Whether by spear or plough, humans have been homogenizing North American mammal communities for more than 10,000 years, according to new research led by the Canadian Museum of Nature and the University of Nebraska-Lincoln.
Danielle Fraser of the museum, Kate Lyons of Nebraska and international colleagues analyzed 8,831 fossils representing 365 species of mammals from 366 sites across North America. Drawing on this fossil record allowed the team to assess homogenization: the extent to which specific mammalian species in an ecological community match those in surrounding communities, such that few species are unique. to one community or another.
A few earlier studies—those examining North American mammals tens of millions to millions of years ago—generally identified climate as the primary culprit for the homogenization and heterogenization they found. Other research, focusing only on the past century to recent decades, has chronicled recent human influences of land conversion, poaching, and territorial encroachment.
But no team had established a baseline of homogenization, or the true scale of human contributions to it, by examining the phenomenon both before and after the arrival of Homo sapiens. So Fraser, Lyons and their colleagues turned their attention to the past 30,000 years, a period encompassing the absence of Homo sapiens on the continent, their migration across it, and their transition from hunting-gathering to intensive agriculture.
Homo sapiensthe team found, are likely primarily responsible for the unprecedented rates and levels of homogenization seen in North American mammal communities – to flatten their distinctiveness by increasing the similarity between many of them.
“Our conclusion is that it has to do with early human activities and the arrival of humans in the Americas,” said Lyons, an assistant professor of biological sciences at Nebraska.
North American mammal communities today are more than twice as homogeneous as they were around 10,000 years ago, the study found. This change, the researchers say, is equivalent to the current difference in homogenization between the subtropics of central Mexico and the relatively uniform mammal communities of the Arctic.
The trend appeared earlier and was particularly pronounced in mammals weighing at least 1 kilogram or 2.2 pounds. Also tell? Homogenization began to accelerate around 12,000 years ago, around the time humans hunted mammoths, saber-toothed cats, giant wolves and other large mammals until extinction.
Together, Lyons said, these findings suggest the wave 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 his colleagues showed that these extinctions also push smaller species to expand their range, filling in the geographic gaps left by their larger counterparts. The expansion would have led to more territorial overlap, Lyons said, further homogenizing communities in the process.
But homogenization in North America has accelerated further over the past 5,000 years – a period marked by a 10-fold increase in the human population and the emergence of widespread agriculture, especially in what would become the central and eastern United States.
“It happened much later in North America than in other continents,” Lyons said. “But that’s really when humans in North America went from being hunter-gatherers to being more sedentary and dependent on agriculture.”
The proliferation of human settlements across the continent attracted species of mammals – coyotes, raccoons, rats and other rodents – that would come to thrive on the byproducts of these settlements and profit from the removal of predators by the people who live.
The conversion of grasslands and forests for agriculture, meanwhile, has reduced the number of plant species in a given habitat from hundreds or thousands to a few tens or less, reducing the habitable territory for more difficult herbivores and carnivores or omnivores that feed on them. Cultivated fields, roads and other human-made boundaries would also have acted as “dispersal barriers”, Lyons said, which also locked some species into smaller territories.
“You still have narrow-range species, but now they’re in fewer communities, so their overall contribution to the difference between communities is much smaller than perhaps it was before,” Lyons said.
As for the potential effect of climate? The team found little evidence from between 10,000 and 500 years ago. About 20,000 to 15,000 years ago, warming North America saw the retreat of glaciers that had enveloped most of modern Canada and much of the northern United States. Warmer climates generally produce more gradual north-south gradients in temperature and precipitation. This warming-induced climatic homogeneity, Lyons said, also tends to engender homogeneity in mammal communities.
If climate had contributed to the homogenization of mammal communities, the team would have expected this homogenization to accelerate before 10,000 years ago. The fact that this is not the case indicates that the climate probably had nothing to do with it, she said.
“What we find when we look at climate models is that all of this happened very early, before we saw this dramatic homogenization,” Lyons said.
For all the speed and severity of homogenization over the past 5,000 years, it has only increased in the past 500 years, the team concluded. To the extent that it stems from the continued extinctions of keystone species whose behaviors and abilities are particularly important, this homogenization could be a danger to ecosystems, Lyons said.
“A lot of what we’re seeing is that when we lose species – especially when we lose large species that tend to be what we call ecosystem engineers – there’s a sea change in the ecosystem that remains,” she said. “Large mammals do all kinds of things in ecosystems.
“Elephants eat a lot, they move around a lot, and they poop a lot, so they move a lot of nutrients around in ecosystems. What we’re seeing, then, is that nutrients are essentially being lost from ecosystems (in their absence). »
With fewer keystone species, homogenized mammal communities can also boast fewer ways to respond to, and possibly survive, the current challenges of climate change and human encroachment, Lyons said.
“Communities will likely be less resilient to future disturbances and potential extinctions,” she said. “It also makes the world less interesting, because there are fewer wonderful variations out there.”
The team reported their findings in the journal Nature Communications. Fraser and Lyons are the authors of the study along with Alex Shupinski, a doctoral candidate in biological sciences in Nebraska; Amelia Villasenor of the University of Arkansas; Anikó Tóth from the University of New South Wales; Meghan Balk of the Battelle Memorial Institute; Jussi Eronen from the University of Helsinki; W. Andrew Barr of George Washington University; AK Behrensmeyer, Gary Graves, Richard Potts, and Laura Soul of the Smithsonian Institution; Matt Davis of the Natural History Museum of Los Angeles County; Andrew Du of Colorado State University; J. Tyler Faith of the University of Utah; Nicholas Gotelli of the University of Vermont; Advait Jukar of Yale University; Cindy Looy of the University of California at Berkeley; Brian McGill of the University of Maine; Joshua Miller of the University of Cincinnati; Silvia Pineda-Munoz of Indiana University.
The researchers received funding in part from the National Science Foundation and the Smithsonian Institution.