The loss of large plant-eaters, such as elephants and bison, might not just stop with the animals in question. The removal of these so-called megafauna from ecosystems could have permanent impacts on the plants and small animals that are left behind. New research has looked at the effect that the extinction of large herbivores had on the environment, and found that when the megafauna were gone certain ecosystems were forever lost with them.
Starting around 15,000 years ago, when humans arrived in the Americas, the large herbivores that lived there started to decline. Roaming the landscape at the time were mammoths, mastodons, wild horses and giant ground sloths, which all exerted an influence on the environment in different and varying ways. What eventually killed them off is still hotly debated, but the effect their absence had on the landscape during the following thousands of years can still be examined.
“Vegetation change was assessed from studies of fossil pollen from lake cores near the megafauna sites,” explained Professor Anthony Barnosky, who led the study published in the Proceedings of the National Academy of Sciences, in an email to IFLScience. “The changes in vegetation abundances in the North American sites tended to be more consistent with removing big browsers and grazers from the region than from climate change alone, although certainly climate change explains a good deal of the vegetation change as well.”
The effect of large herbivores grazing and trampling the ground can keep trees at bay and churn up the soil, maintaining areas of grassland. Lee Prince/Shutterstock
Large herbivores can influence the ecosystem in a myriad of ways. The most obvious is still seen with modern elephants, which uproot and tear down small trees and shrubs as well as churning the soil, which has the effect of keeping landscapes open and free of trees. Mammoths and mastodons in North America probably played a similar role, and in fact this is exactly what the pollen studies showed. But the impact might not only extend to the plant communities, but also the small mammals that were scurrying around the mammoths’ feet.
Certain sites containing hundreds to thousands of small mammal fossils record how these animals responded over time to the loss of the larger herbivores. What the researchers found in California was that there was a decrease in small rodent diversity once the larger animals became scarce, allowing the most widespread and adaptable mouse species to dominate.
The effects of megafaunal removal were not, however, uniform across the whole of the Americas. For example, when the giant ground sloth and mastodon went extinct in the South American pampas of Argentina, there was not the expected spread of forests. The researchers suspect that this is probably related to the environment of the region, with the weather and rainfall not suitable to support forests.
“It's not a simple story, where if you pull out a big beast you see major changes in the landscape,” says Barnosky, in a statement. “It's actually dependent on how big a beast you pull out, and also how that beast interacts with the plants and animals in the area, and what other plants and animals are there. It depends on what the animal does for a living.”
The overall message of the study, however, is that by removing the large herbivores from an environment, the ecosystem in which they once lived can be dramatically altered. These changes can then last for thousands of years, permanently shifting the dynamics of environments to such an extent that it may no longer be possible to return it to how it once was. With the dramatic and steep decline of large herbivores seen in the modern era, from elephants in Africa to rhinos in Asia, we might not only be witnessing the extinction of single species, but the entire ecosystems in which they live.