The fight over whether rangeland in the American West is threatened by wild horses because they constitute an “invasive” species should now be over. Or it should be, if some new science is listened to and converted into policy. A recent paper* in the leading journal Science has demonstrated that bulk-feeding megafauna (species with body weights greater than 100 lb/44 kg) are instrumental in maintaining and even increasing plant diversity in natural contexts, and that their value in this particular regard has nothing to do with their status as “native” or “introduced”. By comparing the results of 221 separate investigations, the authors were able to document that all the megafauna under study—ranging from pigs to deer to cattle to horses—exhibit environmentally beneficial traits, like the ability to disperse seeds of native plants. Notably, they also found that larger-bodied megafaunal communities had more positive effects on native plant diversity than did smaller-bodied ones. This helps to contradict the claim that large megafauna like free-ranging “wild” horses disproportionately cause environmental harm:
“[N]ot all introduced megafauna are considered equally problematic. “Invasive” megafauna are thought to have uniquely detrimental effects on ecosystems, and some argue that feral megafauna (wild but descending from domestic populations) have distinct effects due to human selection on their ancestors. However, there was no evidence that the effects of “invasive” megafauna species or of feral megafauna on native plant abundance and diversity were different from the effects of other megafauna.
“If…it were impossible to determine the nativeness of an organism from their actual effects, then nativeness would remain a description of dispersal history but would not be a meaningful way to understand ecological interactions.”
The new study shows that it is objectively irrelevant whether free-ranging horses are regarded as native or introduced, as their impacts, under equivalent circumstances, cannot be quantitatively or qualitatively distinguished from those of cattle or elk or any other big herbivore—and that their removal might be positively harmful. But that of course is not the conventional wisdom, and that’s what needs to change.
Let’s consider one objection to the new findings that is bound to be brought up by many people, whatever their attitude is toward free-ranging horses. This is the notion that the all-megafauna-are-the-same position overlooks obvious differences among species in terms of their “value”. Although it may be distasteful to rate species exclusively in terms of their apparent value to humans, that is in fact how the natural world is frequently perceived and there is no point in ignoring this viewpoint. The way to constructively change minds in this regard is to show that even greater value can be obtained by following a different course, one that need not involve massive expenditures, just the application of common sense. From the value standpoint, the chief benefit that cattle confer is that they convert indigestible, high-fiber plants (grasses) into meat, which is a dominant source of protein for consumers in this country and therefore of undeniable importance. Of course horses and other megafaunal grazers do the same thing physiologically, but as they are not farmed for meat as cattle are, where does their value lie? As the new study makes clear, all megaherbivores—including horses—possess traits that make them indispensable for maintaining or restoring ecosystem health. But for goals like improving the quality of public rangelands in the American West, maximizing the presence of native plant species, ensuring that carbon sequestration takes place while erosion doesn’t, it is crucial to evaluate all relevant traits found in megafaunal grazers, not just their phylogenetic origin, because they are indeed not all the same.
For example, the most desirable kind of pasture for cattle is flat-lying land with few or no obstacles between them and their forage. Nor is much movement encouraged, as that could affect optimum slaughter weight. Due to centuries of selection for quick growth and relatively immense size, most commercial breeds of cattle do not have the locomotor capacity to negotiate steep slopes or broken ground. Existing horse breeds are also the result of human selection, but the capacity for cursoriality (running at high speed) has been retained and even emphasized. Legs with long limb segments enable horses to nimbly cross difficult terrain without having to maintain a rigid stance. (Consider a horse’s canon bones compared to your metacarpals, which make up the skeleton of your palm; in an evolutionary sense these are the same elements, but utterly different in the locomotor behaviors they enable.) In short, horses can thrive where cattle cannot.
Alleged propensity for overgrazing, the argument that comes up all the time in discussions of the environmental impact of wild horses, would not occur if ample land were available for them, including land that is otherwise undesirable for agriculture or other commercial activities. Free-ranging horses will migrate over the course of the year depending on where the best grass is, without much regard to terrain. With cattle, much more active management is required to ensure that overgrazing does not occur.
The bison, a close relative of domestic cattle, should not be left out of consideration here. During the Pleistocene they occupied the West together with horses, mammoths, and many other megaherbivores, and by virtue of the ecosystem services they delivered our grasslands were superdiverse and highly productive. No single species, nor any one combination of species, can maintain all the functions of a diverse ecosystem, especially one that has become highly degraded. To some degree we can still recover that ancient level of ecosystem health by deploying our surviving megafauna in circumstances to which they are best suited to achieve the best outcomes, for them as well as us.
Ross MacPhee PhD, RAN Director of Science
Lundgren, E et al. (2024) Functional traits—not nativeness—shape the effects of large mammalian herbivores on plant communities. Science: DOI: 10.1126/science.adh2616. www.science.org/doi/10.1126/science.adh2616
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