Humans have been homogenizing the mammal communities of North America for 10,000 years, whether by spear or plow, according to new research led by the University of Nebraska-Lincoln and the Canadian Museum of Nature.
Kate Lyons of the University of Nebraska, Danielle Fraser of the Canadian Museum of Nature, and international colleagues examined 8,831 fossils representing 365 mammal species from 366 sites across North America. Using those fossil records, the team was able to assess homogenization: the degree to which the specific mammal species in one ecological community resembled the species composition of its surrounding communities.
A few previous studies, which looked at North American mammals from tens of millions to millions of years ago, generally blamed climate for the homogenization and heterogeneity they discovered. Other studies, focusing on the last century to a few decades, have documented recent human influences such as land conversion, poaching, and territorial encroachment.
However, no team had established a baseline of homogenization, or the true magnitude of human contributions to it, by studying the phenomenon both before and after Homo sapiens arrived. So Fraser, Lyons, and their colleagues focused their attention on the last 30,000 years, a time span that encompassed Homo sapiens’ absence from the continent, their migration across it, and their transition from hunting-gathering to intensive agriculture.
Homo sapiens, the team found, are probably most responsible for the unprecedented rates and levels of homogenization seen in North American mammal communities — for flattening their distinctive character by escalating the similarity among many of them.
“Our conclusion is that this does have to do with early human activities and the arrival of humans into the Americas,” said Lyons, assistant professor of biological sciences at Nebraska.
What we find when we look at the climate patterns, is that all of that happened very early on, before we see this dramatic homogenization. It happened much later in North America than it did on the other continents. However, that is when humans in North America transitioned from hunter-gatherers to more settled and reliant on agriculture.
Asst. Prof. Lyons
According to the study, modern North American mammal communities are more than twice as homogeneous as they were roughly 10,000 years ago, and could be nearly four times as homogeneous by the end of the twenty-first century. The researchers claim that this shift is equivalent to the current disparity in homogenization between the subtropics of central Mexico and the comparatively uniform mammal communities of the Arctic.
The trend began earlier and was most pronounced in mammals weighing at least 1 kilogram (2.2 pounds). Also instructive? Around 12,000 years ago, when humans were hunting mammoths, saber-toothed cats, dire wolves, and other massive mammals to extinction, homogenization began to accelerate.
Together, Lyons said, those findings suggest that the spate of large-mammal extinctions contributed to homogenization. The disappearance of large mammals unique to individual communities would have directly increased their similarity, she said. And in a 2019 study, Lyons and colleagues showed that those extinctions also drove smaller species to expand their ranges, filling the geographic voids left by their larger counterparts. Expansion would have led to more territorial overlap, Lyons said, further homogenizing communities in the process.
However, homogenization in North America has accelerated over the last 5,000 years, a period marked by a 10-fold increase in human population and the emergence of widespread farming, particularly in what would become the central and eastern United States.
“It happened much later in North America than it did on the other continents,” Lyons explained. “However, that is when humans in North America transitioned from hunter-gatherers to more settled and reliant on agriculture.”
The proliferation of human settlements across the continent attracted mammal species — coyotes, raccoons, rats and other rodents — that would come to thrive on the byproducts of those settlements and enjoy the elimination of predators by the people inhabiting them.
Meanwhile, agricultural conversion of prairies and forests has reduced the number of plant species in a given habitat from hundreds or thousands to dozens or fewer, narrowing the habitable territory for pickier herbivores and the carnivores or omnivores that prey on them. Cultivated fields, roads, and other human-made boundaries would have also acted as “barriers to dispersal,” hemming certain species into smaller territories, according to Lyons.
“You still have narrow-ranging species, but they’re in fewer communities now, so their overall contribution to the difference in communities is much smaller than it might have been before,” Lyons explained.
As for the potential effect of climate? The team found scant evidence for it between 10,000 and 500 years ago. From about 20,000 to 15,000 years ago, a warming North America saw the retreat of glaciers that had enveloped nearly all of modern-day Canada and much of the northern United States. Warmer climates generally yield more gradual north-south gradients in temperature and precipitation. That warming-driven homogeneity in climate, Lyons said, tends to breed homogeneity in mammal communities, too.
If climate had contributed to the homogenization of mammal communities, the team would have expected that homogenization to accelerate prior to 10,000 years ago. The fact that it didn’t indicates that climate probably had little to do with it, she said.
“What we find when we look at the climate patterns,” Lyons said, “is that all of that happened very early on, before we see this dramatic homogenization.”
The team concluded that the speed and severity of homogenization over the past 5,000 years has only increased in the last 500. Lyons believes that homogenization could pose a threat to ecosystems if it is caused by the continued extinction of keystone species whose behaviors and capabilities are especially important.
“A lot of what we’re finding is that when we lose species, especially large species that tend to be what we call ecosystem engineers,” she said, “there’s a dramatic change in the ecosystem that’s left.” “Large mammals do a variety of things in ecosystems.
“Elephants eat a lot, move a lot, and poop a lot, so they move a lot of nutrients around ecosystems. So, we’re discovering that nutrients are essentially lost from ecosystems (in their absence).”
Lyons believes that because there are fewer keystone species, homogenized mammal communities may have fewer ways to respond to, and possibly survive, the ongoing challenges of climate change and further human encroachment.
