The Mustang as an Invasive Species · 9:05pm Sep 16th, 2023
Now, you don't need me to tell you that we humans have greatly affected Mother Nature in multiple negative ways. From poaching animals for their body parts to the destruction of habitats. Among the many harmful things we do to the world's ecosystems is introduce invasive species into foreign ecosystems. Whether we do it by accident or if we don't want to keep an animal anymore and instead of being responsible we dump that animal in the wilderness. As a result of the ecological damage caused by invasive species, we then try to control them via lethal means...for the most part.
And I say that because with lethal invasive species control in the US, there are a couple of exceptions to the typical way we manage invasive species: feral horses and burros. You see, in the year of 1971, the federal government decided to give the mustangs and feral burros protection from being killed by people due to pressure from the public. And it's been that way ever since and since that time, mustang populations have increased and are under the management of the Bureau of Land Management or BLM. The BLM manages mustangs with fertility control and large roundups which of course are controversial because here in America it seems we argue over everything so why wouldn't we argue over mustang management?
One argument often made is the fact that North America actually did have native equine species in the prehistoric past, and because the horses of the last ice age died out pretty recently in geologic terms, the introduction of the mustangs should really be considered a reintroduction of a long lost species to a continent. Basically, it's seen by many as an example of "Pleistocene Rewilding". And thus mustangs are native wildlife.
Personally, I don't buy this argument. But I used to.
What one has to remember is that the ecologies of the last ice age and the modern day were very different. True, wolves and cougars are effective horse predators, but the Pleistocene landscape had predators like the Scimitar-Toothed Cat that would've been even better suited to regulate horses. And no, I'm not saying to introduce lions into North America because that would bring its own ecological chaos. And while we humans think the idea of free-roaming previously domestic horses is cool, mule deer, elk, pronghorn, and desert bighorns disagree. It turns out that mustangs can actually be pretty aggressive towards these ungulates and will displace them from water sources and thus reduce water access to these species.
1.) https://academic.oup.com/jmammal/article/89/2/459/932883?login=false&fbclid=IwAR3i3hPtLB9jLOAcCLWNFhttucm8EEiaJrhBC3vqLM6Rd7zt0R9rgmeI-rc
"We studied sympatric populations of native bighorn sheep (Ovis canadensis) and feral horses (Equus caballus) to quantify their spatial and temporal overlap and to determine whether horses interfered with use of water by bighorn sheep. We observed no evidence of direct competition, but our field experiment, which involved placing desert-acclimated domestic horses near watering sites used by bighorn sheep, demonstrated that bighorn sheep avoided sites with horses nearby. The presence of domestic horses near a watering site preferred by bighorn sheep resulted in a 76% reduction in the number of groups of bighorn sheep coming to water at that location and a concomitant increase in the number of bighorn sheep watering at other sites. An experimental approach to studying competition between large mammals has been problematic and to our knowledge this study constitutes the 1st manipulative field experiment to test for competitive interactions between feral horses and native ungulates."
2.) https://esajournals.onlinelibrary.wiley.com/.../ecs2.2096
"The horse (Equus caballus) is a feral ungulate that currently exceeds target population sizes in many areas of western North America. Horses are behaviorally dominant over native ungulates and outcompete the latter for access to water sources. However, a better understanding of the broader spatial and temporal implications of horse-induced competition on access to water by native ungulates will enable better conservation and management of native species. Our objective was to determine whether pronghorn (Antilocapra americana) and mule deer (Odocoileus hemionus) spatially or temporally altered their use of water to minimize interactions with horses. From 2010 to 2014, we used remote cameras to monitor ungulates at 32 water sources in the Great Basin Desert. We evaluated spatial and temporal partitioning by these species at water sources using generalized linear models, mixed-effects models, and Mann–Whitney U tests. We found that both native ungulates used water sources less often where horse activity at water sources was high, indicating that spatial avoidance occurred. Further, we observed significant differences in peak arrival time for pronghorn, but not mule deer at horse-occupied sites versus sites where horses were absent or uncommon, indicating that temporal avoidance may be more important for pronghorn than mule deer. Because mule deer are primarily crepuscular and nocturnal whereas horses are largely diurnal, we did not expect to observe a temporal shift for mule deer. We also found strong support for the interactive negative effect of elevated temperature and subsequent increased activity of horses at water sources on drinking patterns of pronghorn and mule deer. Our findings indicate that feral horses further constrain access to an already limited resource for native species in a semi-arid environment."
"We used a motion-triggered camera to document and qualify the nature of interspecific interactions between elk (Cervus elaphus) and feral horses (Equus caballus) at an isolated, natural water source in Mesa Verde National Park, Colorado. The camera was active for at least 110 of 145 potential trap-nights between 24 April 2012 and 16 September 2012. Elk and horse interactions were observed 51 times, and elk were averted from accessing water 42 times (82%). Feral horses exhibited dominant behavior over native elk during the driest time of the year (May–June), often preventing acquisition of water by elk."
"Feral horses (Equus callabus) occur throughout the world on all continents except Antarctica. In North America, feral horses occupy 31.6 million acres throughout western North America. Throughout their range, feral horses often share habitat with American pronghorn (Antilocapra americana). Since horses are larger and more aggressive than pronghorn, they are considered socially dominant. In the Great Basin of western North America, pronghorn often access water sources where horses occur since habitat preferences are similar. If pronghorn are excluded where water is used by both species, pronghorn fitness may be impaired, especially during dry or droughty periods. The purpose of this study was to investigate interference competition between pronghorn and feral horses at water sources within the Great Basin. We observed horses and pronghorn at high-use water sources and recorded all occurrences and outcomes of pronghorn/horse interactions. We assessed differences in pronghorn behavior in the presence or absence of horses. Pronghorn invested more time on vigilance behavior and less time foraging or drinking in the presence of horses than in their absence. Nearly half of pronghorn/horse interactions resulted in pronghorn exclusion from water. We conclude that as feral horse numbers increase, competition for water will subsequently increase."
"Introduced species can impact native communities by altering competition dynamics. Large exotic species, such as the horse (Equus caballus), may have a competitive advantage over smaller native species and could exclude them from access to limited resources. Our objective was to determine the influence of the exotic horse on the use of water by native species in a semi-arid environment where availability of water is limited. From July 2010 to August 2011, we used remote cameras to monitor water sources in the Great Basin Desert where horses had drinking access and where horses were excluded (with fencing) to compare 1) composition of native communities and 2) water usage by native species. We captured 96,601 images representing 40 species of birds (29,396 images) and 13 species of mammals (67,205 images). Of the 67,205 images of mammals, 79% contained horses. Horses were associated with decreased richness and diversity of native species at water sources. Furthermore, native species had fewer visits and spent less time at water sources frequented by horses. Our results indicated that horses displaced other species at water sources providing evidence of a negative influence on how communities of native wildlife access a limited resource in an arid environment."
That being said, the mustang is indeed an invasive species in need of management.
