Potential effects of incorporating fertility control into typical culling regimes in wild pig populations

Evaluating hunting and capture methods for urban wild boar population management

Wild ungulates are expanding in range and number worldwide leading to an urgent need to manage their populations to minimize conflicts and promote coexistence with humans. In the metropolitan area of Barcelona (MAB), wild boar is the main wildlife species causing a nuisance, from traffic accidents to health risks. Selective harvesting of specific sex and age classes and reducing anthropogenic food resources would be the most efficient approach to dealing with overpopulation. Nonetheless, there is a gap in knowledge regarding the age and sex selectivity of the capture methods currently applied in the MAB for wild boar population control. Thus, this study aimed to evaluate the performance and age and sex bias of different hunting and capture methods and the seasonal patterns in their performance (number of captured individuals per event). From February 2014 to August 2022, 1454 wild boars were captured in the MAB using drop net, teleanaesthesia, cage traps, night stalks, and drive hunting. We applied generalized linear models (GLM) to compare the performance of these methods for the total number of wild boars, the wild boars belonging to each age category (i.e., adult, yearling, and juvenile), and for each season. The studied capture methods showed age-class bias and sex bias in adults (>2 years). Drive hunting and drop net removed mainly adult females and yearlings (1-2 years), with drive hunting having the highest performance for adult males. Instead, cage traps and drop net were the best methods to capture juveniles (<1 year). Overall, global performance was higher in summer, decreasingly followed by autumn and spring, winter being the worst performing season. Wildlife managers and researchers should consider the different performance and sex and age bias of each hunting and capture method, as well as the associated public cost, to improve efficiency and achieve the best results in wild boar population management.

Changes in wild pig (Sus scrofa) relative abundance, crop damage, and environmental impacts in response to control efforts

BACKGROUND

As the population and range of wild pigs (Sus scrofa) continue to grow across North America, there has been an increase in environmental and economic damages caused by this invasive species, and control efforts to reduce damages have increased concomitantly. Despite the expanding impacts and costs associated with population control of wild pigs, the extent to which wild pig control reduces populations and diminishes environmental and agricultural damages are rarely quantified. The goal of this study is to quantify changes in wild pig relative abundance and subsequent changes in damages caused by invasive wild pigs in response to control.

RESULTS

Using a combination of wild pig population surveys, agricultural damage assessments, and environmental rooting surveys across 19 mixed forest-agricultural properties in South Carolina, USA, we quantified changes in wild pig relative abundance and associated damages over a 3-year period following implementation of a professional control program. Following implementation of control efforts, both the number of wild pig detections and estimated abundance decreased markedly. Within 24 months relative abundance was reduced by an average of ~70%, which resulted in a corresponding decline in environmental rooting damage by ~99%.

CONCLUSION

Our findings suggest that sustained wild pig control efforts can substantially reduce wild pig relative abundance, which in turn resulted in a reduction in environmental rooting damage by wild pigs. Ultimately this study will help fill critical knowledge gaps regarding the efficacy of wild pig control programs and the effort needed to reduce impacts to native ecosystems, livestock, and crops. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Fertility Control for Wildlife: A European Perspective

Trends of human population growth and landscape development in Europe show that wildlife impacts are escalating. Lethal methods, traditionally employed to mitigate these impacts, are often ineffective, environmentally hazardous and face increasing public opposition. Fertility control is advocated as a humane tool to mitigate these impacts. This review describes mammalian and avian wildlife contraceptives' effect on reproduction of individuals and populations, delivery methods, potential costs and feasibility of using fertility control in European contexts. These contexts include small, isolated wildlife populations and situations in which lethal control is either illegal or socially unacceptable, such as urban settings, national parks and areas where rewilding occurs. The review highlights knowledge gaps, such as impact of fertility control on recruitment, social and spatial behaviour and on target and non-target species, provides a decision framework to assist decisions about the potential use of wildlife fertility control, and suggests eight reasons for Europe to invest in this area. Although developing and registering contraceptives in Europe will have substantial costs, these are relatively small when compared to wildlife's economic and environmental impact. Developing safe and effective contraceptives will be essential if European countries want to meet public demand for methods to promote human-wildlife coexistence.

The status of fertility control for rodents-recent achievements and future directions

Management of overabundant rodents at a landscape scale is complex but often required to sustainably reduce rodent abundance below damage thresholds. Current conventional techniques such as poisoning are not species specific, with some approaches becoming increasingly unacceptable to the general public. Fertility control, first proposed for vertebrate pest management over 5 decades ago, has gained public acceptance because it is perceived as a potentially more species-specific and humane approach compared with many lethal methods. An ideal fertility control agent needs to induce infertility across one or more breeding seasons, be easily delivered to an appropriate proportion of the population, be species specific with minimal side-effects (behavioral or social structure changes), and be environmentally benign and cost effective. To date, effective fertility control of rodents has not been demonstrated at landscape scales and very few products have achieved registration. Reproductive targets for fertility control include disrupting the hormonal feedback associated with the hypothalamic-pituitary-gonadal axis, gonad function, fertilization, and/or early implantation. We review progress on the oral delivery of various agents for which laboratory studies have demonstrated efficacy in females and/or males and synthesize progress with the development and/or use of synthetic steroids, plant extracts, ovarian specific peptides, and immunocontraceptive vaccines. There are promising results for field application of synthetic steroids (levonorgestrel, quinestrol), chemosterilants (4-vinylcyclohexene diepoxide), and some plant extracts (triptolide). For most fertility control agents, more research is essential to enable their efficient and cost-effective delivery such that rodent impacts at a population level are mitigated and food security is improved.

Effects of a Recombinant Gonadotropin-Releasing Hormone Vaccine on Reproductive Function in Adult Male ICR Mice

Gonadotropin-releasing hormone (GnRH) regulates the reproductive endocrine system in mammals. The GnRH immunocontraception vaccine can aid animal population control and management. We evaluated a recombinant GnRH fusion protein with the adjuvant MONTANIDE ISA 206 VG as a GnRH vaccine in adult male ICR mice by evaluating anti-GnRH antibodies; concentrations of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone; testis size and histomorphology; and semen quality. Response was assessed after intramuscular administration of the vaccine to mice in weeks 0, 4, and 8. The vaccine induced specific antibody response by week 5, with peak of antibody levels observed by week 13 and a declining level thereafter until the end of the study at week 24. Furthermore, it reduced serum FSH, LH, and testosterone concentrations. The vaccinated mice exhibited testicular atrophy and reduced sperm quality, concentration, morphology, and viability compared to control males. The outcomes of pairings of treated males with untreated females revealed reduced mating, pregnancy rates and number of litters compared to control pairings. Assessment of this GnRH vaccine in different species could assist its development for future applications.

The scale-dependent effectiveness of wildlife management: A case study on British deer

Impacts of herbivory by wild ungulates represent a significant issue world-wide. To be effective, management of populations and impacts needs to be coordinated above the site scale, yet little research has investigated the appropriate spatial scale over which management should be integrated to be fully effective. In consideration of reduction of impacts in deciduous or mixed woodland habitats, we tested scale-specific management effectiveness in a lowland area of UK where moderate- to high-density populations of four deer species were the target of deliberate control programmes, and nonhuman predators were absent. We modelled the annual impact recorded between 2009 and 2015 in 98 woodlands as a function of cumulative culls of deer taken since the commencement of management. Analysis was repeated at different spatial scales by increasing the circular area around each focal woodland, from 2.5 km-radius up to 100 km-radius. Our findings suggest for the first time the geographical scale over which deer management needs to be coordinated for optimum effectiveness in decreasing their impact on woodland across relatively homogenous landscapes. For small bodied and relatively sedentary species (roe deer Capreolus capreolus; Reeves' muntjac Muntiacus reevesi), reductions in impacts within woodlands can be achieved by culling at the immediately local level, but some modest increase in effectiveness (probably relating to reductions in the degree of source-sink movement) may be expected with an increase in spatial scale of culling to around 30-70 km-radius. For larger-bodied, herding species with more extensive home-ranges (fallow deer Dama dama; red deer Cervus elaphus) management for reduction of woodland impacts was only really effective when coordinated above the single woodland-scale, with marked increases shown again up to a scale of 100 km-radius. Whilst future studies for different landscape types are still needed, our work emphasises that the spatial scale at which control plans are conducted can determine the effectiveness of wildlife management, possibly providing an advance on how to manage wildlife populations more effectively.

Too many wild boar? Modelling fertility control and culling to reduce wild boar numbers in isolated populations

Wild boar and feral swine number and range are increasing worldwide in parallel with their impact on biodiversity and human activities. The ecological and economic impact of this species include spread of diseases, vehicle collisions, damage to crops, amenities and infrastructures and reduction in plant and animal abundance and richness. As traditional methods such as culling have not contained the growth and spread of wild boar and feral pigs, alternative methods such as fertility control are now advocated. We used empirical data on two isolated wild boar populations to model and compare the effects of different regimes of culling and fertility control on population trends. We built a Bayesian population model and applied it to explore the implications for population control of various management options combining culling and/or contraception. The results showed that, whilst fertility control on its own was not sufficient to achieve the target reduction in wild boar number, adding fertility control to culling was more effective than culling alone. In particular, using contraceptives on 40% of the population to complement the culling of 60% of the animals, halved the time to achieve our target reduction compared with culling only. We conclude that, assuming the effort of adding fertility control to culling was found to be cost-effective in terms of population reduction, these two methods should be used simultaneously if a rapid decrease in wild boar number is required for a closed population.

Optimal spatial prioritization of control resources for elimination of invasive species under demographic uncertainty

Populations of invasive species often spread heterogeneously across a landscape, consisting of local populations that cluster in space but are connected by dispersal. A fundamental dilemma for invasive species control is how to optimally allocate limited fiscal resources across local populations. Theoretical work based on perfect knowledge of demographic connectivity suggests that targeting local populations from which migrants originate (sources) can be optimal. However, demographic processes such as abundance and dispersal can be highly uncertain, and the relationship between local population density and damage costs (damage function) is rarely known. We used a metapopulation model to understand how budget and uncertainty in abundance, connectivity, and the damage function, together impact return on investment (ROI) for optimal control strategies. Budget, observational uncertainty, and the damage function had strong effects on the optimal resource allocation strategy. Uncertainty in dispersal probability was the least important determinant of ROI. The damage function determined which resource prioritization strategy was optimal when connectivity was symmetric but not when it was asymmetric. When connectivity was asymmetric, prioritizing source populations had a higher ROI than allocating effort equally across local populations, regardless of the damage function, but uncertainty in connectivity structure and abundance reduced ROI of the optimal prioritization strategy by 57% on average depending on the control budget. With low budgets (monthly removal rate of 6.7% of population), there was little advantage to prioritizing resources, especially when connectivity was high or symmetric, and observational uncertainty had only minor effects on ROI. Allotting funding for improved monitoring appeared to be most important when budgets were moderate (monthly removal of 13-20% of the population). Our result showed that multiple sources of observational uncertainty should be considered concurrently for optimizing ROI. Accurate estimates of connectivity direction and abundance were more important than accurate estimates of dispersal rates. Developing cost-effective surveillance methods to reduce observational uncertainties, and quantitative frameworks for determining how resources should be spatially apportioned to multiple monitoring and control activities are important and challenging future directions for optimizing ROI for invasive species control programs.

Factors and costs associated with removal of a newly established population of invasive wild pigs in Northern U.S

The human-mediated spread of exotic and invasive species often leads to unintentional and harmful consequences. Invasive wild pigs (Sus scrofa) are one such species that have been repeatedly translocated throughout the United States and cause extensive damage to natural ecosystems, threatened and endangered species, agricultural resources, and private lands. In 2005, a newly established population of wild pigs was confirmed in Fulton County, Illinois, U.S. In 2011, a state-wide wild pig damage management program involving federal, state, and local government authorities directed a concerted effort to remove wild pigs from the county until the last wild pig (of 376 total) was successfully removed in 2016. We examined surveillance data from camera traps at bait sites and records of wild pig removals during this elimination program to identify environmental and anthropogenic factors that optimized removal of this population. Our results revealed that wild pigs used bait sites most during evening and nocturnal periods and on days with lower daily maximum temperatures. Increased removals of wild pigs coincided with periods of cold weather. We also identified that fidelity and time spent at bait sites by wild pigs was not influenced by increasing removals of wild pigs. Finally, the costs to remove wild pigs averaged $50 per wild pig (6.8 effort hours per wild pig) for removing the first 99% of the animals. Cost for removing the last 1% increased 84-fold, and averaged 122.8 effort hours per wild pig removed. Our results demonstrated that increased effort in removing wild pigs using bait sites should be focused during periods of environmental stress to maximize removal efficiency. These results inform elimination programs attempting to remove newly established populations of wild pigs, and ultimately prevent population and geographic expansion.

Effect of vaccination with a novel GnRH-based immunocontraceptive on immune responses and fertility in rats

1. As human-wildlife conflicts increase worldwide, novel methods are required for mitigating these conflicts. Fertility control, based on immunocontraceptives, has emerged as an alternative option to lethal methods for managing wildlife.

2. Immunocontraceptives are vaccines that generate an immune response to key components of an animal's reproductive system. Some of these vaccines target the gonadotropin-releasing hormone (GnRH) and have been used successfully as contraceptives for many wildlife species. However, the need to capture animals for treatment limits the field applications of injectable vaccines. The availability of orally delivered immunocontraceptives would increase the breadth of applications of fertility control for wildlife management.

3. This study explored a new approach to developing an oral immunocontraceptive, exploiting the bioadhesive and immunologically active properties of killed Mycobacterium avium cell wall fragments (MAF). The MAF was conjugated to a GnRH recombinant protein called IMX294, used as a GnRH-specific immunogen.

4. An initial trial using the MAF-IMX294 conjugate provided the first evidence that an orally delivered immunocontraceptive vaccine could generate anti-GnRH antibody titres in laboratory rats.

5. Increasing the dose and frequency of vaccine administered to rats, in a second trial, enhanced the immune response, eliciting titres that reduced the proportion of females giving birth. This provided the first evidence of the contraceptive effect of an oral anti-GnRH vaccine.

6. Future work is required to further increase the immunogenic effect of the oral vaccine and to establish a dosing schedule that is effective for practical field applications.

Ecological drivers of African swine fever virus persistence in wild boar populations: Insight for control

Environmental sources of infection can play a primary role in shaping epidemiological dynamics; however, the relative impact of environmental transmission on host-pathogen systems is rarely estimated. We developed and fit a spatially explicit model of African swine fever virus (ASFV) in wild boar to estimate what proportion of carcass-based transmission is contributing to the low-level persistence of ASFV in Eastern European wild boar. Our model was developed based on ecological insight and data from field studies of ASFV and wild boar in Eastern Poland. We predicted that carcass-based transmission would play a substantial role in persistence, especially in low-density host populations where contact rates are low. By fitting the model to outbreak data using approximate Bayesian computation, we inferred that between 53% and 66% of transmission events were carcass-based that is, transmitted through contact of a live host with a contaminated carcass. Model fitting and sensitivity analyses showed that the frequency of carcass-based transmission increased with decreasing host density, suggesting that management policies should emphasize the removal of carcasses and consider how reductions in host densities may drive carcass-based transmission. Sensitivity analyses also demonstrated that carcass-based transmission is necessary for the autonomous persistence of ASFV under realistic parameters. Autonomous persistence through direct transmission alone required high host densities; otherwise re-introduction of virus periodically was required for persistence when direct transmission probabilities were moderately high. We quantify the relative role of different persistence mechanisms for a low-prevalence disease using readily collected ecological data and viral surveillance data. Understanding how the frequency of different transmission mechanisms vary across host densities can help identify optimal management strategies across changing ecological conditions.

The risk of foot-and-mouth disease becoming endemic in a wildlife host is driven by spatial extent rather than density

In the past 20 years, free living populations of feral wild boar have re-established in several locations across the UK. One of the largest populations is in the Forest of Dean where numbers have been steadily increasing since monitoring began in 2008, with estimates from 2016 reporting a population of more than 1500. Feral wild boar have significant ecological and environmental impacts and may present a serious epidemiological risk to neighbouring livestock as they are a vector for a number of important livestock diseases. This includes foot-and-mouth disease (FMD) which is currently absent from the UK. We developed an individual-based spatially explicit modelling approach to simulate feral wild boar populations in the Forest of Dean (England, UK) and use it to explore whether current or future populations might be sufficient to produce long-lived outbreaks of FMD in this potential wildlife reservoir. Our findings suggest that if you exclude the spread from feral wild boar to other susceptible species, the current population of boar is insufficient to maintain FMD, with 95% of unmanaged simulations indicating disease burn-out within a year (not involving boar management specifically for disease). However, if boar are allowed to spread beyond their current range into the adjacent landscape, they might maintain a self-sustaining reservoir of infection for the disease.

Fertility Control in Wildlife: Review of Current Status, Including Novel and Future Technologies

Wildlife conservation requires varying degrees of management of endangered species as well as management of their potential predators and competitors. Conservation also depends on ameliorating human-wildlife conflict, especially where there is a threat to the safety of people and of their pets and livestock. In addition, sustainability of wildlife populations can be affected by feral domestic animals or invasive species, that require removal or control. With the increasing concern for animal welfare, non-lethal methods of limiting population size and distribution, such as fertility control, are gaining favor. Breeding programs in zoos depend on highly effective and selective contraception to manage sustainable insurance populations. This review covers fertility control methods currently in use, those that have not lived up to past promises, and others that are under development and present hope for addressing remaining challenges.

Stochastic assessment of management strategies for a Mediterranean peri-urban wild boar population

Wild boar (Sus scrofa) population spread into urban and periurban areas has exacerbated conflicts with humans. There is a need for planned wild boar management strategies, and Population viability analysis (PVA) combined with perturbation analyses allow the assessment of the management effort of control methods. Our study aims to develop stochastic predictive models of the increasing wild boar population of the 80 km² peri-urban Mediterranean area of Collserola Natural Park (CNP), located near Barcelona, Spain, as well as assessing specific management measures (including reduced food availability, selective harvest, and reduction in fertility). Population parameters were estimated from previously published census and hunting data provided by the CNP and the local hunting administration. The results revealed that under the current conditions the CNP wild boar population will continue to increase. The most efficient strategy to reduce wild boar abundance was a combination of reducing supplementary anthropogenic food resources and selective removal of juvenile (<1 year) and yearling (1–2 years) wild boar. These strategies will probably be also the most efficient ones in other oversupplemented increasing wild boar populations in similar situations, although specific studies will be needed to fine-tune the best management option for each context. PVA allows the prediction of future population trends and the assessment of the efficacy and efficiency of potential management strategies before implementing management measures.

Could current fertility control methods be effective for landscape-scale management of populations of wild horses (Equus caballus) in Australia?

Context Fertility control is seen as an attractive alternative to lethal methods for control of population size and genetic diversity in managed animal populations. Immunocontraceptive vaccines have emerged as the most promising agents for inducing long-term infertility in individual animals. However, after over 20 years of scientific testing of immunocontraceptive vaccines in the horse, the scientific consensus is that their application as a sole management approach for reducing population size is not an effective strategy. Aims The purpose of this review is to evaluate currently available non-lethal fertility-control methods that have been tested for their contraceptive efficacy in Equidae, and to assess their suitability for effective management of wild (feral) horses in an Australian setting. Key results (1) Fertility-control agents, particularly injectable immunocontraceptive vaccines based on porcine zona pellucida (PZP) or gonadotrophin-releasing hormone (GnRH), can induce multi-year infertility (up to 3 years) in the horse. Some formulations require annual or biennial booster treatments. Remote dart delivery (on foot) to horses is possible, although the efficacy of this approach when applied to large numbers of animals is yet to be determined. (2) The proportion of females that must be treated with a fertility-control agent, as well as the frequency of treatment required to achieve defined management outcomes (i.e. halting population growth in the short term and reducing population size in the long term) is likely to be >50% per annum. In national parks, treatment of a large number of wild horses over such a broad area would be challenging and impractical. (3) Fertility control for wild horses could be beneficial, but only if employed in conjunction with other broad-scale population-control practices to achieve population reduction and to minimise environmental impacts. Conclusions In Australia, most populations of wild horses are large, dispersed over varied and difficult-to-access terrain, are timid to approach and open to immigration and introductions. These factors make accessing and effectively managing animals logistically difficult. If application of fertility control could be achieved in more than 50% of the females, it could be used to slow the rate of increase in a population to zero (2–5 years), but it will take more than 10–20 years before population size will begin to decline without further intervention. Thus, use of fertility control as the sole technique for halting population growth is not feasible in Australia.