- A new study maps out the original distribution of 145 large mammal species, showing how their ranges have been reduced, sometimes to just 1% of their original extent, by human activity.
- In South America, the marsh deer and the jaguar are among the species that lost the most distribution, at 76% and 40% of their original range, respectively.
- Some species, like the Javan rhino, confined to a single humid forest in Indonesia, are considered “climate refugees” because their current range is different from the habitats they historically roamed.
- The study’s authors say these changes in historical distribution areas must be considered when planning conservation actions or reintroducing locally extinct species back into the wild.
“Anywhere you find humans, you find the extinction of species,” says biologist Mathias Pires, a professor of biology at Campinas State University (UNICAMP) in Brazil.
The near-eradication of the American bison (Bison bison), the largest land animal in North America, is a case in point. These one-ton bovines once roamed the prairies in herds numbering in the millions. The bison’s distribution was so vast that it was found from Alaska all the way down to northern Mexico.
But with the arrival of the first European colonialists in North America, the bison was hunted near to extinction. In the 19th century, fewer than 100 of the animals remained in the wild. Today, their numbers have recovered slightly, and they can be found in small herds mostly in protected areas.
Pires’s observation of the destructive power of humans isn’t new, but now he and several colleagues have put into numbers the impact that we as a species have had on the other species that share this planet since Homo sapiens first migrated out of Africa 50,000 and 11,000 years ago.
“The late Pleistocene was the period when the great human migrations out of Africa began to take place,” says Lilian Sales, a UNICAMP researcher and the lead author of the new study that maps out the original and current distributions of 145 large mammal species. “Man’s arrival and the disappearance of some species were synchronous.”
The study, published in March in the journal Global Change Biology, shows how species such as bison were wiped out from much of their range — nearly a third, on average — because of human activity and forced to move into other habitats and climates.
Some, including the greater one-horned rhino (Rhinoceros unicornis) and the Sumatran orangutan (Pongo abelii), are today found in less than 50% of the habitats where they once occurred. Others, like the Javan rhino (Rhinoceros sondaicus) and the European bison (Bison bonasus) are restricted to an area that’s just 1% of their original range.
“The impact of human presence on the reduction of mammal megafauna species distribution has been well documented in fossil records,” Sales says. “The study’s main objective was to analyze whether the changes in these species’ geographic distribution led to changes in the niches they occupy.”
Species in Asia the most affected
The reason that large mammals were chosen for the study — megafauna defined as weighing more than 44 kilograms (97 pounds) — is that their large bodies are more likely to leave a fossil record, and hence identify patterns of change. They’re also more vulnerable to humans because of their size.
According to the study, large mammals in Asia have experienced the greatest impact from human activity. The list of those whose ranges were reduced the most includes the two rhino species, as well as the wild water buffalo (Bubalus arnee), Père David’s deer (Elaphurus davidianus) and the Arabian oryx (Oryx leucoryx). These last two were at one point declared extinct in the wild, but thanks to reintroduction programs, have once again begun to populate parts of their original ranges.
In South America, the marsh deer (Blastocerus dichotomus) and the jaguar (Panthera onca) are among those species whose distribution shrank the most, by 76% and 40% of their original territory, respectively. The latter, the largest cat in the Americas, was originally found from the southeastern U.S. through to northern Argentina. Today, jaguar sightings in the U.S. are rare, and the species is also seldom in Mexico.
The changes in species’ niches that Sales speaks of are driven by factors such as the temperature and its variability, and rainfall. Many of the species that have gone locally extinct across much of their original range are now confined to regions where the climate is no longer optimal for them.
This is the case with the Javan rhino, which today is restricted to a single national park with humid forest on the western tip of the Indonesian island of Java. In the past, the species occupied a wide range of habitats, from lowland forest to high-altitude forest, marshes to humid prairies, throughout much of Southeast Asia.
Another species, the spotted hyena (Crocuta crocuta), an animal associated today with the savannas of Africa, originally also occurred in the steppes and prairies of Europe and Asia — a range much larger than the African continent.
Sales says large animals need suitably large ranges to live. They also tend to have low reproduction rates, which also leaves them more vulnerable to change.
This is why researchers already refer to some species like the Javan rhino and spotted hyena as “climate refugees.” This term, however, is used in a different context than when referring to human populations forced to leave regions rendered inhospitable by climate change.
“Many species we know today are actually found in less-than-ideal climates, different from those in which their ancestors lived,” says Mauro Galetti, a co-author of the article and faculty member at Brazil’s Paulista State University and assistant professor at the University of Miami. “This is due to the fact that the populations of these species in perfect climates were made extinct by humans.”
Given this scenario, the study authors stress the importance of taking these historical distribution changes into account when making projections for the species’ future and to better plan conservation actions. Without considering these animals’ original ranges, they warn, we could be making mistakes about choosing the best environments for them where they can avoid extinction.
“If we ignore the past and look only at the present, we have only the perspective of already-impoverished environments,” Pires says. “We must also consider the habitats of these species before the arrival of Homo sapiens.”
The vast majority of models in use today for predicting species’ responses to climate change are based on current events. But if they show only a small part of the ranges that these animals originally occupied, they may be making an incorrect diagnosis.
“All of Earth’s species are restricted in space by just a few variables like temperature and humidity,” Galetti says. “This is easy to understand. If you want to know where to find a polar bear [Ursus maritimus], you will look in cold, high-latitude regions with little rainfall. But if we kill off 90% of all the polar bears and scientists in the future try to restore their distribution based only on those in existence, we will have an erroneous ‘map’ as to the ideal climate in which they lived.”
That’s because climate conditions across the Arctic are not homogenous. There are warmer and colder regions, others in which the temperature varies more during the year, and regions with more rainfall. In a future conservation project for reintroducing polar bears in the event they become nearly extinct, the map of their current occurrence won’t represent the species’ true distribution area over the past centuries. And this could affect the success of any conservation or reintroduction program.
The authors say the survival of Earth’s existing fauna depends on humans keeping a careful eye on the past — as in, thousands of years ago — and not maintaining a myopic focus on today’s “environmentally compromised” situation for most of these threatened and iconic species.
Sales, L. P., Galetti, M., Carnaval, A., Monsarrat, S., Svenning, J., & Pires, M. M. (2022). The effect of past defaunation on ranges, niches, and future biodiversity forecasts. Global Change Biology. doi:10.1111/gcb.16145