Whether with the spear or the plow, humans have homogenized North America’s mammalian communities for more than 10,000 years, says 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 conducted an analysis of 8,831 fossils representing 365 mammal species at 366 sites in North America. By relying on those fossils, the team was able to assess homogenization: the degree to which the specific mammal species in an ecological community match those of the surrounding communities, so that few species are unique to one community or the other.
A few previous studies — studies that examined North American mammals from tens of millions to millions of years ago — generally cited climate as the primary culprit of the homogenization and heterogenization they discovered. Other research, focusing only on the past century to past decades, has mapped the recent human influences of land conversion, poaching and territorial encroachment.
But no team had established a baseline of homogenization, or the true magnitude of human contributions to it, through the phenomenon both before and after the advent of Homo sapiens. So Fraser, Lyons, and colleagues turned their attention to the past 30,000 years, a time span that marked the absence of Homo sapiens on the continent, their migration across the continent and their shift from hunting-gathering to intensive farming.
Homo sapiensthe team found, are probably most responsible for the unprecedented rates and levels of homogenization seen in North American mammal communities — for smoothing out their distinctiveness by increasing the similarity between many of them.
“Our conclusion is that this has to do with early human activities and the arrival of humans to the Americas,” said Lyons, assistant professor of biological sciences in Nebraska.
North American mammal communities of the modern age are more than twice as homogeneous as they were about 10,000 years ago, the study revealed. That shift, the researchers said, is equivalent to the current difference in homogenization between the subtropics of central Mexico and the relatively uniform mammalian communities of the Arctic.
The trend occurred earlier and was especially pronounced in mammals weighing at least 1 kilogram or 2.2 pounds. Also tell? Homogenization began to accelerate about 12,000 years ago, around the time humans were on the brink of extinction on mammoths, saber-toothed cats, dire wolves and other large mammals.
Together, Lyons said, these findings suggest that the wave of large mammal extinctions contributed to homogenization. The disappearance of large mammals unique to individual communities would have directly increased their resemblance, she said. And in a 2019 study, Lyons and colleagues showed that those extinctions also prompted smaller species to expand their range, filling in the geographic voids left by their larger counterparts. Expansion would have led to more territorial overlap, Lyons said, further homogenizing communities.
But homogenization in North America accelerated even more over the past 5,000 years — a period marked by a whopping 10-fold increase in human population and the rise of widespread agriculture, particularly in what would become the central and eastern United States. become.
“It happened much later in North America than on other continents,” Lyons said. “But that’s really when people in North America went from hunter-gatherers to more settled and dependent on agriculture.”
The proliferation of human settlements across the continent attracted mammalian species — coyotes, raccoons, rats and other rodents — that would come to thrive on those settlements’ byproducts and enjoy the elimination of predators by the people who inhabit them.
The conversion of prairies and forests to agriculture, meanwhile, reduced the number of plant species in a given habitat from hundreds or thousands to just tens or fewer, reducing the habitable area for more finicky herbivores and the carnivores or omnivores that prey on them. . Cultivated fields, roads and other man-made borders would also have acted as “dispersion barriers,” said Lyons, who also confined certain species into smaller territories.
“You still have species with a small range, but now they’re in fewer communities, so their overall contribution to the difference in communities is much smaller than maybe before,” Lyons said.
As for the possible effect of the climate? The team found little 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 almost all of present-day Canada and much of the northern United States. Warmer climates generally produce more gradual north-south gradients in temperature and precipitation. That warming-induced homogeneity in climate, Lyons said, also tends to breed homogeneity in mammalian communities.
If climate had contributed to the homogenization of mammalian communities, the team would have expected that homogenization to accelerate until 10,000 years ago. The fact that it didn’t indicates that climate probably had little to do with it, she said.
“What we see when we look at the climate patterns is that all of that happened very early, before we see this dramatic homogenization,” Lyons said.
Despite all the speed and severity of homogenization over the past 5,000 years, it has only increased over the past 500, the team concluded. To the extent that it stems from the ongoing extinction of keystone species whose behavior and capabilities are particularly consistent, that homogenization could endanger ecosystems, Lyons said.
“A lot of what we find is that when we lose species — especially when we lose large species that are usually what we call ecosystem engineers — there’s a dramatic change in the ecosystem that remains,” she said. “Large mammals do all kinds of things in ecosystems.
“Elephants eat a lot, they move a lot and they poop a lot, so they actually move nutrients through ecosystems a lot. So what we’re finding is that nutrients are essentially lost from ecosystems (in their absence).”
With fewer keystone species, homogenized mammalian communities also boast fewer ways to respond to, and potentially survive, the ongoing challenges of climate change and further human encroachment, Lyons said.
“Communities will likely be less resilient to future disruptions and possible extinctions,” she said. “It also just makes the world less interesting, because you have less wonderful variety there.”
The team reported its findings in the journal Nature Communications. Fraser and Lyons co-authored the study with Nebraska doctoral student of biological sciences Alex Shupinski; Amelia Villaseñor of the University of Arkansas; Anikó Tóth of 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, Berkeley; Brian McGill of the University of Maine; Joshua Miller of the University of Cincinnati; Silvia Pineda-Munoz of Indiana University.
The researchers received partial funding from the National Science Foundation and the Smithsonian Institution.