Over the past many thousands of years, nature and animal life have experienced massive changes. For example, hunting of megafauna (large animals) has meant that many species are now extinct. When these species became extinct, we lost the important functions performed by these animals for Earth’s ecosystems over the millions of years in which the biodiversity we see today was accumulated.

However, the past is not necessarily lost forever. A new international study shows that introduced species of large herbivores, for example feral horses, dromedary camels and wild boars, actually help re-establish prehistoric conditions in ecosystems. And they do this even though their distant cousins or totally unrelated domestic large native animals are long gone. This can spur us to look at non-native large animals in a new light. Focus is often only on the downside of the camels in Australia or feral horses in North America; that is, as invasive species that only have a negative impact on nature.

"Our study compares ecologically important, so-called functional, traits for the large non-native herbivore species found around the world as a result of previous introductions of animals, especially for hunting purposes, or animals that have escaped captivity, to those of the extinct species as well as surviving native species. Overall, we find that the extinctions of the past have partially been compensated by today’s non-native large herbivores because of similarities in their functional traits, and this is very uplifting from the perspective of biodiversity," explains Professor Jens-Christian Svenning.

Can we put the “sins” of the past behind us?

Nature as we know it today looks very different from the nature of the past 30-45 million years. During all of this period and until recently, large mammals flourished and were diverse and abundant in ecosystems across the world, from warm to cold climates. For example, in the warm period just before the Last Ice Age, c. 125, 000 years ago, climate in Denmark was similar to today, but in addition to familiar animals and plants such as red deer, oak, ash, hornbeam, spruce and heather, there were also species of elephant and rhinoceros.

Paleo-biological findings show that the large herbivores started to become extinct about 50,000-100,000 years ago, at the same time as Homo sapiens spread over the world and most likely as a result of hunting pressure and increasing human population densities.

The research team has compared key functional traits, such as body size, diet, and habitat, for all the large herbivore species that have existed over the past 130,000 years, both extinct species and surviving species. This has enabled the researchers to directly compare the species across millennia based on their functional traits and thus their impact on ecosystems.

“We’ve been able to quantify the traits of a given non-native species, and compare these traits with those of the surviving native species and extinct native species. Surprisingly, it turns out that non-native animals match the extinct animals fairly well, and overall they contribute to partially restoring ecosystem conditions to pre-extinction conditions,” says Jens-Christian Svenning.

In other words, what has surprised the researchers is that the non-native herbivores of today are responsible for an increasing resemblance between today’s fauna and the fauna we would have had, had it not been for the massive extinctions of large animals of the past.

Distant ancestry with the same habits

It may seem strange that animal species that have lived thousands of years apart and come from very different areas are comparable ecologically. This is partly reflect convergent evolution of the same adaptations, and partly that some groups only became extinct in some areas, but survived in others. As a result, 64% of non-native herbivores have traits more similar to extinct native species in their new habitat than to the surviving native species.

This means that the non-native large herbivore species can be perceived as a form of surrogate – or replacement – for species that have become extinct. We see this, for example, in Australia where dromedary camels functionally substitute extinct giant marsupials, and in North America, where wild horses have substitute extinct horse breeds, and wild boars substitute extinct peccary. In Denmark, we also have examples: fallow deer, introduced in the Medieval Period for hunting, but which would probably still be found naturally in Denmark today, had it not been for their prehistoric extirpation from most of Europe. In this case, the extinct species has been reintroduced directly, and not just substituted; this is also true of the feral horses in America, replacing extirpated wild horses of the same species.

“Natural conditions for ecosystems are often being defined by the short period of time for which we have registered and documented history. However, this does not give us the full picture of the conditions that extant species have evolved under and adapted to, as most species are hundreds of thousands of years old or older. Important, this near-sighted perspective ignores that major changes that have taken place over the past 100,000 years, in step with the prehistoric dispersal of modern humans all over the planet. If we adopt a broader approach and a more evolutionary time perspective, we can ask more relevant questions and work with more long-term solutions to the current biodiversity crisis,” explains Jens-Christian Svenning.

In the article, the group points to major benefits for both nature management and ecological research if we look beyond the past few hundred years. The relatively recent past is simply too narrow a focus, notably  in relation to finding solutions that are long-lasting:

“We really need to reconsider our approach to introduced non-native animal species, and not just automatically assume that they have a negative effect. Some species are problematic, others are neutral. However, non-native large herbivores typically contribute positively to restoring the conditions that have created and maintained biodiversity through hundreds of thousands to millions of years despite major climate change. Our study is an important step towards seeing non-native species in nature in a more neutral, functional light,” says Jens-Christian Svenning.

Contact

Professor Jens-Christian Svenning
Center for Biodiversity Dynamics in a Changing World (BIOCHANGE)
Institut for Biologi
Aarhus Universitet

The study was published in the scientific journal PNAS, and is a collaboration between the University of Technology Sydney (UTS), Australia; University of Massachusetts Amherst, University of Kansas, University of California Davis, Natural history Museum of Los Angeles County, USA; University of Sussex, UK; Universidad de Alcalá, Spain; and Aarhus University, where professor Jens-Christian Svenning has participated in collaboration with PhD students Simon Schowanek and postdoc Matt Davis, Center for Biodiversity Dynamics in a Changing World (BIOCHANGE) at the Department of Biology.

The study was carried out with grants from the Carlsberg Foundation and the VILLUM FOUNDATION.