U.S. Department of Agriculture U.S. Government Publication Animal and Plant Health Inspection Service Wildlife Services

ORIGINAL PAPER

Historical, current, and potential population size estimatesof invasive wild pigs (Sus scrofa) in the United States

Jesse S. Lewis . Joseph L. Corn . John J. Mayer . Thomas R. Jordan .

Matthew L. Farnsworth . Christopher L. Burdett . Kurt C. VerCauteren .

Steven J. Sweeney . Ryan S. Miller

Received: 5 February 2018 / Accepted: 30 March 2019 / Published online: 4 April 2019

� Springer Nature Switzerland AG 2019

Abstract To control invasive species and prioritize

limited resources, managers need information about

population size to evaluate the current state of the

problem, the trend in population growth through time,

and to understand the potential magnitude of the

problem in the absence of management actions. This

information is critical for informing management

actions and allocating resources. We used two

national-scale data sets to estimate historical, current,

and future potential population size of invasive wild

pigs (Sus scrofa; hereafter wild pigs) in the United

States. Between 1982 to present, the Southeastern

Cooperative Wildlife Disease Study mapped the

distribution of wild pigs in the United States. In

addition, recent research has predicted potential

population density of wild pigs across the United

States by evaluating broad-scale landscape character-

istics. We intersected these two data sets to estimate

the population size of wild pigs in 1982, 1988, 2004,

2010, 2013, and 2016. In addition, we estimated

potential population size if wild pigs were present at

equilibrium conditions in all available habitat in each

state. We demonstrate which states have experienced

recent population growth of wild pigs and are

predicted to experience the greatest population

increase in the future without sufficient management

actions and policy implementation. Regions in the

For inquiries about how this work informs the National Feral

Swine Damage Management Program please contact

ryan.s.miller@aphis.usda.gov.

Electronic supplementary material The online version ofthis article (https://doi.org/10.1007/s10530-019-01983-1) con-tains supplementary material, which is available to authorizedusers.

J. S. Lewis (&) � M. L. Farnsworth

Conservation Science Partners, 5 Old Town Sq, Suite 205,

Fort Collins, CO 80524, USA

e-mail: jslewi10@asu.edu

J. L. Corn

Southeastern Cooperative Wildlife Disease Study,

Department of Population Health, College of Veterinary

Medicine, University of Georgia, 589 D.W. Brooks Drive,

Athens, GA 30602, USA

J. J. Mayer

Savannah River National Laboratory, Savannah River

Nuclear Solutions, LLC, Aiken, SC 29808, USA

T. R. Jordan

Center for Geospatial Research, Department of

Geography, University of Georgia, Athens, GA 30602,

USA

C. L. Burdett

Department of Biology, Colorado State University,

Fort Collins, CO 80524, USA

K. C. VerCauteren

USDA/APHIS/Wildlife Services, National Wildlife

Research Center, 4101, Laporte, Fort Collins, CO 80521,

USA

123

Biol Invasions (2019) 21:2373–2384

https://doi.org/10.1007/s10530-019-01983-1(0123456789().,-volV)( 0123456789().,-volV)

western, northern, and northeastern United States

contain no or few wild pig populations, but could

potentially support large numbers of these animals if

their populations become established. This informa-

tion is useful in identifying regions at greatest risk if

wild pigs become established, which can assist in

prioritizing management actions aimed at controlling

or eliminating this invasive species across broad to

local scales.

Keywords Feral swine � Invasive species �Introduced species � Population size �Range expansion

Introduction

Invasive species are one of the leading threats to

ecosystems and biodiversity around the world and

negatively impact myriad landscapes and ecological

communities, many of which support sensitive, threat-

ened, or endangered species (Mack et al. 2000;

Wilcove et al. 1998; Yiming and Wilcove 2005). In

the United States alone, invasive species are respon-

sible for over $120 billion in damage and control costs

each year (Pimentel 2007). Importantly, land man-

agement agencies have implemented aggressive man-

agement plans to combat the spread and impacts of

introduced plants and animals (Centner and Shuman

2015) through early detection and rapid response

strategies, eradication efforts, control actions, and

ecological restoration (NISC 2016; USDOI 2016).

Impacts of invasive species are expected to intensify

and expand to new areas over the next several decades

in response to expanding populations, new introduc-

tions, and shifting vegetation communities resulting

from land-use and climate change (Early et al. 2016).

To control invasive species and prioritize limited

resources, managers need information about

population size to evaluate the current state of the

problem, the trend in population growth through time,

and to understand the potential magnitude of the

problem without management actions. This informa-

tion is critical for informing management actions and

allocating resources.

Invasive wild pigs (Sus scrofa; hereafter wild pigs;

other common names include wild boar, wild/feral

swine, wild/feral hog, and feral pig) (Keiter et al.

2016) are one of the 100 most destructive invasive

species in the world (Lowe et al. 2000) with far

reaching economic, ecological, and social impacts

across local, national, and global scales (Barrios-

Garcia and Ballari 2012; Bevins et al. 2014). In the

United States, wild pigs are responsible for over $1.5

billion in damage annually (Pimentel 2007) that has

resulted in the creation of a national program to

mitigate damages to natural ecosystems, residential

developments, agricultural, and rangelands (Miller

et al. 2018). For damage to six crops across ten states

with robust wild pig populations, estimated annual

losses from wild pigs equal nearly $200 million

(Anderson et al. 2016). In natural ecosystems, they can

severely damage broad areas and sensitive ecological

communities, especially riparian areas, grasslands,

and deciduous forests (Barrios-Garcia and Ballari

2012; Bevins et al. 2014; Hone 2012). In addition, wild

pigs can impact a variety of rare, threatened, and

endangered species through habitat destruction, direct

predation, and competition for resources (Barrios-

Garcia and Ballari 2012). Finally, wild pigs can host a

suite of viruses, bacteria, and parasites, many of which

can be transmitted to other wildlife, humans, and

livestock (e.g., classical swine fever, pseudorabies

virus, Brucella spp., Trichinella sp., Toxoplasma sp.,

E. coli, Mycobacterium tuberculosis complex) (Bev-

ins et al. 2014; Jay et al. 2007; Meng et al. 2009; Miller

et al. 2017).

Wild pigs are currently experiencing global range

expansion due to translocations by humans, natural

dispersal, and favorable changes in environmental

conditions. In particular, widespread and illegal

releases of wild pigs for the purposes of sport hunting

continue across numerous countries today (Long

2003), especially in the United States (Bevins et al.

2014; Gipson et al. 1998; Hernandez et al. 2018;

Tabak et al. 2017). In addition, the geographic

distribution and abundance of wild pig populations

within their native and non-native ranges have

S. J. Sweeney � R. S. Miller

United States Department of Agriculture, Animal and

Plant Health Inspection Service, Veterinary Services,

Center for Epidemiology and Animal Health, Fort Collins,

CO 80524, USA

Present Address:

J. S. Lewis

College of Integrative Sciences and Arts, Arizona State

University, Mesa, AZ 85212, USA

123

2374 J. S. Lewis et al.

exhibited a substantial increase over the last two

decades, which is driven in part by human land-use

patterns and changing climatic conditions (Brook and

van Beest 2014; Frauendorf et al. 2016; Massei et al.

2015; Vetter et al. 2015). In particular, agriculture can

potentially facilitate expansion of wild pig populations

(Brook and van Beest 2014; Lewis et al. 2017;

McClure et al. 2015; Snow et al. 2017). Further,

climate change is reported to be creating milder winter

temperatures and increased forest mast production,

which is potentially increasing population growth of

wild pigs across broad spatial extents (Frauendorf

et al. 2016; Vetter et al. 2015). Thus, wild pig

populations are both increasing in areas where they

occurred at relatively low densities in the past and

invading new, formally unoccupied areas. Ultimately,

invasive wild pigs are causing greater negative

impacts, as well as introducing new problems to

regions unaccustomed to this destructive species.

Because of the broad geographic range of wild pigs

and their destructive behavior, their populations have

been widely studied across the United States. The

Southeastern Cooperative Wildlife Disease Study

(SCWDS) has mapped the distribution of wild pigs

in the United States from 1982 to the present (Corn and

Jordan 2017). In addition, recent research has pre-

dicted the potential population density of wild pigs in

the United States by evaluating broad-scale landscape

characteristics (Lewis et al. 2017). Thus, by combin-

ing these efforts, we can evaluate broad-scale patterns

of wild pig populations across the United States

through time. No studies have estimated the popula-

tion size of wild pigs across states in the US using a

consistent methodology, evaluated the trend in the

population size of wild pigs over the last several

decades, or predicted the potential population size of

wild pigs if all available habitat was occupied in the

US. Ultimately, this information can help guide and

prioritize management actions and resources aimed at

controlling or eradicating this invasive species.

The overall goal of our current research is to

estimate the population size of wild pigs in the United

States through time. Using information about wild pig

distribution (Corn and Jordan 2017) and population

density (Lewis et al. 2017), we estimated the potential

population size of wild pigs in the United States (1)

given their historic distributions (1982, 1988, 2004,

2010, 2013), (2) for their current (2016) distribution,

and (3) if all habitat was colonized in each state and

populations reached maximal potential (equilibrium)

density. Using this information, we identified states

that have recently experienced population expansion

and are likely to experience the greatest future growth

in wild pig populations and conflicts with people if

management and policy actions are not undertaken.

These results fill an important gap in our knowledge of

wild pig populations in the United States, which can

inform management plans designed to address the

spread of this highly invasive species across national,

state, and local levels.

Methods

Data sets

To estimate the abundance of wild pigs in the United

States through time we used two national-scale data

sets of (1) geographic distribution and (2) population

density. The distribution of wild pigs in the United

States has been recorded since 1982 by SCWDS

through a consistent and national effort (Corn and

Jordan 2017). Populations were included in the

SCWDS distribution if wild pigs were present for

two or more years and there was evidence of

reproduction. Populations were removed from the

SCWDS map when state and federal agencies deter-

mined that wild pigs no longer occupied an area. Data

quality likely varies from 1982, 1988, and 2004 when

states provided information on hand-drawn maps,

compared to 2008 to present, where data were entered

by each state using an interactive web-based mapping

system. Despite these limitations, these are the best

available data describing invasive wild pig distribu-

tion over time and have been used to estimate wild pig

probability of occurrence (McClure et al. 2015) and

the potential range expansion of wild pigs (Snow et al.

2017).

Population density is an informative and funda-

mental metric for the management of species (Brown

et al. 1995; Williams et al. 2002). In particular,

predictions of population density can be used to

estimate abundance of species across local to national

scales. To estimate abundance, we used the density (#

of animals per km2) of wild pigs in the United States

estimated by Lewis et al. (2017), which was related to

a suite of biotic and abiotic factors (See Lewis et al.

(2017) for complete details of this approach) (Fig. 1).

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Historical, current, and potential population size estimates of invasive wild pigs 2375

Population density estimates from 129 areas across the

global range of wild pigs revealed that the most

important landscape characteristics explaining popu-

lation density included potential evapotranspiration

(?), large carnivore richness (-), agriculture (?),

unvegetated areas (-), and precipitation during the

wet and dry seasons (?). These covariates were then

used to estimate the density of wild pigs throughout

the United States in relation to habitat characteristics.

The estimates of wild pig population density

represent the best current estimates on a broad scale,

both globally and in North America. However, there

are specific caveats concerning the interpretation of

these broad-scale estimates of population density. Our

method of estimating potential population size

assumes that wild pig populations reach their maximal

potential population density (i.e., biological carrying

capacity) within the boundaries of their distribution.

This species was first introduced into the continental

United States in the early 1500 s and populations of

wild pigs have been established for extended periods

of time in many regions of the southern United States

and California; populations in other western states and

the northern United States typically have been estab-

lished more recently. The assumption of equilibrium

population density will likely best reflect abundance

for populations that have been established for

extended periods of time (e.g., Texas and Florida)

because wild pigs would have the opportunity to

colonize available habitat and the populations have

had the opportunity to grow to equilibrium densities.

The assumption will likely lead to overestimates of

abundance in areas where wild pigs have recently

established populations (e.g., Oregon) or are managed

to suppress their population size (e.g., New Hamp-

shire). Especially for newly invaded populations, it

can take several generations for populations to reach

equilibrium densities. With these considerations in

mind, we highlight states that are more recently

invaded by wild pigs and identify states that are likely

to experience the greatest increase in wild pig

populations if given the opportunity.We also highlight

states that are likely at or near equilibrium densities

and would not expect additional large increases in wild

pig abundance. We calculated 95% prediction inter-

vals (PI) of population density to estimate uncertainty

in our estimates of abundance using the ‘‘predict’’

function in the ‘‘raster’’ package in program R (R

Development Core Team 2017). Because our response

variable, density, was log transformed, when we back

transformed estimates, the upper and lower bounds of

prediction intervals were not symmetrical.

Fig. 1 Predicted density of

wild pigs throughout the

United States based on

biotic and abiotic factors if

populations were present at

equilibrium conditions in all

available habitat. The results

of this figure are from the

analyses of Lewis et al.

(2017). Predicted population

density ranges across values

of low (yellow: 0–2 animals/

km2), medium (orange: 3–5

animals/km2), and high (red:

6–8 animals/km2)

123

2376 J. S. Lewis et al.

Estimating abundance

To estimate the abundance of wild pigs in the United

States, we intersected predictions of population den-

sity (Lewis et al. 2017) and the associated lower and

upper bounds of the 95% prediction interval with

boundaries for multiple scenarios. First, we considered

the distribution of wild pigs based on SCWDS data for

1982, 1988, 2004, 2010, 2013, and 2016. Next, for

each of the 48 contiguous states in the United States,

we intersected each SCWDS GIS layer of predicted

wild pig density with state boundaries. Lastly, to

estimate the potential maximal abundance of wild pigs

across all available habitat within the entire extent of

each state (i.e., not considering the SCWDS distribu-

tion), we intersected the map of predicted wild pig

density with state boundaries. We then estimated

abundance within states by summing density for each

1 km 9 1 km cell for each scenario:

NState ¼Xn

i¼1

density � boundary ð1Þ

where n represents all 1 km 9 1 km cells for each

state, density is the predicted population density for a

cell, and boundary is a binary indicator variable

describing the presence (1) or absence (0) of pigs for

each scenario. To estimate population size across the

United States (NNational) for each scenario, we summed

estimates across states

NNational ¼Xn

i¼1

NState ð2Þ

Spatial analyses were conducted in QGIS (QGIS

Development Team 2016) and Google Earth Engine

(Google Earth Engine Team 2016) and statistical

analyses were conducted in program R (R Develop-

ment Core Team 2017). We used the Alber’s Equal

Area geographic projection because our analyses

occurred over the broad extent of the United States.

To identify states that have experienced relatively

recent growth in wild pig populations (NNew), which is

based on the expanding range of populations, we

calculated the proportion of the 2016 population

(N2016) that was added since 2004 (N2004) as

NNew ¼ N2016�N2004ð Þ=N2016 ð3Þ

where values range from 0 to 1 for populations that

have been increasing, with values closer to 1 indicat-

ing states with greater levels of recent population

increase. Newly established populations are likely to

exhibit values closer to 1.

To evaluate the potential for population growth of

wild pigs in each state (NUnrealized), based on the

expansion of wild pig distribution, we compared

estimates of abundance from 2016 (N2016) to those

from all available terrestrial habitat within state

boundaries (NEntireState). We calculated the proportion

of the potential population size that was unrealized by

NUnrealized ¼ NEntireState�N2016ð Þ=NEntireState ð4Þ

where values range from 0 to 1; values closer to 0

indicated states that are close to reaching their

maximal potential population density and values

closer to 1 indicated states that have greater opportu-

nities for wild pig populations to increase.

Lastly, to evaluate the percent change in the United

States population size of wild pigs through time

(NChange), we compared time periods by

NChange ¼Ntþ1�Ntð Þ

Nt

� 100% ð5Þ

where Nt represents the starting population size and

Ntþ1 indicates the ending population size.

Previous work that cross-validated the model of

population density demonstrated good predictive

ability on a global scale (Lewis et al. 2017). At a

national scale, to evaluate the results of population

size from this study to other reported results, we

compared population density estimates of wild pigs

between (a) the model from Lewis et al (this study)

intersected with the 2013 SCWDS data and (b) Mayer

(2014) who used multiple methods and contacted

experts in each state. We used estimates of population

size based on 2013 SCWDS data to increase compa-

rability with Mayer (2014). We compared estimates

between these two approaches using linear regression

with and without log transformed data.

Results

Wild pig populations have expanded and increased

across the United States from 1982 to 2016 (Fig. 2,

Online Resources 1–7). Nearly all states where wild

123

Historical, current, and potential population size estimates of invasive wild pigs 2377

pigs occur exhibited increased population size during

the last several decades (Online Resources 1–7).

Nationally, wild pig abundance has increased from

approximately 2.4 million (95% PI = 0.65–6.9 mil-

lion) in 1982 to 6.9 million (95% PI = 1.8–19.8

million) in 2016 (Fig. 3, Online Resources 1–7).

Several states have exhibited relatively recent

population expansion, indicating recent invasions

(Figs. 4, 5). Some states with relatively small popu-

lations exhibited reduced population sizes during this

time period due to state and federal control efforts

(e.g., Indiana, Nebraska, New Mexico).

Despite widely expanding wild pig populations

across the United States, many states (especially in the

Fig. 2 Predicted potential population density of wild pigs

across the United States for 1982 (a), 1988 (b), 2004 (c), 2010(d), 2013 (e), and 2016 (f) . Each map was created by

intersecting the predicted potential population density of wild

pigs presented by Lewis et al. (2017) with the respective year of

SCWDS data. Predicted population density ranges across values

of low (yellow: 0–2 animals/km2), medium (orange: 3–5

animals/km2), and high (red: 6–8 animals/km2)

123

2378 J. S. Lewis et al.

northern contiguous United States) experienced no or

very small and scattered populations of wild pigs

(Fig. 2). However, there is high potential for wild pigs

to increase their range and abundance, although the

degree of potential population increase varies by state

(Fig. 6, Online Resources 9 and 10). States that have

had large wild pig populations for extended periods of

time typically exhibit low potential for expanding wild

pig populations (e.g., Texas, California, Florida).

However, the vast majority of states contain expansive

areas of available habitat for wild pigs that is

unoccupied, which could support large populations;

thus, these states, primarily occurring in the western,

northern, and eastern United States, have high poten-

tial for expanding wild pig populations if animals are

introduced and allowed to become established (Fig. 6,

Online Resources 9 and 10). If all available habitat

was occupied across each state (Fig. 1), the estimated

population size of wild pigs in the United States

(NNational) would reach approximately 21.4 million

animals (95% PI = 4.6–65.6 million). The percent

change (NChange) comparing the population estimates

if all available habitat was occupied in the Unites

States to the population size in 2016 is 210% and 1982

is 784%.

The comparison of population size estimates across

states from this study (Lewis et al.) to that of Mayer

Fig. 3 There is an increasing trend in the predicted population

size of wild pigs in the United States from 1982 to 2016.

Estimates were obtained by intersecting the potential population

density layer from Lewis et al. (2017) with the associated

SCWDS distribution data. See Online Resources 1–6 for

measures of uncertainty associated with estimates of predicted

abundance

Fig. 4 States with recently increasing wild pig populations

(NNew), based on the expansion of wild pig distribution, which

was defined as the proportion of 2016 population size that was

added since 2004. Values closer to 1 indicate newly invaded

states and states that have experienced relatively high popula-

tion growth and range expansion since 2004. Values were

calculated as the estimated population size for 2016 minus the

estimates of population size for 2004, divided by the estimated

population size for 2016. Note that for the states not shown, IN,

NE, and NM exhibited reduced population size from 2016

compared to 2004 due to eradication efforts and CT, DE, ID, IA,

ME, MD, MA, MN, MT, NY, RI, SD, and WY exhibited zero

wild pigs in 2004 and 2016, but some states contained small

populations between these years that were eradicated within this

time period. Note that some populations contain few individuals

(e.g., CO, MI, and NH), so we stress that these results are

relative, and it is likely that many such populations will be

eradicated within the near future

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Historical, current, and potential population size estimates of invasive wild pigs 2379

(2014) (Online Resources 8) indicated that results

from these two studies were similar in most cases,

including with non-transformed data (b = 1.03, SE =

0.06; adjusted R2 = 0.90) and with a natural log

transformation (b = 0.96, SE = 0.08; adjusted

R2 = 0.80) on estimates of abundance (Fig. 7).

Fig. 5 Map identifying wild pig populations that have

increased since 2004 across states in the United States (NNew),

based on expanding distributions of wild pigs. The relative

population increase in wild pig population size is displayed on a

continuous scale from 0 to 1. Values closer to 1 indicate newly

invaded states and states that have experienced relatively high

population growth and range expansion since 2004. States not

included in these analyses are depicted by white. Note that for

the states not shown, IN, NE, and NM exhibited reduced

population size from 2016 compared to 2004 due to eradication

efforts and CT, DE, ID, IA, ME, MD, MA, MN, MT, NY, RI,

SD, and WY exhibited zero wild pigs in 2004 and 2016, but

some states contained small populations between these years

and were eradicated within this time period

Fig. 6 Estimated potential population size of wild pigs for each

state if all available habitat was occupied within a state’s

boundary and for their 2016 SCWDS distribution. Estimates

were obtained by intersecting the population density layer from

Lewis et al. (2017) with the boundaries for the entire state

(labeled ‘‘State boundary’’) and the 2016 SCWDS data within

each state’s boundaries (labeled ‘‘2016 distribution’’). Only the

point estimates are reported for clarity of presenting the figure.

Please see Online Resources 1–7 for measures of uncertainty

associated with estimates of predicted abundance

123

2380 J. S. Lewis et al.

Discussion

There is an increasing trend in wild pig distribution

and abundance in the United States from 1982 to 2016,

where wild pig population size was estimated to nearly

triple nationally. Several states have recently been

invaded by wild pigs and exhibited large increases in

abundance over a relatively short period of time.

Although many new populations have been estab-

lished in the last 15 years, several states, primarily in

the northern United States, exhibit no or few isolated

populations. However, natural dispersal and human-

mediated translocations from their existing distribu-

tion in the United States and Canada threaten to

expand wild pig populations across all states in the

United States (Snow et al. 2017). Unless proactive

management plans are established and implemented,

wild pigs are predicted to substantially expand their

populations in the future. Given the challenges with

controlling and eradicating wild pig populations once

they become established into a new area (Mayer and

Brisbin Jr 2009; Tabak et al. 2018), wild pigs threaten

to permanently expand their distribution throughout

the United States and lead to further widespread

economic, social, and ecological impacts.

Our research provides the first estimates of histor-

ical, current, and future population size of wild pig

populations across the United States using a consistent

methodology. Although population sizes are predicted

to vary across states, our predictions indicate that wild

pig populations could invade habitat in all regions of

the continental United States. Wild pigs can persist in

a range of environments across their native and non-

native ranges, including cold northern climates, arid

regions, and mixed forests (Lewis et al. 2017).

Although the United States government, states, and

the public are increasingly addressing this growing

issue (Centner and Shuman 2015; USDA 2015), wild

pig populations continue to expand in many areas

(Corn and Jordan 2017; Snow et al. 2017). Our results

can be used to identify states that have high potential

for wild pig populations to expand if they are

introduced and become established; thus, high priority

should be given to quickly identifying and eradicating

populations that invade unoccupied habitat to limit the

spread of invasive wild pigs into new areas. In

addition, states that harbor low population sizes of

wild pigs could be prioritized to control populations

and reduce the threat of their expansion. In some states

(e.g., Texas and Florida), wild pigs have been

established for long periods of time and have invaded

the majority of available habitat, which limits their

potential for future population growth and options for

eradication efforts.

Importantly, although our results provide managers

and policy makers with information at state and

national levels, which is critical for broad-scale

landscape planning, our results can also be applied

to local levels. For example, within regions of states,

land managers can use the SCWDS data (Corn and

Jordan 2017) to identify areas where wild pigs are

currently absent and use the predicted population

density data (Lewis et al. 2017) to understand the

potential for wild pig abundance if populations were

established in these areas. Land managers could then

use this information to prioritize local areas to focus

public education and outreach, implement regulations,

and conduct surveillance to counter invasions. Such

strategies have been used for other invasive species

that are spreading to new regions and threaten to

rapidly expand into unoccupied habitat (Hawthorne

et al. 2015; Herborg et al. 2007).

Results of predicted population density are consis-

tent with previous research evaluating niche relation-

ships and occurrence patterns of wild pigs in the

United States (Lewis et al. 2017). Using these results,

predictions of population density (from this study) and

local watershed-level occurrence (McClure et al.

2015) appear consistent for wild pigs in the United

States, including high predicted density and

Fig. 7 Comparison of population size estimates across states in

the United States by Lewis et al (this paper) and Mayer (2014)

when estimates are on a natural log scale. The correlation of

Lewis et al. (this study) predicting Mayer (2014) was relatively

high (b = 0.96, SE = 0.08; adjusted R2 = 0.80). Lewis et al.

used SCWDS data from 2013 for comparison to Mayer (2014).

States were included in analyses if either Lewis et al (this paper)

or Mayer (2014) recorded a state population size estimate

greater than 0 (i.e., 37 states)

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Historical, current, and potential population size estimates of invasive wild pigs 2381

occurrence in the southeast and moderate to high

predictions along the west coast. Our estimates also

predict moderate potential density in the western, mid-

western, and northeastern regions of the United States,

which likely is related to our model incorporating

estimates from the diverse range of wild pigs across

Eurasia and globally, including arid and cold climates.

There are several considerations when interpreting

these results. First, our estimates of abundance using a

consistent methodology across states were similar to

estimates presented by previous research. For exam-

ple, our estimate of population size of wild pigs in the

United States in 2013 was 6.3 million animals (95%

PI = 1.6–18.0 million), and the estimate from Mayer

(2014) was 6.3 million animals (range 4.4–11.3

million). Similarly, our estimate of population size

of wild pigs in Texas, the state with the largest

population, in 2013 was 2.5 million animals (95%

PI = 0.6–7.3 million), whereas the estimate of Tim-

mons et al. (2012) was 2.6 million animals (range

1.8–3.4 million). However, some states exhibit closer

relationships than others (see below). Second, the

model used to estimate population density in our

analyses assumes that the density of wild pigs is in

equilibrium with their environment. We expect that

our abundance predictions are most accurate for states

with populations of invasive wild pigs that have been

established for long periods of time and thus closer to

equilibrium. Indeed, this is supported when comparing

our estimates of abundance for states such as Texas

and Florida (Mayer 2014). Our model likely overes-

timated density for populations that were recently

established, rapidly expanding, or maintained by low

numbers of escaped animals because populations have

not reached equilibrium (e.g., Michigan and New

Hampshire). Especially for newly invaded areas, it can

take several generations for the population to reach

equilibrium conditions. However, because these pop-

ulations are relatively small, they likely do not

substantially inflate estimates of abundance at the

national scale. Importantly, population abundance will

increase or decrease based on seasonal and annual

variation in environmental conditions, which can lead

to varying equilibrium conditions of population den-

sity through time. And third, as reported elsewhere

(Street et al. 2017), the 95% prediction intervals

associated with estimates were very wide, which

might limit their usefulness in some cases. Although

the overall patterns of our study provide important

information on wild pig populations through time,

future work could aim to improve the precision of

predicted population size, such as by explicitly

including harvest and removal effort (Davis et al.

2016).

Results from this study can be applied to address

other important impacts of wild pig invasions in the

US. First, estimates of population size could be

evaluated in relation to ecological and agricultural

damage across the United States. This information

could be used to update our understanding of the

economic impact that wild pigs cause across the

United States. Second, our results could be used to

identify areas that are at risk of disease transmission to

humans and wildlife by evaluating how population

density of wild pigs is predicted to intersect with

regions of agriculture and landscapes of important

conservation value. Finally, although our research is

focused on the United States, the spread, population

size, and impacts of wild pigs could similarly be

evaluated across North America and worldwide (Corn

and Jordan 2017).

In the long run, it can be more effective and

efficient to proactively address wildlife management

issues, such as invasions by non-native species, before

they become established and cause significant eco-

nomic, social, and ecological impacts (Hulme 2006;

Jerde et al. 2011; Rosatte et al. 2009; Simberloff

2009). It is important to not only track populations

through time, but also understand the potential for

future population growth and impacts considering

‘‘how bad could it get?’’. Once an invasive species is

established, it can be expensive or even impossible to

eliminate, as is the case with invasive wild pigs in

many U.S. states. It can be more cost effective to be

proactive rather than to retroactively manage a chronic

issue, which might result in limited effectiveness and a

greater expenditure of resources. Thus, proactively

designing and implementing management plans to

confront potential threats can be the most effective

management strategy. Wildlife managers were able to

rapidly respond to wild pig invasions in Colorado,

New Mexico, Michigan, and Nebraska to control or

eradicate populations and limit the spread of wild pigs

within these states. Because management actions were

conducted when these populations were relatively

small, wildlife managers were able to successfully

control wild pig invasions.

123

2382 J. S. Lewis et al.

Our research provides information that supports the

call to action to manage the growing problem of

invasive wild pigs. Invasive species can be success-

fully managed using an early detection and rapid

response strategy (NISC 2016; USDOI 2016). The

implementation of this strategy will require a coordi-

nated effort among federal, state, and local govern-

ments and the public. One of the greatest contributions

to reducing the spread of wild pigs is through state

regulations classifying them as an invasive and

harmful species that is undesirable. A priority for

addressing the spreading invasion of wild pigs is to

halt the translocation of wild pigs for the purposes of

recreational sport hunting (Tabak et al. 2017). Imple-

menting proactive wild pig control and management

strategies across states in the United States at national,

state, and local levels has been identified as a necessity

for successful control of these populations (Centner

and Shuman 2015). Using information from the

distribution of wild pigs (i.e., SCWDS) and population

abundance such as ours, managers can track trends in

future population abundance to inform control strate-

gies and evaluate progress in addressing this nation-

ally growing problem.

Acknowledgements This study was funded and supported by

the Wildlife Services/National Wildlife Research Center and

Veterinary Services/Center for Epidemiology and Animal

Health programs of the US Department of Agriculture/Animal

and Plant Health Inspection Service, the National Feral Swine

Damage Management Program, Colorado State University,

Conservation Science Partners, and Arizona State University.

Funding for preparation of the SCWDS distribution maps and

development and maintenance of the NFSMS was through

Cooperative Agreements with the U.S. Department of

Agriculture, Animal and Plant Health Inspection Service,

Veterinary Services. Support for this study was provided by

the Department of Energy to the Savannah River National

Laboratory under contract DE-AC09-08SR22470. J. Anderson

assisted with calculation of prediction intervals. We thank B.

Dickson and reviewers for providing thoughtful feedback that

improved earlier versions of this paper.

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