Introduced megafauna are rewilding the AnthropoceneSubmitted by editor on 16 May 2018. Get the paper!
A wild donkey (Equus asinus) in the Sonoran Desert, Arizona. Photograph by Michael Lundgren.
By Erick Lundgren et al.
The world was once home to an astounding diversity of large herbivorous mammals (>100 kg), or megafauna. The majority of these species went extinct at the end of the Pleistocene as modern humans spread from Africa. Most research points to overhunting by humans as the most likely culprit for these extinctions (1, 2).
Unfortunately, the world’s remaining megafauna continue to face steep declines in their native ranges. In 2015, our co-author William Ripple (3) found that 60% of the world’s remaining megafauna are threatened in their native ranges due to exploitation and habitat loss. The plight of these populations is very concerning and has led to global calls to conserve the remaining members of this functional group (4).
However, in analyzing the ranges of these native megafauna, an interesting idiosyncrasy emerges: a significant portion of the world’s megafauna is not included! A whole ‘invisible’ biodiversity of megafauna (5) exists outside the umbrella of conservation concern because they were introduced by humans in modern times.
Including introduced megafauna shifts how we understand the distribution of this important and declining functional group. While many native populations of megafauna are declining, all continents now have more megafauna species than they have had since the Late Pleistocene.
To quantify this paradox, for our recent article (6) we mapped all documented introduced populations of megafauna. We found that, of 76 remaining megafauna species, 22 have wild introduced populations (~29%). Of these, 64% are threatened, extinct, or decreasing in their native range.
By including introduced megafauna, two species are resurrected in the wild, cattle (Bos taurus), descendants of the extinct aurochs (Bos primigenius), and dromedary camels (Camelus dromedarius), a species which went extinct in the wild in its native range thousands of years ago but now roams the deserts of Australia.
In addition to ‘rewilded’ post-domestic species, 15 non-domesticated species have found new homes through introductions. This includes the ‘cocaine hippos’ of Colombia (Hippopotamus amphibius), which are listed as Vulnerable in their native range but are spreading in their introduced range despite efforts to control them.
Introduced megafauna are established on every continent and in some cases have created novel species-rich megafauna communities that bring many continents closer in species richness to the environments of the Late Pleistocene. For example, while Australia lost all of its megafauna at the end of the Pleistocene, the introduction of eight megafauna species has restored Australia’s species richness by 67%.
This paradox – that local conservation goals to preserve ecosystems in a historic state can be at odds with global conservation goals to protect the persistence of species and their evolutionary potential – raises deep questions on how to conserve life in an age of pervasive global change. Accepting, studying, and valuing these ‘invisible megafauna’ (5) offers the possibility of a new relationship with the globalized but still wild natural world.
Wild introduced donkeys (Equus asinus) have established in North America, South America, Africa, and Australia. Their pre-domestic ancestor (Equus africanus) is critically endangered with an estimated population of 50-200. Eradicating the introduced populations of this species may restore local communities to historic states but would also engender the extinction of this form of life in the wild. Photograph by Michael Lundgren.
Both native and introduced megafauna can have suppressive effects on other species through herbivory and disturbance. However, these influences are often contingent on the presence and protection status of apex predators. Apex predators shift the ecological functions of megafauna by reducing their densities through predation and by modifying their behavior through fear (7). Evidence suggests that apex predators exert similar influences on introduced megafauna (8, 9).
Megafauna exert significant ecological influences that can reduce fire frequency, restructure plant communities, facilitate smaller herbivores, increase nutrient cycling, and disperse seeds and nutrients great distances and increase water availability (3, 10-12). Indeed, there is growing evidence that introduced megafauna may have restored lost Pleistocene functions or have introduced important new ones.
For example, grazing by introduced wild water buffalo (Bubalus bubalis) in Australia increases tree growth rates by suppressing grasses (13); the removal of wild introduced donkeys (Equus asinus) and other megafauna appears to have lead to the extinction of endemic fish populations in desert springs (14); and our ongoing research has found that well digging by wild burros (Equus asinus) in the Sonoran Desert of North America creates unique water sources that are used by more than 40 other vertebrate species and in some conditions become vegetation nurseries for foundational riparian trees.
Wild donkeys (or burros as they are known in the USA) dig wells up to more than a meter in depth to access groundwater in the Sonoran Desert of Arizona. We have documented more than 40 other bird and mammal species using these wells.
A young cottonwood (Populus fremontii), a foundational riparian tree species, growing from an abandoned donkey well on the Bill Williams River, Arizona. Donkey wells appear to mimic the abiotic conditions that local flood-adapted riparian tree species are dependent upon.
While many introduced megafauna populations are thriving, others face similar threats as native wildlife. For example, the wildlife trade threatens more than 300 native mammals, including megafauna (15). What is less known is that introduced megafauna have also been targeted; 1.5 million domestic and wild donkeys were slaughtered in 2017 to fuel a growing Chinese market for the traditional medicine ejiao. The trade converges with and drives networks that also traffic in CITES listed species, creates serious animal welfare harms, and exacerbates poverty in rural communities (16). Currently, the Australian government is considering capitalizing on this trade to incentivize the eradication of wild introduced donkeys. Meanwhile, the pre-domestic native donkey (the African wild ass, E. africanus) has a population estimated between 50 and 200 adult individuals. While protecting this population is paramount, to beget or ignore the extinction of their ‘rewilded’ brethren may doom this form of life in the wild.
For conservation to be effective in the Anthropocene – an age of unprecedented global change – it may need to embrace the inadvertent rewilding presented by introduced megafauna. Further research on the ecological functions of these organisms, and their relation to apex predators, will shine a light on the wondrous unseen wilds of our globalized world and perhaps offer more ethical and pragmatic responses (17) to the ecological changes of the Anthropocene.
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