Emerging Research to Support Management of Free-Roaming Feral Equids


The feral horse and burro population on western rangelands in the USA continues to grow, surpassing 90,000 animals on Bureau of Land Management Herd Management Areas in 2020. This problem is not unique to the United States, with feral equids being found across the globe. We edited a special issue of the Journal of Wildlife Management to highlight recent research on feral horses and burros worldwide. In this symposium authors present their research covering topics including effects of feral horses on rangelands and sympatric wildlife, feral horses as prey species, fertility control and stress measurements, and results of studies measuring and monitoring feral horse habitat use. Authors will present their results, and we will have opportunities for questions and discussion about both presented research and future avenues of research.

The Wildlife Profession’s Duty in Achieving Science-Based Sustainable Management of Free-Roaming Equids
Kate A. Schoenecker, Sarah King, Terry A. Messmer
Equidae originated in North America approximately 10–20 million years ago. Ancient equids included a broad assemblage of species, exhibiting the greatest species radiation of all North American ungulates. Eventually, all equids went extinct in North America 10,000–12,000 years ago during the late-Pleistocene, likely due to climate and environmental change, disease, and the arrival of human hunters. Horses survived on the steppes of Eurasia and continued to evolve, ultimately being domesticated by humans for milk and meat about 5,500 years ago. Horse domestication is an important component of human history and significant in humans’ emotional attachment to horses. Horses enabled cultures to disperse and advance agriculture, transportation, industry, commerce, and warfare. Domestication and artificial selection by humans for certain traits over thousands of years led to horses that were optimized for certain size, color, and reproductive characteristics. Today, all domesticated and feral horses differ genetically and phenotypically from their non-domesticated ancestors and are morphologically different from their only extant wild relative, the Przewalski’s horse. Domestic horses were first brought to North America in 1493 on the second expedition of Christopher Columbus. After industrialization demand for horses and burros by the military and individuals declined resulting in intentional releases in the mid-twentieth century. As free-ranging horses became more visible across western rangelands, they were captured by mustangers who sold them for slaughter, re-sale, or other economic purposes. This treatment of horses (and other animals) was questioned, eventually leading to passage of legislation to protect them. In 1971 the United States Congress passed the Wild Free-Roaming Horse and Burro Act and the Bureau of Land Management and United States Forest Service assumed statutory obligation to manage them. Today feral equids are found on every continent except Antarctica and managing them is a substantial challenge that requires more input and investment from wildlife professionals.
Lentic Meadow Habitat and Riparian Functions Impaired By Season-Long Free Roaming Horse and Cattle Grazing
Sherman Swanson, Jacob Burdick
ABSTRACT Perennially and intermittently available water in lentic, spring-fed herbaceous meadows supports wetland plants that build and maintain absorbent soil organic matter, capture sediment, prevent erosion, and thereby sustain riparian ecosystems.  Forage and available water attract a diversity of wildlife and grazing animals that can degrade riparian functions and conditions through prolonged overuse.  We tested novel monitoring methods and adapted existing methods to quantify vegetation data consistently linked to riparian functionality and ungulate species use.  We quantified ungulate use with time-lapse cameras at 10 Nevada spring-fed herbaceous meadows where core greater sage-grouse (Centrocercus urophasianus) habitat, public land grazing allotments, and free-roaming horse (Equus caballus) herd management areas (Bureau of Land Management) or territories (U.S. Department of Agriculture Forest Service) overlapped.  Free-roaming horses used most study sites over longer periods and grazed more intensely than cattle or domestic sheep.  Native ungulate grazing intensity was comparatively negligible.  Free-roaming horses selected spring-fed meadow sites on average 51 (± 33 [SE]) times more than the surrounding rangelands, whereas cattle selected them 30 ± 23 times more. Within the wettest area of a site, study sites with longer duration of use, and higher intensity of use, had more soil alteration and bare ground. Riparian ecosystem degradation occurred where meadows had limited rest or recovery time from grazing during the growing season.  At all sites we noted loss of riparian function, especially within the hydric areas, those with the highest potential to grow abundant cover of stabilizing wetland plant species.  Transects for monitoring soil alteration and vegetation cover in this hydric location can provide a leading indicator of riparian recovery or degradation and help in adjusting free-roaming horse population levels or livestock management strategies. Random transects across the meadow were less diagnostic.
Distribution of Competition Potential between Native Ungulates and Free-Roaming Equids on Western Rangelands
David Stoner, Michael Anderson, Cody Schroeder, Cole Bleke, Eric Thacker
Free-roaming horses (Equus caballus) and burros (Equus asinus) are widely distributed and locally abundant across the western United States. The 1971 Wild Free Roaming Horse and Burro Act specifies that feral equids must be managed in concert with native wildlife, but equid population control measures have failed to keep pace with intrinsic growth rates, raising questions about potential competition with native ungulates. Mule deer (Odocoileus hemionus), pronghorn (Antilocapra americana), elk (Cervus canadensis), and bighorn sheep (Ovis canadensis) are of ecological and economic value to the states where they occur, and all exhibit some distributional, habitat, or dietary overlap with horses or burros. Notwithstanding the scale of the problem, there have been no range-wide assessments of competition potential between native and feral ungulates for space, forage, or water. To address this need, we used species occurrence data collected by federal and state agencies to map the distributions of 4 native ungulate species across Federal Equid Management Units (FEMUs) in 10 western states (n = 174; 2010-2019). Collectively, FEMUs covered approximately 18% of all BLM and USFS lands in affected states. Each FEMU supported ≥ 1 native ungulate and 14% contained all 4. Overlap between native and feral species varied by state, ranging from < 1% for mule deer in Montana, to 40% for bighorn sheep in Nevada. Oregon had the largest proportion of units supporting all 4 native ungulates (58%), whereas Montana and New Mexico had the fewest equids, but all populations were over target densities. Despite the perception that the problem of equid abundance is limited to the Great Basin states, high growth rates and social constraints on management practices suggest all affected states should monitor range conditions and native ungulate demography in areas where forage and water resources are limited and expanding equid populations are a concern.
Temporal Overlap Among Feral Horses, Cattle, and Native Ungulates at Water Sources
Jacob Hennig, Jeffrey Beck, Derek Scasta, Caleb Gray
Feral horse (Equus ferus caballus) presence at water sources can alter watering patterns of wild ungulates in arid regions, potentially resulting in deleterious effects. The few studies examining these interactions have infrequently occurred within areas specifically managed for feral equids and have not occurred in sites with concurrent cattle grazing (Bos taurus). We used motion‐sensitive cameras at 8 watering sources to document watering activity patterns and examine temporal overlap among feral horses, cattle, elk (Cervus canadensis), mule deer (Odocoileus hemionus), and pronghorn (Antilocapra americana) within the Adobe Town Herd Management Area of southern Wyoming, USA, between June and September 2018 and 2019. Feral horses, cattle, and pronghorn exhibited a high degree of temporal overlap (>79%) in water use, with feral horses and pronghorn exhibiting the highest estimated percent overlap (88.1%, 95% CI = 86.5–89.6%). Feral horses spent a mean of 16.7 minutes during a watering event and were present at a given water source on average 4.5% and up to 34.9% of the day. Meanwhile, cattle spent on average 23.5 minutes during a watering event, and were present on average 4.2% and up to 42.4% of the day at a single water source. Results of generalized linear mixed‐effects models indicated that number of conspecifics was the strongest predictor of visit duration for pronghorn and horses with hour of the day and group size of heterospecifics as less important variables. There was no difference in peak visitation time for any species between sites of high versus low horse or cattle use. Despite temporal overlap, we did not find evidence of interference competition between feral horses, cattle, and pronghorn. We recommend future examination of interference competition and its biological consequences between introduced and native ungulates at water sources of varying size across sites, equid population levels, and livestock stocking rates. 
Ecological Interactions Involving Feral Horses and Predators: Consequences to Food-Web Dynamics and Biodiversity
Philip McLoughlin, Paul Boyce
For many ecosystems, feral horses are increasingly becoming an important if not dominant component of ungulate biomass and hence influence on community dynamics. Yet we still know little of how horses contribute to key ecological interactions including predator-prey and indirect competitive relationships at a community level. Notably, feral horses can exhibit life-history traits that differ from that of native (mainly artiodactyl) herbivore competitors. Artificial selection for traits like increased, early, or extended reproduction that have yet to be reversed by natural selection, coupled with naturally selected differences in anatomy and behavior plus unique management objectives for horses compared to other species, means that the dynamics of feral horse populations are not likely to align with what might be expected of other large herbivores. Unexpected population dynamics and inherent biological asymmetries between native ungulates and feral horses may therefore influence the former via direct competition for shared resources and through enemy-mediated interactions like apparent competition. In several localities feral horses now co-exist with multiple native prey species, some of which are in decline or are species at risk. Compounding risks from direct or indirect competitive exclusion by horses is the unique nature and socio-political context of feral horse management, which tends towards allowing horse populations to be limited largely by natural, density-dependent factors. We summarize the inherent asymmetries between feral horse biology and that of other ungulate prey species with consequences for conservation, focusing on predator-prey and emerging indirect interactions in multi-prey systems, and highlight future directions to address key knowledge gaps in our understanding of how feral horses may now be contributing to the (re)structuring of food webs. Our findings apply not only to the conservation of game species but also species-at-risk, necessitating an ecological understanding of the role of horses in natural environments that goes beyond that of population control.
Feral Horse Space Use and Genetic Characteristics from Fecal DNA
Sarah King, Kate A. Schoenecker, Jennifer Fike, Sara Oyler-McCance
Small, isolated populations can be at risk of extirpation from stochastic events and deleterious genetic effects resulting from inbreeding and reduced heterozygosity. The genetic diversity of discrete feral horse (Equus ferus caballus) herds on Bureau of Land Management or Forest Service lands is periodically monitored using blood or hair samples collected during management gathers. We conducted a study to examine genetic characteristics of the feral horse population at the BLM Little Book Cliffs Herd Management Area (HMA) in Colorado, USA, using non-invasively collected fecal samples. Additionally, we explored whether genotypes could be used to document space use and potential sub-population development. We used a random sampling scheme, walking transects throughout the HMA to find and collect >1,800 fecal samples during May, August, and October 2014. We then identified unique individuals using a suite of microsatellite loci. Our estimates of genetic diversity from fecal samples were higher than those reported from blood and hair samples taken during horse gathers, likely because our sample size and spatial distribution was larger. Genotypes revealed that some individuals were found only in certain parts of the study area and at a higher proportion than random; thus, they could be considered residents in those sampling areas. Using discriminant function analyses, we detected 5 genetic groups in the sample population, but these did not correspond to individuals in specific parts of the study area. Our results support the use of fecal DNA to augment direct observations of horse presence and could be used to detect habitat use and areas of high density. Non-invasive techniques such as fecal DNA sampling can help managers decide whether new individuals need to be translocated to a closed population to maintain genetic diversity without the human safety and animal welfare concerns associated with gathers and invasive techniques.
Spatial Ecology, Seasonal Resource Selection and Movement Ecology of Free-Ranging Horses in the Western USA
Kate A. Schoenecker, Sarah King, Saeideh Esmaeili
Understanding factors driving resource selection and habitat use of different species is an important component of management and conservation. Feral horses (Equus ferus caballus) are free ranging across various vegetation types and habitats in the western USA, yet almost no studies have measured their habitat selection and seasonal habitat use. We conducted a study to determine effects of vegetation community, distance to water, and topographic variables on seasonal habitat selection of feral horses in two independent populations in a Great Basin sagebrush ecosystem of west-central Utah, USA. We deployed GPS radio collars on 38 female horses in Conger and Frisco Herd Management Areas (HMA) and attached GPS transmitters to the tail hair of 14 stallions, with 2-hour fixes for 1-4 years. We calculated individual home ranges and core areas using auto-correlated kernel density estimators for each biologically defined season (breeding, fall, and winter). We examined home range size by sex, social status, and season, seasonal home range overlap, and compared seasonal habitat selection of horses in a use-availability framework using RSFs. At Conger there was no difference between sex in home range size but home range was affected by season. At both HMAs horses selected for herbaceous habitat types and avoided forest relative to shrubland throughout the year. Mares at Frisco were consistently located closer to water sources, whereas selection in mares and stallions at Conger varied seasonally. We found resource use and movements of mares and stallions is partially influenced by season, and horses are not constrained by HMA boundaries. Frisco mares had over half of their home ranges outside the HMA. Our results highlight the importance of water sources, which were a key driver of horse movement patterns. We emphasize the utility of telemetry devices to understand habitat use at a fine scale, enabling management and to be more targeted.
Feral Horse Population Model and Body Condition: Useful Management Tools in Tornquist Park, Argentina?
Alberto Scorolli
Feral invasive mammals challenge management strategies worldwide including the feral horse (Equus caballus) population in Tornquist Park, Argentina. In this study, I constructed population matrix models using demography data collected between 1995 and 2002. I validated the models with independent counts from 2008 to 2016. I individually identified feral horses and recorded them as I walked a fixed path that covered 20 km2. I performed an elasticity analysis to estimate the influence of the demographic parameters on population growth rate. In addition, I explored whether body condition score could be a proxy for carrying capacity. Model projections with the highest values of fecundity and survival exhibited by females of all ages during 1995–2002 fit the counts from 2008 to 2016, when growth rate was moderate (λ = 1.10), very well. Elasticity of adult female survival (0.54) was 4 times higher than for fecundity and survival of all other ages. Body condition of adult females at the end of winter, 2012–2013, after reductions in density, was improved compared with 2001–2002 when the population approached carrying capacity. The validated population model together with body condition score could allow managers to take more informed decisions when planning the control of feral horses in Tornquist Park. This approach could also be applied to other small, closed, feral horse populations and other large‐mammal populations.
Habitat Productivity and Predictability Drive Resource Selection by Feral Burros
Saeideh Esmaeili, Kate A. Schoenecker, Sarah King
Feral burros (Equus asinus) were domesticated from African wild ass (E. africanus) and introduced to North America in 1495. To inform management of feral burro populations, we evaluated space and resource use patterns in two populations using GPS-telemetry data. We conducted our study in two Herd Management Areas (HMAs), managed by the Department of Interior, Bureau of Land Management in Utah and Arizona, USA. The two HMAs differed in topographic features, annual precipitation, availability of surface water, vegetation communities, and level of human activity. The Lake Pleasant (LP) HMA, Arizona was characterized by high levels of availability and less variability in surface water and plant productivity across a year, whereas Sinbad HMA, Utah, lacked permanent water sources and was characterized by variable surface water availability and plant productivity in a more seasonal pattern. We used GPS locations of female burros (n = 52 in LP, n = 25 in Sinbad) to delineate seasonal home ranges and evaluate seasonal resource selection patterns. We expected that burros in LP would require smaller home ranges to meet nutritional needs because of more constant patterns of resource availability across a year. In Sinbad, we predicted larger home ranges and more fluctuating patterns of resources selection, due to variable resources in that HMA. Combining resource selection of the two populations, we expected to detect a functional response in resource selection where selection for a resource would be a function of availability of that resource within the home range of each burro. We found burros in Sinbad HMA had home ranges seasonally distinct and larger than twice of those in Lake Pleasant HMA. Selection for surface water and plant productivity were associated with availability within the home range of each burro; however, we did not find similar functional response in selection for topographic and anthropogenic variables.
Faecal Glucocorticoid Metabolites as Biomarkers in Equids: Assay Choice Matters
Susanne Shultz
Free ranging animals are exposed to environmental, demographic, and ecological challenges over time, which can affect their health and fitness. Non-invasive biomarkers can provide insight into how animals perceive challenges. We evaluated how different assays impact on the ecological inferences from two systems: 1) free ranging ponies (Equus ferus caballus) on the Carneddau Mountain range, North Wales, 2) Cape Mountain Zebra (Equus zebra zebra) in South Africa. We evaluated fecal glucocorticoid metabolites using two enzyme immunoassays (EIAs): an 11-oxoetiocholanolone EIA (measuring 11,17-dioxoandrostanes [11,17-DOAs]) and a corticosterone EIA. The former has been validated in equids, whereas there is limited evidence for the suitability of the latter. We also used a parent testosterone EIA to measure fecal androgen metabolites. The inferred response to ecological and management stressors varied between the assays. For the ponies, the metabolites measured by the corticosterone assay showed no response to a roundup event, whereas there was a pronounced increase in metabolite concentration measured by the 11-oxoetiocholanolone assay. Metabolite concentrations measured by the corticosterone and testosterone assays were highly correlated and were temporally associated with the onset of the breeding season, whereas the 11,17-DOAs were not. In the mountain zebra, metabolites measured by the testosterone and corticosterone assay were again highly correlated and associated with the onset of the breeding season. The ecological factors associated with elevated glucocorticoid metabolite concentrations varied across the glucocorticoid assays. The co-variance of metabolites measured by the corticosterone and testosterone assays, and the lack of an acute response in the corticosterone assay to the roundup, suggests that metabolites detected by the corticosterone assay were not primarily associated with increased glucocorticoid production. We highlight the risks associated with not using well-validated fecal biomarker assays of hypothalamus-pituitary-adrenal axis activity to evaluate and compare the effect of different management interventions and environmental change. 
Popequus: A Predictive Model to Evaluate Effects of Management Actions on Feral Horse Populations
Brian Folt, L. Stefan Ekernas, David Edmunds, Mark Hannon, Kate A. Schoenecker
In western North America, feral horse populations have rapid population growth rates, and populations have largely exceeded desired sizes in management areas. To achieve a mission of sustainable, multi-use rangelands in western ecosystems, wildlife managers need better understanding of how management tools, such as fertility-control treatment, might be used to decrease feral horse population growth and size, while minimizing cost. Here we describe a fully-stochastic, stage-based population modeling framework used to evaluate the effects of management actions on feral horse population growth. We used available data describing horse survival and reproduction to model horse demography and then simulated how four management actions (removals for adoption, treatment with intra-uterine devices [IUDs], treatment with fertility vaccines [GonaCon, PZP-22, or ZonaStat-H], and mare sterilization), used alone or in concert with other actions, influenced population growth, while accounting for costs associated with management actions. We simulated 19 management strategies (no management [n = 1]; a single action of removals, GonaCon, PZP-22, ZonaStat-H, IUDs, or mare sterilization [n = 6]; removals with GonaCon, PZP-22, ZonaStat-H, IUDs, or mare sterilization [n = 5]; mare sterilization with GonaCon, PZP-22, ZonaStat-H, or IUDs [n = 4]; IUDs with GonaCon, PZP-22, or ZonaStat-H [n = 3]) and measured the effectiveness of each strategy for reducing populations to desired management levels and costs. We built a prototype web application (“PopEquus”), with which wildlife managers can simulate management alternatives and evaluate tradeoffs among management and cost objectives in an easy-to-use website interface.

Location: Virtual Date: November 4, 2021 Time: 3:00 pm - 4:00 pm