Rapid eradication of feral pigs (Sus scrofa) from Santa Cruz Island, California

Biological Conservation 143 (2010) 634–641

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Rapid eradication of feral pigs (Sus scrofa) from Santa Cruz Island, California John P. Parkes a,*, David S.L. Ramsey b, Norman Macdonald c, Kelvin Walker c, Sean McKnight c, Brian S. Cohen d, Scott A. Morrison d a

Landcare Research, PO Box 40, Lincoln 7640, New Zealand Arthur Rylah Institute for Environmental Research, PO Box 137, Heidelberg 3084, Victoria, Australia c Prohunt, Inc., 2102 Business Center Drive, Suite 130, Irvine, CA 92612, USA d The Nature Conservancy, 201 Mission St., 4th Floor, San Francisco, CA 94105, USA b

a r t i c l e

i n f o

Article history: Received 18 February 2009 Received in revised form 23 November 2009 Accepted 28 November 2009 Available online 19 January 2010 Keywords: Biodiversity conservation Control Invasive alien species Island restoration

a b s t r a c t Eradication of invasive alien species from islands is often necessary to protect native biota. The rapidity in which eradication projects are implemented can help reduce risk they will fail. We describe the eradication of feral pigs (Sus scrofa) from Santa Cruz Island, California, highlighting those control techniques that removed the most pigs and those that removed the last pigs. In 411 days, a total of 5036 pigs were removed from the 25,000-ha island. Before the eradication began, the island was fenced into five zones. Within each zone, the same general sequence of control methods was used: trapping (16% of dispatches in 1660 trap-nights); aerial shooting from a helicopter (77% of dispatches in 13,822 km of flight path); and then ground-based hunting with trained dogs (5% of dispatches in 1111 hunter-days). Sterilized adult pigs fitted with radio collars were subsequently used to aid in the location of surviving wild pigs and to monitor the success of the project. Female telemetered pigs were more effective than males at locating remaining wild pigs. Only 10% of the last 102 pigs (the last 20 or so present in each fenced zone) were dispatched as a result of being found with a telemetered pig, but telemetered animals were responsible for finding 43% of the very last pigs once normal hunting had ceased. The time taken to eradicate pigs on Santa Cruz Island was about half that taken on a neighbouring island of similar size (Santa Rosa Island) and 12 times as fast as that on Santiago Island (58,465 ha), Galapagos Islands. The deliberate sequencing of control methods, using first those that taught surviving pigs the least, and the intensive implementation of those methods, represent important advances in the practice of eradication and so biodiversity conservation. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Invasive alien species pose one of the greatest threats to biodiversity conservation (Vitousek et al., 1997) with island biotas being especially susceptible (Mulongoy et al., 2006). Eradication of invasive alien species from islands is increasingly recognized as prerequisite to restoring insular ecosystems and protecting native species (Hutton et al., 2007). However, to successfully eradicate a population a variety of biological, logistical, social and other challenges must be overcome (Parkes, 1984). Fortunately, recent decades have seen considerable advances in strategies and tactics for achieving eradication goals (e.g. Veitch and Clout, 2002). Still, eradication projects may fail for a variety of reasons. Some are doomed because one or more of the obligate conditions for success (Parkes, 1990) cannot be met, e.g. immigration may be certain and unmanageable (Broome, 2007). If a project is not planned or implemented in a way to render eradication feasible, an eradica* Corresponding author. Tel.: +64 3 321 9768. E-mail address: [email protected] (J.P. Parkes). 0006-3207/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2009.11.028

tion goal may evolve, whether consciously or not, into a de facto sustained control strategy (Parkes and Panetta, 2009). Conversely, the goal to eradicate an invasive alien species sometimes emerges after it is realised that previous sustained control strategies have failed to meet conservation goals and that eradication is thus required (e.g. Garcelon et al., 2005). Eradication projects that evolve out of sustained control efforts may be especially challenged to succeed, because at that point the target species may have learned to avoid detection (Morrison, 2008). The best strategy for achieving eradication goals differs by circumstance and also by taxa. For some species, eradication might best be attempted using a single control event, such as aerial poison baiting for rats and mice (Howald et al., 2007). For such species, the cost to monitor for survivors for the purpose of controlling them would be relatively and prohibitively great (Donlan and Wilcox, 2007). Instead, one intensive event is meticulously planned, even over-engineered, to maximize the probability of reaching 100% of the target population. About 10% of such attempts on rats have failed (Howald et al., 2007). It is nevertheless a costefficient strategy for these species to plan a project’s monitoring

J.P. Parkes et al. / Biological Conservation 143 (2010) 634–641

to detect survivors only after the population has had time to rebound, not in the immediate-term with intent to pursue any individuals detected. In other words, if the event failed to eradicate the population, the optimal strategy is to wait to ascertain the failure of the first attempt, and then to mobilize another. For most species, however, eradication requires combining an initial cull with immediate-term follow-up control because the available culling techniques are incapable of removing all of the individuals in a single event, e.g. for rabbits (Parkes, 2006), or repeated applications of the technique or a sequence of techniques is required to reduce the population to zero, e.g. for most large vertebrates such as feral goats or pigs (Campbell and Donlan, 2005). This ‘successive cull’ strategy also has risks of failure, perhaps mostly because the target animals may learn to avoid the control (Morrison et al., 2007) or compensate for reduced densities by increasing their productivity (Parkes, 1984). Also, if the eradication program becomes protracted it may become difficult to sustain logistically, financially, or programmatically. Critical to the success of a successive cull strategy is that it be implemented rapidly, as that can reduce risks due to biological, logistic and other institutional factors. Rapidity can also improve efficiency and enhance the return on investment of limited conservation funds (Morrison, 2008). Here we report on a project to eradicate feral pigs (Sus scrofa) on Santa Cruz Island, California, where efficiency, at least in terms of the deadlines to achieve success, was an explicit goal of the sponsors of the project and of the contractor conducting the work. In general, the sponsors sought rapid abatement of the threats posed by pigs, and it was in the contractor’s interest to complete the eradication quickly and efficiently to maximize their profit under the fixed-price contract agreed with the sponsors. Both parties also recognized that efficiency and speed would result in fewer individual pigs being dispatched because recruitment would be minimized, and that minimizing the total number of pigs needing to be dispatched had animal welfare advantages. Pigs have been deliberately introduced to and established feral populations on many islands around the world (Lever, 1994; Long, 2003), where they have direct and indirect adverse impacts on native species. Turtle eggs, for example, are devoured by pigs (Cruz et al., 2005), and pig rooting of the soil while foraging can fundamentally alter vegetation community dynamics (Mitchell et al., 2007). However, feral pigs have been eradicated from several islands (e.g. Kessler, 2002; Lombardo and Faulkner, 2000; McIlroy, 2005) with Santiago Island in the Galapagos at 58,465 ha being the largest to date (Cruz et al., 2005). Santa Cruz Island is approximately 40 km off the southern California coast. The Nature Conservancy (TNC) owns 76% of the 25,000-ha island and the United States National Park Service (NPS) owns the remainder. Pigs were introduced to the island in the mid-19th century (Sterner and Barrett, 1991), and have since caused extensive damage to the island’s natural and cultural resources. Prior to the arrival of pigs, the island was free of burrowing mammals so pig rooting could erase an otherwise intact archaeological record of human habitation spanning millennia (Schoenherr et al., 1999). Adverse effects on the island’s ecosystem were similarly pervasive. Rooting by pigs altered native vegetation communities through the facilitation of invasion of non-native pest plants, seed predation, and direct destruction of native plants (e.g. Peart et al., 1994). The pigs themselves also provided a food resource that subsidized the colonization of the island by golden eagle (Aquila chrysaetos), which through hyperpredation drove the endemic island fox (Urocyon littoralis) population to near extinction (Roemer et al., 2002). All told, ten species on the island are listed as federally threatened or endangered, with pigs being implicated as a direct and or indirect cause of the decline of all of them (NPS, 2002).

635

In response to this environmental degradation, island managers analysed various alternatives for abating the threat to island resources posed by pigs (NPS, 2002). The preferred alternative was eradication of pigs, facilitated by dividing the island into zones with pig-proof fencing. TNC issued a request for proposals from professional vertebrate pest management providers and selected Prohunt Inc. to conduct the work. The contractor initiated the program in 2005. Here we evaluate the techniques and strategies used to remove the pigs. 2. Methods 2.1. The island Santa Cruz Island is transected by two mountain ranges (741 and 464 m a.s.l.) flanking a fault-dominated central valley. Steepsided canyons traverse both ridge systems. The island is characterized by a Mediterranean climate of cool wet winters and warm dry summers. Vegetative cover on the island is varied, due to soil and slope conditions, as well as historical land use. North-facing slopes tend to be covered by dense evergreen shrub communities (chaparral) dominated by scrub oak (Quercus dumosa) and manzanitas (Arctostaphtlos spp.). South-facing slopes tend to be covered by lower statured deciduous sagebrush. Patches of higher statured vegetation communities occur across the island, including densecanopy oak (Quercus spp.) woodland, which is predominant in canyons, and Bishop pine (Pinus muricata) forest. The road network on the island is minimal and, following rains, unreliable. 2.2. Strategic approach Prior to the launch of the eradication effort the island was divided into five zones using a 9-wire 1-m-high fence. This required 42.6 km of fencing and cost US$42,000 per km for the direct establishment costs. A sequence of control techniques was deployed in each zone. Generally control began with trapping, and then aerial hunting was added, then ground hunting, followed by the use of telemetered pigs to locate remaining pigs. The control began in the western-most zone in late March 2005 and progressed eastwards through the remaining zones as a rolling front of intensifying control effort. Thus, soon after the program began there were different techniques being applied across all zones. The extent that each technique was used in each zone depended on the observed efficacy of each technique in the particular zone, and on the contractor’s perception of the likely vulnerability of pigs to the method in the particular habitats (such as forest cover) in the zone. The eradication team used only firearms to dispatch pigs and only shot when they were assured of a clean kill – a method that meets the standards for euthanasia in the USA (American Veterinary Medical Association, 2001). A critical component of this program is that comprehensive data on all effort and outcomes were collected throughout the program. Hunters, dogs and the helicopter all carried GPS recorders when hunting to measure effort and search patterns. The location where pigs were dispatched was also recorded. The age and sex of most pigs caught in traps was recorded, while the sex of most pigs shot by the ground hunters and about 60% of those shot from the helicopter were recorded. 2.3. Field methods 2.3.1. Trapping Walk-in traps were used to capture pigs, particularly in the more densely vegetated areas. The traps were circular wire mesh pens about 5 m in diameter. The doors were left open and the traps

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baited with cracked corn every second day for between 6 and 109 days depending on whether pigs were still using the trap. They were then set for between 2 and 95 days and all pigs caught were either dispatched, or fitted with radio-transmitters and monitored to locate other wild pigs. 2.3.2. Aerial shooting A Schweizer 269C helicopter was used to locate pigs which were then dispatched by a hunter shooting from the machine. This was done for a daily total of 2–2.5 h just after dawn and just before dusk. Generally, the morning flights were used to support ground hunting teams, while the evening flights were used to dispatch pigs directly from the helicopter. Using a helicopter meant that all areas of the island were covered, usually many times, during the operation. 2.3.3. Ground hunting with dogs Twelve hunters using 23 trained dogs were used during the project. Each zone was divided into hunting blocks (n = 79) with recognisable boundaries such as catchments that could be hunted in 1 day by the hunting team. Each block was hunted twice, with extra effort afforded to places where pigs were known or suspected to be present. Generally, four or five hunters each with one or two dogs spaced themselves no more than 150 m apart and remained in radio contact with one another and with the helicopter pilot. The helicopter was used to position and reposition the hunters and dogs, transport water for the dogs, and replace tiring dogs. The helicopter would land on some vantage point to overlook the hunting block and report to the ground hunters on any pigs seen, or could fly off and dispatch any pigs that breached the hunting line or advanced too far ahead of the ground hunting team. The dogs were trained to hunt in an arc of up to 200 m from their master and to find and bail (i.e. not attack) the pigs. Only the dogs that made first contact would bail the pig; the rest would remain on their station to prevent any pigs escaping back through the hunting line. The nearest hunter would go to the place where the pig was bailed and dispatch it. The dog would then return to its master and the hunting line would advance. All of the dogs underwent aversion training to avoid the endemic fox. 2.3.4. Telemetered animals Radio-telemetered individuals, usually sterilized, were released into an area suspected to contain surviving wild individuals. It is assumed that in social species telemetered animals will associate with wild animals which can then be located and dispatched (e.g. Campbell et al., 2005). Radio-collars or GPS-collars were attached to 71 pigs that had been caught on the island. Variable numbers of these were deployed at any one time in different zones which, in retrospect, had at least one wild pig. The telemetered pigs were located every 2–3 days, using the helicopter, and any associated wild pigs were dispatched. The 44 telemetered females were sterilized and induced into oestrus by injecting them with estradiol at release and some subsequent recaptures to enhance their attractiveness to males (e.g. see Campbell et al., 2005 for development of this method in feral goats). The 27 males were either sterilized when caught or at subsequent captures. These and additional telemetered pigs were left in place and located periodically long after the last ‘wild’ pig was dispatched, to act as a surveillance method. The last of these ‘sentinel’ pigs was dispatched in January 2007. Note that in the Section 3 on the numbers of pigs dispatched we account for telemetered pigs as if they were dispatched at their date of first capture. We estimated the probability of a telemetered pig associating with another telemetered pig for the subset of all telemetered pigs (66 females and 39 males) for which we had at least five locations. This was done by noting the time between the release of the tele-

metered animal in a zone and when it was first seen with other telemetered animals, i.e. the time to first detection. We assumed each telemetered pig in a zone was potentially capable of associating with any other telemetered pig in the zone. We analysed whether the probability of a telemetered pig associating with another telemetered pig depended on the sex of the pig as well as the distance between their centres of activity. Activity centres for each pig were calculated as the arithmetic means of the easting and northing coordinates. The effects of sex and distance between activity centres on the time to first detection were analysed using parametric survival analysis. In this context, a telemetered pig entered a zone at time zero and its ‘survival’ time was estimated. If a telemetered pig was found associating with another telemetered animal at a time after entering the zone, the telemetered animal was deemed to have ‘died’ at that point. Any further associations after the first one did not enter the analysis. Telemetered pigs that were never found associating with another telemetered pig up to the last observation date ‘survived’ until the end of the time period and were right censored (McCallum, 2000) in the analysis. We fitted Weibull and exponential distributions to the time to first detection data to estimate the hazard or detection rate. The Weibull distribution was used to determine whether the association rate changed dependent on the length of time a telemetered pig spent in a zone, while the exponential distribution assumes the association rate was constant over the same time. The effects of the covariates, sex and distance between activity centres were assessed for significance using Wald tests. Analysis was undertaken separately for each zone and by combining data from all zones. The location of most pigs dispatched by aerial hunting (n = 3768) or by ground hunting (n = 260) was later matched with the dominant vegetation type at that GPS point, using a digital map of major vegetation types provided by NPS. The six types we chose were based on our perception of the likely vulnerability of pigs to the two main hunting methods – aerial shooting and ground hunting with dogs. The six types were those dominated by evergreen trees (usually Bishop pine), deciduous trees (usually live oak), evergreen shrublands (usually island scrub oak and manzanita), deciduous scrub (usually sagebrush or buckwheat), grasses and forbs (annual grasses and the exotic weed fennel (Foeniculum vulgare)), and others (usually unvegetated areas). 3. Results 3.1. Number of pigs dispatched A total of 5036 pigs were dispatched (or captured and radio-collared and later dispatched) over 411 days between 29 March 2005 and 10 May 2006; a density of 0.2 pigs/ha (Table 1). 3.2. Trapping Sixteen percent of the pigs finally dispatched were removed in the 102 traps (Table 1) set for 1660 trap-nights giving a catch rate of 0.5 pigs per trap-night. Most traps were set along the central valley of the island and so not all pigs in each zone were likely to encounter the traps. Expected home ranges of pigs was 290 ± 125 ha (TNC, unpubl. data). So while the efficacy of the traps in zone 1, for example, was approximately 20%, that efficacy increased to 45% if only the pigs in an area estimated to be within a homerange of the traps were considered. 3.3. Aerial hunting Seventy-seven percent of pigs were dispatched from the helicopter (Table 1). The total search effort covered 13,822 km (Table

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J.P. Parkes et al. / Biological Conservation 143 (2010) 634–641 Table 1 Number of pigs dispatched by four control methods in five fenced zones on Santa Cruz Island, California. Zone 1 2 3 4 5 Totals a

Area (ha) 5163 4539 6953 2530 5953

Control period (days)

No. pigs trapped

No. pigs dispatched by aerial hunting

168 293 430 251 246

160 121 455 46 33

572 415 1358 73 1450

815

3868

25,138

No. pigs dispatched by ground hunting

No. pigs dispatched by unknown methods

Total No. pigs dispatched

63 41 59 9 89

6 2 16 52 7

801 579 1888 180 1579

261

83

5027a

Does not include nine pigs dispatched without recording the zone.

Table 2 Aerial hunting effort and intensity in five fenced zones on Santa Cruz Island. Zone

Area (km2)

Search effort – flight path lengths (km)

Hunting intensity (km/km2)

Efficiency (pigs dispatched/km flown)

1 2 3 4 5

51.63 45.39 69.53 25.30 59.53

1995 2053 6108 1951 1715

38.6 45.2 87.8 77.1 28.8

0.29 0.20 0.22 0.04 0.85

because close inspection was not usually possible. The sex ratios of trapped and ground-shot pigs, which were mostly sexed (96% and 95%, respectively), were also male biased (Table 4). The sex ratio of the 227 piglets and weaners trapped also favoured males, although the difference was not significantly different from unity (1 male: 0.79 female; P = 0.084), suggesting the larger overall bias in favour of males must be due to differential natural mortality among females. 3.6. Relative vulnerability of pigs to aerial or ground hunting in different vegetation types

2). At least one pig was dispatched on 221 days in about 442 h flying, with the maximum rate being 241 pigs in a day. 3.4. Ground hunting Five percent of the pigs were dispatched by the ground hunters (Table 1). About five hunters operated at any time for a total of 1111 hunter-day’s effort. The mean area hunted each day was 219.5 ± 42.0 ha in the first sweep and 257.2 ± 104 ha in the second sweep with a total distance travelled of 3583 km in the first sweep, 3204 km in the second sweep, and 696 km in the targeted hunts (Table 3). Of the 261 pigs killed, 210 were dispatched in the first sweep of the hunting blocks, 47 during the second sweep and only four when the hunters were targeting known or suspected remaining pigs (Table 3). As it turned out, there were 79 pigs left on Santa Cruz Island after the first ground sweep, 51 of which were dispatched during the subsequent ground hunting (Table 3). 3.5. Sex ratios There was a significant bias towards males (1: 0.56; P < 0.001) among the 3457 pigs whose sex was recorded (Table 4). This bias appears to be real and not an artefact of observer biases, which might be expected among animals dispatched from the helicopter

Table 3 Hunting effort and numbers of pigs dispatched by ground hunters in five fenced zones on Santa Cruz Island.

No. hunting blocks Hunting intensity (hunter-days/km2) No. pigs dispatched Sweep 1 Sweep 2 Targeted sites Pigs left in the zone after Sweep 1

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

22 4.74

16 5.46

19 3.08

8 4.94

14 4.69

52 11 0

29 12 0

48 9 2

9 0 0

72 15 2

22

18

12

1

26

The proportions of pigs dispatched in the six vegetation types differed significantly from that expected if pigs were distributed according to the areas of each type for both aerial hunting (v2 = 509, P < 0.0001) and ground hunting (v2 = 39.8, P < 0.0001). In particular pigs were more vulnerable to ground hunters than aerial hunters in the evergreen forests but more vulnerable to aerial hunters in the open herbaceous and grassland habitats (Table 5). 3.7. Telemetered–telemetered pig associations The numbers of radio-telemetered pigs in each zone used in the analysis of the tagged–tagged association rate varied between 15 and 37 individuals. The results of the survival analysis on the time to first detection for the data combined over all zones indicated

Table 4 Sex ratios of pigs dispatched on Santa Cruz Island. Method

No. males

Trapping Ground hunting Aerial hunting

432 166 1615

No. females 347 82 815

1: 0.80; P = 0.005 1: 0.49; P < 0.001 1: 0.50; P = 0.001

Sex ratios

Totals

2213

1244

1: 0.56; P < 0.001

Table 5 Relative vulnerability of pigs to aerial or ground hunters in six main vegetation types, Santa Cruz Island. Vegetation type

% of island

% of aerial dispatches

% of ground dispatches

Evergreen forest Deciduous forest Evergreen shrubland Deciduous shrubland Herbaceous/grass Other

3.7 2.7 24.9

2.1 4.1 26.6

7.7 4.2 33.5

44.5

32.2

27.3

21.1 3.1

33.8 1.3

23.5 3.9

Total ha/numbers

25,064

3768

260

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J.P. Parkes et al. / Biological Conservation 143 (2010) 634–641

that the fit using the Weibull distribution (non-constant hazard) was not significantly better than the fit using the exponential distribution (constant hazard) (likelihood ratio test; v2 = 1.05, df = 1, P = 0.31). Hence we considered only the exponential distribution for further inference. Using the exponential model, only the effect of distance between activity centres significantly affected the time to detection, with the effect of sex being non-significant (Table 6). Predictions from this model indicated that a telemetered pig had at least a 20% probability of associating with another telemetered pig within 120 days, when their activity centres coincided. However, this dropped away rapidly when activity centres were further away than about 2 km, such that at greater than 6 km separation the probability of association after 120 days was less than 1% (Fig. 1). 3.8. Telemetered–wild pig associations The 27 male telemetered pigs were located on 193 occasions and the 44 female telemetered pigs on 569 occasions. A total of 81 wild pigs were dispatched when found associating with these radio-tagged animals (Table 7).

Table 6 Results of the survival analysis from fitting an exponential distribution to the time to detection data for the combined telemetered pig data. Log(k) – the natural log of the association rate (per day). Term

Log(k)

Intercept Sex Distance

6.2 0.058 0.62

SE

z

P

0.213 0.199 0.084

0.29 7.41

0.77 <0.001

Although female telemetered pigs were deployed in zones at times when, on average, more wild pigs were present (mean = 258) than for the male telemetered pigs (mean = 60.2), female telemetered pigs were still clearly better at attracting wild pigs than male telemetered pigs (Table 7). For the female telemetered pigs, 92% of the wild pigs dispatched when associating with them were males, about twice the expected percentage based on the biased sex ratio in the whole population. 3.9. The last pigs Trapping had ceased in most zones before the population was reduced to very low levels, but both ground and aerial hunting continued long after the last pigs had been (in retrospect) dispatched. Most of the final 105 pigs (i.e. the last 2% on average in each zone) dispatched (ignoring the last radio-collared pigs being used as surveillance animals) were usually dispatched by the ground hunters, although the very last few pigs were generally dispatched by aerial hunting. Only 9% of the last 105 animals were dispatched as a result of their association with a telemetered pig (Table 8). However, once the ground hunting had ceased in each zone, the telemetered pigs found three of the seven wild pigs that remained, and were responsible for the dispatch of the last wild pig in two zones. Of the 93 pigs sexed among this last 105 animals, 55 were males and 38 females which is not significantly different from unity (v2 = 3.11, P = 0.08) and not significantly different from the sex ratio of pigs dispatched earlier (v2 = 0.99, P = 0.32 in a two-sample test for equality of proportions). Hunting units in which most pigs were dispatched, and so presumably where most pigs preferred to be, were more likely to have harboured these last 105 pigs than hunting units in which fewer pigs were dispatched (71.7 ± 12.4 versus 46 4 ± 13.9, t = 2.71, P = 0.008). This and the lack of a strong bias towards either the control method used or the sex of the pigs dispatched suggests luck played a large role in which pigs survived until the last. 3.10. Surveillance

Fig. 1. Probability of a telemetered pig associating with another telemetered pig with distance between their activity centres. Lines represent predictions for different exposure periods.

As the hunters became more confident that no wild pigs remained in each zone (generally about 2 months after they had dispatched the putative last pig) the radio-telemetered pigs became part of a surveillance program as sentinel pigs which, along with ongoing aerial and ground searches, was used to claim the success of the eradication (Table 9). Final aerial searches for pigs or their sign were made in all zones in January and March 2007 after all sentinel animals had been removed. No pigs or fresh sign were found. A Bayesian analysis of the hunters’ belief they had succeeded and the results of the hunting prior to the start of this certification phase gave a probability that wild pigs remained despite the absence of pigs in recent hunting, and showed the extra monitoring required using both ground and/or aerial searches to decrease this probability to levels acceptable to managers – given their perceptions of the consequences of falsely declaring success – is reported elsewhere (Ramsey et al., 2008).

Table 7 Number and type of wild (i.e. without radio-collars) pigs found associating with telemetered pigs.

Male Female Total

No. pigs

No. times located

Mean No. wild pigs present to be detected at each location event

No. wild female associates dispatched

No. wild male associates dispatched

Total associates dispatched

27 44 71

193 569 762

60.2 ± 54.8 258.0 ± 85.9

6 5 11

5 61 66

11 70 81

639

J.P. Parkes et al. / Biological Conservation 143 (2010) 634–641 Table 8 Characteristics of the last 20 pigs dispatched in each of five fenced zones, Santa Cruz Island.

No. pigs dispatched by aerial hunting No. pigs dispatched by ground hunting No. pigs dispatched by trapping Miscellaneous No. of pigs dispatched in association with a telemetered pig Sex ratio Pigs remaining in the zone after last pig dispatched by ground hunters

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Total (%)

13 7 0 0 3 11:7 5

6 14 0 1 0 11:7 5

2 22 0 0 1 12:12 1

10 7 4 0 5 10:8 1

5 13 1 0 0 11:4 0

35 60 4 1 9.8 55:38

Table 9 Surveillance of sentinel pigs in five fenced zones on Santa Cruz Island. Data were not collected for Zone 1. Zone

Date last wild pig dispatched

1

18 September 2005 24 January 2006 6 April 2006 20 June 2006 23 June 2006

2 3 4 5

Time sentinel pigs monitored (months)

No. times located

% of times two or more sentinels were together

No. times a telemetered pig was associated with the last wild pig

Date last sentinel pig removed

5

NA

NA

0

January 2006

11 9 5 9

619 629 253 571

28 30 47 31

0 1 0 0

November 2006 January 2007 December 2006 December 2006

4. Discussion The Santa Cruz Island pig eradication project had risks that are typical of projects requiring successive applications of control. The project was going to take an unknown time, but the longer it took, the higher the risk of failure. Previous successful feral pig eradications over similar scales had taken between 31 and 360 months. The removal of pigs from Pinnacles National Monument took 31 months to remove 200 pigs (McCann and Garcelon, 2007). Eradication of pigs from 21,450-ha Santa Rosa Island (also in the California Channel Islands) took 33 months to remove 1175 pigs (Lombardo and Faulkner, 2000). Eradication of pigs from 58,465ha Santiago Island took 360 months to remove 18,000 pigs (Cruz et al., 2005). On Santa Cruz Island, 14 months lapsed between the first and the last pig to be dispatched. The managers of Santa Cruz Island had many incentives to complete the project as quickly as possible. Aside from the urgency to abate threats to endangered species, legal challenges to halt the eradication persisted throughout the project (Morrison, 2008). Yet, managers were also risk-averse and required a high level of certainty that the last pig dispatched was indeed the last pig on the island. The systems used by the contractor to ensure few pigs escaped their first encounter (Morrison et al., 2007) and their demonstrated hunting and technical skills gave some credence to their claims of success when they could no longer find pigs. Nevertheless, the managers’ risk aversion meant that extensive surveillance (5–11 months in each zone) was required after the putative claim of success. This was done via an ‘independent audit’ of the data collected during the course of the hunting and monitoring phases of the project. This audit quantified the likelihood of pigs remaining after the ‘last’ was dispatched given none were detected during the surveillance phase, and recommended how much extra surveillance (and where it should be located) was required to reduce this likelihood to levels acceptable to the managers (Ramsey et al., 2008). This analysis allowed the managers to make informed and transparent decisions on their risks of demobilizing the control effort. A number of additional factors contributed to the rapid success of the project, and so might be considered by others contemplating large-scale eradication. The contract itself, for example, was a deliberate incentive to efficiency. The fixed-price contract tied pay-

ments to the contractor to measurable performance milestones, i.e. the contractor was paid for completion of milestones irrespective of how long it took or how much it actually cost them. Also, the use of only professional hunters to design and then deploy a sequence of control methods minimized the chance of pigs either learning or escaping from their first encounter with the control technique. Both the sequence of control methods and the humaneness standards minimized the opportunity for a pig to learn to avoid the hunters and become at best a costly animal to ultimately find and dispatch towards the end of the operation (Parkes, 1993) or at worst a survivor that would cause the eradication to fail (Choquenot et al., 1999; Morrison et al., 2007). Comprehensive effort and outcome data were collected and evaluated on a daily basis, which allowed for appropriate short-term adaptive implementation by the hunting team and for informed strategic decisions by the program managers and the contractor in the medium term. The team approach to hunting was also a key to efficiency. The integration of ground and aerial hunting, and the use of highly trained dogs, all supported by communications and GPS technology optimized the chance that all of the pigs in the area being hunted were at risk during any single hunting event. Finally, fencing the island into smaller zones certainly facilitated planning and implementation of the project, and judging by the results from radio-telemetered animals they were largely pig-proof (only a single radio-collared female was known to have breached a fence; TNC, unpubl. data). The sequence and intensity of control methods used was not replicated across the five fenced zones because each zone had different proportions of open and forested habitats (which were predicted to affect the efficacy of different methods), different densities of pigs, different constraints imposed by the land manager’s ability to restrict public access (for safety reasons), and because the hunters learned as they proceeded from one zone to the next. Therefore, although all methods were used in each zone, the results (e.g. in Table 1) from the five zones on Santa Cruz Island cannot be used to explore whether, for example, the fencing or trapping was essential to success. All we can say is that both ground hunting and aerial hunting appeared necessary to dispatch the last pigs in each zone. And, that the helicopter greatly enhanced efficiency and access to all parts of the rugged island. In addition to its use as a shooting platform, it was used to support

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virtually all the other control and field activities which enabled the team to focus their energies on productive tasks. The sex ratio of pigs was strongly biased towards males. Similar biased overall sex ratios (734 males to 628 females; v2 = 8.25, P = 0.004), but unbiased in the youngest age classes, were present among the 1362 pigs shot, mostly in zone 4, on Santa Cruz Island in 1990 (Sterner and Barrett, 1991). This result and our data show that females must have higher mortality rates than males, suggesting that the habitat was not ideal for pigs to raise young – or at least not ideal in some seasons or years. It is possible that the effect is a year-effect since the density of pigs in the 2250-ha fenced exclosure used in Sterner and Barrett’s study in 1989/1990 (in the presence of sheep) was 0.63 pigs/ha compared with only 0.07 pigs/ha in the present study. They suggested the high density was caused by a large acorn mast event in the preceding year, and certainly the age structure of their population was heavily biased towards young pigs (51%). We do not have data on ages of most of the pigs dispatched in 2005/2006, although among those trapped young animals also made up 50% of the sample – but we suspect this is biased as sows and their piglets appeared to be more vulnerable to this control method (Table 3). In contrast, the sex ratio of 455 pigs dispatched by ground hunters and sexed on Santa Rosa Island did not differ from unity (v2 = 0.79, P = 0.37) (Lombardo and Faulkner, 2000). The efficacy of using radio-telemetered animals as a means of finding wild animals depends on the number and nature of the wild animals present when the telemetered animals are deployed. In this case, the contractors did not know how many wild pigs were left in each zone towards the end of the control, and so the contribution of telemetered pigs to the numbers dispatched was modest while the usual hunting methods were continuing. However, once all hunting ceased the telemetered pigs allowed 43% of the last wild pigs to be found and dispatched and accounted for the very last wild pig in two zones. Earlier in the project when there were more wild pigs present, the Santa Cruz Island case showed that adult females in oestrus attracted more wild pigs (mostly males) than male telemetered pigs. The method has been previously tested on a limited number of feral pigs with variable but generally similar results. McIlroy and Gifford (1997) re-located 16 radio-tagged pigs on 419 occasions in a study in Australia and found them to be associated with 109 wild pigs. Again, locally caught adult females in oestrus were on average by far the most effective animals at associating with wild animals, although the single best pig was an adult male. The Santa Cruz Island feral pig eradication project represents an advance in eradication best practice, especially for cases where the eradication is achieved by successive removals of parts of the population using a variety of control methods. The four key factors in this project that, in combination, we think are models for future attempts to eradicate large herbivores are: 1. The NPS and TNC as land managers and project sponsors had completed the substantial consultation and consents processes required under US laws before agreeing to fund and proceed with the eradication (Morrison, 2008). 2. The fixed-price funding model, with a substantial final payment to the contractor based on the absence of feral pigs, drove efficiency – to the benefit of the contractor. The model also shortened the time frame – to the benefit of the sponsors in that it curtailed the adverse publicity and litigation common to such projects and reduced the risks of funder-fatigue seen in projects that drag on with no clear endpoint in sight. 3. The contractor selected by tender had significant experience in eradicating large herbivores. The hunters were skilled in the use of the control tools described in this paper, especially in the use of trained dogs and the ability to service them using a small

helicopter. Their rationale of applying these tools in ways that minimized the opportunities for pigs to escape and learn showed that they had a mind-set to achieve eradication from the start of the project. 4. The contractor also invested in modern technology to collect data on control effort, coverage and success. Many large-scale projects collect such data but few optimize their use to inform either short-term changes in management or to validate success. The Santa Cruz hunters used the data collected on a daily basis to inform their decisions for the following days’ actions, and the data also proved invaluable in providing an objective measure of eradication success and the extent of additional monitoring needed to increase certainty (Ramsey et al., 2008).

Acknowledgements The success of this eradication effort is owed to the efforts of a great many partners and collaborators, including the staffs of NPS, TNC and Prohunt. We especially thank E. Aschehoug, D. Choquenot, K. Faulkner, R. Galipeau, L. Lozier, G. Nugent, A. Saunders, R. Shaw, and L.Vermeer for support and helpful discussions. G. Nugent commented on earlier versions of the paper. J. Barringer analysed the spatial data and G. Forrester advised on the statistical methods. Funding for the eradication program and this paper was provided by TNC and NPS.

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