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Wild horses once ranged over vast swathes of the planet. Is there any sense in returning them to their former rangelands? Two researchers examined the question.
Danish researchers have identified 1.5 million hectares across Europe that would be suitable for reintroducing wild horses to their former habitat, amid growing concern over declines in megafauna around the globe.
Large animals, or megafauna, have been decimated worldwide during the last million or so years, the scientists from Aarhus University said.
Many of these lost animals were keystone species, Pernille Johansen Naundrup and Jens-Christian Svenning reported in the peer-reviewed open-access journal, PLOS ONE.
It was increasingly clear that humans had played a key role in these losses, and human-driven megafauna losses were ongoing, they wrote.
“Their loss likely has had large effects on ecosystems,” they said, which had sparked a growing focus on how large animals could be restored in rewilding projects. Rewilding emphasises species reintroductions to restore ecological function.
“The horse (Equus ferus) is highly relevant in this context as it was once extremely widespread and, despite severe range contraction, survives in the form of domestic, feral, and originally wild horses,” they said.
“Further, it is a functionally important species, notably due to its ability to graze coarse, abrasive grasses.”
The pair, from the university’s Department of Bioscience, used species distribution modelling to link locations of wild-living E. ferus populations to climate to estimate climatically suitable areas for the animals.
These models were combined with habitat information and past and present distributions of equids to identify areas suitable for their reintroduction.
Mean temperature in the coldest quarter, precipitation in the coldest quarter, and precipitation in the driest quarter emerged as the best climatic predictors, they said.
“The distribution models estimated the climate to be suitable in large parts of the Americas, Eurasia, Africa, and Australia and, combined with habitat mapping, revealed large areas to be suitable for rewilding with horses within its former range, including up to 1.5 million hectares within five major rewilding areas in Europe.”
The pair said the widespread occurrence of suitable climates and habitats within the former range of E. ferus, together with its important functions, made it a key candidate for rewilding in large parts of the world.
They acknowledged that such a plan would require important issues to be addressed. “Successful re-establishment of wild-living horse populations will require handling the complexity of human–horse relations, for example, potential conflicts with ranchers and other agriculturalists or with other conservation aims, perception as a non-native invasive species in some regions, and coverage by legislation for domestic animals.”
Naundrup and Svenning noted that only one originally wild subspecies, the Przewalski’s horse, had survived, although much genetic diversity had been preserved in the domesticated forms of the horse.
Wild horses were extremely widespread and common during the Middle and Late Pleistocene, with a distribution that covered most of Eurasia and northern Africa, as well as North and South America, they said.
It evolved in North America 1.1 to 1.2 million years and spread via the Beringia land bridge and the Isthmus of Panama to Eurasia and South America some 800,000 to 900,000 years ago, and to Africa even later still, in the Late Pleistocene.
“Further, E. ferus was just the latest of a more or less long line of grazing equids in these regions,” they said.
“In the Americas, E. ferus went extinct during the latest Pleistocene or early Holocene, whereas it remained widespread in the wild in Eurasia until the late Holocene.”
The last originally wild populations disappeared from Eastern Europe and the southern parts of Russia during the last few hundred years, whereas the Przewalski’s horse survived until 1969 in the wild in Central Asia. It has since been reintroduced to its native habitat through an international breeding effort.
Human activity was clearly the cause of the extinction of wild E. ferus in Eurasia, they said, notably through hunting and domestication.
“The cause of extinction in the Americas is less clear, but the evidence there also points to humans rather than climate being the cause, notably when examined in the context of broader megafauna extinctions.”
They said that, despite the historical range collapse, wild-living horses were found today in many parts of the world, with feral populations found on all continents except Antarctica.
They said while there were large areas of suitable habitat for E. ferus in sub-Saharan Africa, Australia and New Zealand, the species was not native to any of these areas.
They noted that wild horse populations existed in Australia and New Zealand.
New Zealand, they noted, did not naturally harbour land-going mammalian herbivores. “Introduced mammalian herbivores are generally perceived to be highly damaging there.”
Australia’s native mammals were largely marsupials, and feral horses were generally considered to be pests, with big effects on native habitats and agriculture.
“North America likewise harbours large feral populations, and feral horses are there perceived either as an iconic native or semi-native species or as a non-indigenous invasive species.”
The researchers said large herbivores could have profound effects on their habitats.
“Due to the widespread former distribution of E. ferus and earlier grazing equids, grassland biota in much of the world must have evolved and/or persisted under the influence of grazing by horses for millions of years,” they said.
Horses, they noted, were selective grazers, preferring grasses, sedges and herbs, including coarse, highly abrasive grasses.
The researchers said the question of whether reintroduction was desirable depended on several factors, including the level of competition between the reintroduced species and native species with similar ecologies.
Across the former range of E. ferus, there was little reason to suspect general negative effects on other native species as the vast majority would have coexisted with the species for up to a million years, and with other grazing equids for even longer, they wrote.
Naundrup and Svenning said the widespread occurrence of suitable climates and habitats within the very large former range of E. ferus, together with its important functions as a grazer, made wild-living horses a key candidate for rewilding in large parts of the world.
However, it was less clear how much they had previously been limited by predation, and if they may sometimes increase rapidly in numbers in the absence of predators, greatly impacting on vegetation and, in some cases, harming biodiversity.
“There are as yet no definite answers to this issue,” they said. “Clearly, more research is needed to better understand the potential role of top-down regulation in the case of wild-living horses. In some real-world cases, it may be feasible to also reintroduce relevant predators.”
They acknowledged, too that there may also be conflicts with ranchers and other agriculturalists as wild-living horses and domestic livestock may compete for forage, or there may be a risk of disease transmission between species.
“Public views on feral horses range from a pest species to an iconic wild animal that deserves protection, which may render management more difficult.
“Notably, human control of feral horse populations is often controversial as the public may perceive management actions such as culling of excess individuals to be animal cruelty.”
They concluded that large areas of the world could potentially harbour feral horse populations, even when restricting their rangelands to less than 100 square kilometres.
Current feral populations had retained all or most of the species’ former wide ecological adaptability, they said.
“Hence, E. ferus is an obvious species to use in rewilding in much of the world due to its former very wide range, its wide extant ecological tolerance, its particular role as a grazer and our extensive knowledge of its ecology, behaviour and management.”
Naundrup PJ, Svenning J-C (2015) A Geographic Assessment of the Global Scope for Rewilding with Wild-Living Horses (Equus ferus). PLoS ONE 10(7): e0132359. doi:10.1371/journal.pone.0132359
The full study can be read here.