ2 = 1·13, = 0·29).

Table 1.   Number of predation events by large carnivores on Swedish sheep farms 1998–2006
 Brown bearLynxWolf
Number of predation events on sheep farms 1998–200695269121
Number of predation events followed by a second predation event within 12 months193725
Proportion of attacked sheep farms with a second predation event within 12 months0·200·140·21
Number of sheep farms in the area of distribution201881283683
Annual proportion of sheep farms having at least one predation event0·0040·0030·003

The annual risk of a predation event was c. 0·003 for an average sheep farm in our study area. For a sheep farm already having been subject to a predation event, the risk of a repeated predation event within 12 months was on average 0·17. The risk of a second predation event is thus 55 times higher for a farm that has experienced one predation event compared with any sheep farm in the same area.

We classified attacks within 1 km from each other as repeat attacks. We also tested if the distance criterion used (max. 1 km between the first attack and the repeat attack) affected our results by classifying attacks within 3, 5 and 10 km from the first attack as repeat attacks. However, even when using the10-km criterion, the number of attacks differed only marginally compared to using the 1-km criterion. In order to decrease the risk of classifying attacks by other carnivore individuals as repeat attacks, we decided to use only the 1-km criterion.

The risk for a second predation event declines rapidly with time from the initial predation event for wolves and lynx (Figs 1 and 2), whereas the risk for repeated brown bear predation does not show a clear decline with time (Fig. 3). A repeat attack occurred during the week following the first attack in 47% of cases when brown bears were involved and in 32% and 24% of cases when wolves or lynx had attacked sheep (Figs 1, 2 and 4). A repeat attack occurred in the following 5 weeks in 63%, 60% and 50% of cases of attacks on sheep by brown bears, wolves and lynx respectively (Fig. 4).

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Figure 1.  Risk of a repeated attack by wolves for each week after an initial attack on sheep (Kaplan–Meier with a 95% CI).

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Figure 2.  Risk for a repeated attack by lynx for each week after an initial attack on sheep (Kaplan–Meier with a 95% CI).

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Figure 3.  Risk for a repeated attack by brown bear for each week after an initial attack on sheep (Kaplan–Meier with a 95% CI).

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Figure 4.  Number of brown bear, lynx and wolf attacks on sheep for every week following the first attack. Example: eight predation events by brown bear occurred during the first week following the initial predation event, one predation event occurred during the second week, etc.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

On average, the risk of a sheep farm experiencing a second attack by brown bear, wolf or lynx was 55 times higher during the year after a first predation event compared to any farm in the same area. We suggest that the main reason for an increased risk of a repeat attack after a successful first attack is the result of the predator returning to the kill site to feed on the carrion, or possibly to search for new prey. If sheep are still grazing in the area and no deterrents have been put in place, the predator may easily kill further sheep. This pattern of repeat attacks has been reported for carnivore species from different parts of the world (Linnell et al. 1999; Treves et al. 2004). The behaviour leads to a higher risk of predation on livestock in the weeks immediately following the first attack. The behaviour could be a consequence of a number of factors including different sheep husbandry systems at those farms experiencing repeat attacks, or farms being situated in so called ‘hot spots’ for predation (Mech et al. 2000). However, if such factors were a major force behind the pattern of repeat attacks, then these attacks would be more or less evenly distributed over the 12 months following the first attack and not clumped during the first weeks after the first attack. Irrespective of how these two mechanisms contribute to the enhanced risk of a predation event occurring within 5 weeks of the initial attack, it will always be more cost-efficient to implement deterrents on farms that have suffered an initial predation event.

Our analysis considered attacks within a 1-km radius from the first attack as repeat attacks. In addition, we extended that radius to 3, 5 and 10 km without finding significant numbers of repeat attacks. We conclude that farms more than 1 km away from the original attack site do not suffer an increased risk of attacks.

The risk of a repeat attack decreased rapidly over the weeks following a first attack. Approximately 30% of repeat attacks took place within 1 week of the first attack, and 60% within 5 weeks. An effective deterrent may therefore be to use temporary measures such as, scaring devices, keeping livestock inside barns, smaller pastures with better fences, increased herding effort or changing the grazing area used by free roaming livestock.

Many factors affect the effectiveness of different lethal and non-lethal deterrents (Shivik et al. 2003a,b; Herfindal et al. 2005; Treves & Naughton-Treves 2005; Shivik 2006) so selection of appropriate measures will depend heavily on site-specific conditions.

Tigers Panthera tigris, leopards, hyenas, cougars Puma concolor, bears, wolves and lynx are known to return to kills, whereas cheetahs Acinonyx jubatus rarely do so (Ewer 1998; de Ruiter & Berger 2000; Maddox 2003; Bauer et al. 2005). Detailed knowledge of carnivore behaviour is therefore important as not all carnivores will respond to measures applied directly after an attack and the effectiveness of deterrents will vary according to the species involved. In order to maximize effectiveness, it will be important to correctly distinguish between livestock killed by different carnivore species. However, even if the individual predator does not return to the site of an attack, deterrents are still likely to be effective in protecting livestock from other carnivores attracted by the carcass.

In Norway, Odden et al. (2002) found that male lynx are more frequent sheep killers than females, but females returned to kills more often than males. Schaller & Crawshaw (1980) reported that female jaguars Panthera onca frequently return to livestock kills with their cubs. The higher energetic needs of females rearing young may make them more prone to return to kill sites, thereby increasing their risk of carrying out further attacks and becoming vulnerable to culling or removal. If lethal control is used (e.g. sit and wait hunting, traps and snares), there is a clear risk that it will therefore target breeding females more than other age/sex classes. Conservation of small populations of controversial carnivores should therefore consider non-lethal proactive measures being applied directly after an attack, as efforts will then be focused on mitigating conflicts with the individuals having the highest reproductive value, the adult females.

Our results show that the risk of a predator attack on a sheep farm is 50 times higher just after (within 1 week of) a first attack compared to any other sheep farm in the same area. This information should help the management’s response to be carefully targeted to maximize effectiveness. It is possible that past success will lead predators to escalate their efforts when returning to kill sites and to overcome temporary deterrents. Alternatively, predators may focus on neighbouring farms instead, which would not reduce total depredation numbers. Densities of livestock as well as wild prey will be important factors to consider when drawing up management plans. We would expect the benefit from proactive deterrents to be the highest in areas where, for example, wild prey are abundant and carnivores do not depend on livestock for their survival. We strongly encourage more quantitative studies of the risk of predation on livestock in different parts of the world.

Many developing countries do not have the economic means to compensate livestock owners financially or to provide expensive proactive deterrents. For example, only 3% of the actual losses are compensated for in the Ladakh region, India (Mishra 1997). As the risk of a repeat attack decreases rapidly over the first 5 weeks after the initial attack, it may be possible to reduce livestock predation by using simple, low budget solutions such as, for example, scaring devices, keeping livestock inside barns, smaller pastures with improved fencing, increased herding effort or changing the grazing area used by free roaming livestock. In some areas, for example, the Annapurna Conservation Area, in Nepal, livestock were grazed unattended in the same pastures even after several had been killed by snow leopards Uncia uncia (Jackson et al. 1996). These losses could probably have been prevented by moving the livestock to other areas, away from the kill site for a few weeks, or if that is not possible, increasing the herding effort in the weeks immediately after the attack. In other areas, for example Kenya, where sheep are often kept inside fenced enclosures or bomas during the night, deterrents could be applied directly after a predation event to reduce the risk of one or more subsequent attacks.

Some of these methods have been tested. For example, Ogada et al. (2003) and Frank, Woodrofe & Ogada (2005) have showed that reinforcement of bomas as well as increased guarding efforts after attacks was effective in reducing livestock losses by large felids in Africa. The main contribution from our study to the body of knowledge on how to prevent predation on livestock from large carnivores is the quantification of the risk for a repeat attack. This information may be useful when considering governmental mitigation measures and subsidies. Importantly, the results may help to motivate individual livestock owners to invest in simple deterrents for a limited period of time to achieve a high measure of success.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The authors would like to thank Håkan Sand, Henrik Andrén, Michael Schneider, Adrian Treves and Linn Svensson for constructive comments on previous drafts of this manuscript.

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  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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