Ecology and Management of Invasive Wild Pigs

Wild pigs are a highly invasive species that are rapidly expanding in number and distribution throughout the U.S. It is estimated that wild pig populations now exceed 6 million animals in the U.S. alone, causing billions of dollars in ecological and agricultural damages annually. In addition, vehicle collisions with wild pigs represent a growing threat to human safety. Despite the significant ecological and economic impact of expanding wild pig populations, few tools are available to effectively control the rapid spread of this invasive species. Moreover, although estimates of abundance are a central component of any species management program, reliable estimators for free-ranging pig populations are limited due to a number of ecological and economic factors. Our lab is one of the leading academic research programs studying invasive wild pigs in the U.S. and more broadly across the globe. We work closely with various state and federal partners to better understand the ecology and impacts of wild pigs, as well as assess and optimize the efficacy of management programs for controlling this destructive invasive species.

Wild pig social ecology

Describing the genetic and demographic composition of social groupings is important in the conservation and management of species, particularly in modelling disease transmission dynamics. However, social organization can vary across a taxa’s range, and data on wild pig social groups is primarily limited to observational studies. Invasive wild pig social organization is assumed to be composed of two general social units: matriarchal groups (sounders) and solitary males. Sounders have long been believed to be groups of close female kin with their dependent offspring. Males are assumed to always be solitary after dispersing from their natal range, except when courting potential mates, although some groups of young males have also been observed. We are using a large, long term genomic dataset collected from the Savannah River site and South-central Oklahoma to describe patterns in wild pig social group demography and relatedness. Results of this research thus far has revealed that sounders are the most common social unit for wild pigs, but that pseudo-solitary females and male dominated bachelor groups were observed more frequently than previously reported. Also, although primarily composed of close female kin, some sounders also include unrelated females, suggesting not all sounders are intact family units. These data represent one of the most comprehensive genomic assessments of wild pig social organization to date, information that will be used to better inform the management of this species in their invasive range.

Spatial ecology, contact structure, and disease dynamics in wild pigs

We are working with researchers at the USDA-APHIS-WS-National Wildlife Research Center and several collaborators on numerous studies to better understand the movement ecology of wild pigs to better understand how movement behavior may influence disease transmission dynamics. Using a combination of movement data from GPS collared wild pigs, genetic data, and disease data our goal is to elucidate the spatial, genetic, and disease associations of local pig populations, both within and among sounders. By using modern trapping techniques that facilitate sampling of entire sounders, we are able to assess the fine-scale structure and disease dynamics within pig populations at the level of individual sounders. These data coupled with information on the movement behavior of each sampled sounder will be used to develop and refine analytical models to predict the spread of pig populations as well as the dissemination of infectious diseases of concern to human or livestock health. We also are using an extensive network of GPS collars deployed on wild pigs to better understand both inter- and intra-group movement behavior, as well as well as how landscape attributes and wild pig behavioral states influence resource selection patterns. To date we have deployed several dozen GPS collars on wild pigs as well as contact loggers that record inter- and intra-group interactions, representing one of the largest spatial datasets of wild pig movement behavior in the U.S. Results from this research will aid in evaluating and refining the efficacy of control methods to facilitate management of wild pigs in the U.S. and other regions where this species is invasive.

Evaluation of baits and lures in attracting wild pigs for population management

There is a great need to improve the efficiency and efficacy of methods for controlling rapidly expanding populations of invasive wild pigs. Baits such as corn and peanuts are frequently used as attractants for wild pigs when conducting population control. However, examining the efficacy of a broader scope of attractants will lead to more efficient and cost-effective management of wild pigs. Working with the USDA-APHIS-WS-National Wildlife Research Center and the Texas Parks and Wildlife Department, we are studying the efficacy of an extensive range of attractants (from food-based to pheromones) in SC and TX to determine the most effective lures in attracting wild pigs through time, space, and across seasons. Building on the results from this research, we are also collaring wild pigs to examine their fine-scale movements in response to different combinations of lures and food baits. The results of this research ultimately will provide important information on the potential utility of alternative baits and lures and their optimal application in the control of wild pig populations.

Evaluation of changes in wild pig populations as well as agricultural and environmental damage in response to implementation of population control

Wild pigs are a substantial threat to agricultural production throughout their invasive range. As the population and range of wild pigs continues to grow across North America, managers are faced with the mounting challenge of maximizing the efficiency of control and monitoring efforts with limited resources. However, there are currently few published data on the spatial and temporal characteristics of wild pig damage to both native and agricultural systems as well as wild pig responses to management. We are conducting a series of studies to better understand patterns of wild pig damage to resources, as well as the influence of removal efforts on wild pig population size and extent of damages to both agriculture and native ecosystems. In addition, we are quantifying the effectiveness of different trapping techniques as well as changes in wild pig populations, environmental damage, and agricultural damage in conjunction with wild pig control efforts on private lands in SC. This research will ultimately improve management strategies to help reduce the impacts of wild pigs on our native ecosystems, livestock, crops, and human health.

Evaluation of wild pig impacts to native species through dietary studies

Biological invasions are one of the leading causes of global biodiversity loss, second only to habitat loss and deforestation. Invasive wild pigs are one of the most widely distributed invasive vertebrates and pose a threat to numerous taxa and ecosystems globally due to their high reproductive rates, adaptability, destructive rooting habits, and generalist, omnivorous diets. In particular, dietary analyses have revealed wild pigs consume diverse plant and animal taxa, including birds, small mammals, reptiles, and amphibians. With rapidly increasing wild pig populations worldwide and an ever-expanding range, there is rising concern regarding the impacts of wild pigs to sensitive taxa such as ground-nesting birds and reptiles (e.g., sea turtles). Most wild pig diets studies to date have utilized manual observation methods, which have the potential to underrepresent the consumption of soft-bodied animals due to rapid digestion rates. Recent dietary research using molecular methods suggests that in addition to ground-nesting birds, reptiles, and small mammals, amphibians are particularly vulnerable to depredation by wild pigs. We are conducting a series of studies across a range of physiographic regions in the southeastern U.S. to assess the consumption of vertebrates by wild pigs during periods of potential peak vulnerability to predation by wild pigs. The results of this research will produce some of the most substantial assessments of wild pig diets to date using molecular approaches, information that can be applied to inform focused management of wild pigs to better reduce impacts to sensitive wildlife species.

Reproductive ecology of wild pigs in the southeastern U.S.

Wild pigs throughout most of North America are genetic hybrids of feral domestic pigs and wild boar and have the highest reproductive potential of any wild ungulate. Although reproduction in domestic pigs is well-studied, the phenology of reproduction, extent of multiple reproductive events per year, how individual and extrinsic factors contribute to variability in productivity, and impact of genetic lineage on these parameters is not well understood in wild pigs. We are conducting a series of studies to better understand the reproductive ecology of wild pigs through direct counts of fetuses in pregnant sows, ovarian analyses, measurements of fetuses, hormone analyses, and tracking the movements of pregnant sows. We are using data collected from these studies to determine the age at sexual maturity for wild pigs as well as the underlying biotic and abiotic factors influencing reproductive rates, litter sizes, and sex ratios. In addition, we are investigating how sow movements change between pre-parturition, parturition, and post-parturition by fitting pregnant sows with GPS collars and vaginal implant transmitters that signal the exact timing of the birth event as well as the location and composition of the farrowing nest. Collectively, these data will be used to better understand the farrowing ecology of this species as well as the underlying factors most influential in driving reproductive output in wild pigs.

Estimation of wild piglet survival rates

In addition to reproduction, survival is an important driver of population models. Survival can fluctuate with age, size, season, landscape characteristics, resource availability, and genetics. While adult survival is relatively high and constant, juvenile survival may be more sensitive to density dependence. Although recent studies have emphasized the importance of juvenile recruitment in wild pigs to population growth, little research has been performed to estimate piglet survival in their introduced range: vital information for modeling pig population dynamics. We are conducting the first radio-telemetry studies of wild piglet survival in the introduced range of this species, investigating survival as a function of multiple demographic and environmental factors. Increased knowledge of piglet survival rates will aid in the development of more informed population projection models, and therefore help inform management programs to reduce the growth of destructive wild pig populations.

Evaluation of methods to estimate wild pig abundance and improve capture rates of wild pig social groups

Currently, wild pig population sizes are most often estimated through the use of harvest indices, which are frequently biased but often assumed to be the most practical method for surveying populations across broad geographic extents. Improved methods of estimating abundance of this elusive species are necessary to aid researchers and managers in evaluating efficacy of control techniques, predicting potential impacts on ecosystems, and modeling disease risks. Similarly, numerous trap styles and trapping approaches are used across the U.S. to capture wild pig sounders, yet few studies have evaluated performance of these various trap styles in real-world scenarios. In collaboration with the USDA-APHIS Wildlife Services, we are evaluating a broad suite of methods for estimating population size as well as multiple commonly used trap types for capturing wild pigs. These population estimation methods range from capture-based removal models to more labor-intensive genetic capture-mark-recapture and camera-based spatially explicit capture recapture approaches. The results of these studies will have direct implications for the development of robust wild pig population control programs, as well as optimized methodologies for monitoring changes in wild pig populations in response to implementation of control programs.

Movement ecology of translocated wild pigs

Wild pig populations have been increasing in their introduced range as a result of intentional, and often illegal, translocation efforts by pig-hunting enthusiasts. These translocations are particularly harmful because of the ability of pigs to establish new populations in areas to which they are introduced. There has been practically no research, however, on the behavior of wild pigs following translocation. This is a critical data gap as the movement ecology of translocated wild pigs will have substantive implications for their ability to establish new populations and spread diseases to local wildlife, livestock, and humans. We are performing experimental translocations of GPS-collared pigs within the bounds of the Savannah River site to fill this data gap by investigating how the translocation process affects the spatial ecology and survival of wild pigs. This information will be used to model disease transmission risks and a number of other potential impacts of translocated pigs, informing management in areas where a translocation is suspected to have taken place recently, and provide insights into broader processes of invasion ecology.

Population control and impacts of wild pigs on coastal islands

Coastal islands provide critical nesting and stopover habitat for many species of conservation concern, particularly sea turtles and beach nesting shorebirds. However, islands are among the most sensitive ecosystems and thus highly vulnerable to impacts from wild pigs, with wild pigs playing a substantial role in the extinction or extirpation of numerous species from islands. Within the Southeastern U.S. wild pigs are abundant and widely distributed on many coastal islands, where they negatively impact threatened and endangered species directly through predation and indirectly by contributing to beach erosion, spread of invasive plants, and habitat destruction. Despite their extensive distribution and impacts on coastal islands in the Southeastern U.S., management efforts are often insufficient to eliminate populations. Thus, there is an urgent need for proactive and extensive control to eliminate the threat to sensitive habitats and wildlife of conservation concern We are collaborating with the USFWS to develop an adaptive management program for invasive wild pigs that is optimized for coastal island ecosystems. This project involves intensive monitoring of wild pig populations, their movement behavior, and impacts in conjunction with varied population control techniques. Ultimately, this research will yield critical insights into the development of eradication and population control programs that can be optimized for island ecosystems in the southeast.

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