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Extinctions of Massive Mammals Leave 'Missing Pieces' in Ecosystems
Nebraska Ag Connection - 09/28/2022

Welcome to Pocket Science: a glimpse at recent research from Husker scientists and engineers. For those who want to quickly learn the "What," "So what" and "Now what" of Husker research.

What?

For more than 100,000 years, the proliferation and migration of humans across the globe have been followed by the disappearance of large-bodied mammals -- mammoths, mastodons, ground sloths and scores of other species -- that were likely hunted to extinction. And the rate of those extinctions has accelerated considerably, with large-bodied mammalian species vanishing between 10 and 100 times faster since Homo sapiens came to dominate the planet.

Those large-bodied species, many of which evolved within specific ecosystems for millions of years, have carried out multiple roles critical to the identity and functioning of those ecosystems. In consuming massive amounts of leafy vegetation and traversing wide swaths of a landscape, for instance, large-bodied species recycle the nutrients essential to supporting more vegetation and sustaining other, smaller herbivores, which in turn sustain carnivores that prey on them.

So what?

A growing body of evidence suggests that even moderate-sized mammalian species may struggle to fill the ecological niches left when their gigantic peers go extinct. But the effects of ongoing extinctions may take decades to fully manifest -- time that conservationists, and the remaining species they seek to protect, may not have. To better understand the potential consequences of modern-day extinctions, the University of New Mexico's Felisa Smith, Nebraska's Kate Lyons and colleagues resorted to the past. The team focused on the last 20,000 years, when human dispersal across North America triggered the extinctions of more than 65 large-bodied mammalian species on the continent.

Smith, Lyons and their colleagues specifically analyzed molars and other fossils from more than 2,000 mammals in an area of Texas known to have undergone numerous extinctions. Analyzing the concentrations of a particular carbon atom, carbon-13, helped the team determine whether a fossilized specimen was a browser that consumed mostly trees, shrubs and cool-season grasses, a grazer that ate primarily tropical and subtropical grasses, or both. Those fossils, alongside existing knowledge of the species, also allowed the team to estimate the size of mammalian specimens living either before or after the onset of the extinctions.

The research team found that the extinctions of large-bodied species left "missing pieces" in the ecosystem that the smaller, remaining plant-eaters did not fill. The disappearance of mammoth, horse and bison species, for example, opened up opportunities for moderate-sized herbivores to consume the leafier vegetation of the extinct grazers and, in turn, grow to larger sizes themselves. But the researchers found little evidence for it. Instead, the ecosystem transformed from one featuring a wide range of body sizes, as is found in the contemporary savannas of Africa, to one in which many similar-sized herbivores generally competed for the same types of vegetation.

In contrast, the extinction of the community's apex predators, especially its sabertooth cats, led the largest remaining carnivore, the jaguar, to shift from a general to more specialized diet that consisted mostly of the remaining energy-rich grazers previously hunted by the sabertooths. The mountain lion, meanwhile, exploded in numbers by expanding its menu to include prey abandoned by the jaguar.

Now what?

Emerging research indicates that less-complex, more-homogenous ecosystems -- the very sort that the Texas community became following the extinctions of its largest plant-eating mammals -- may be less resilient to climate change, human encroachment and other challenges. From that standpoint alone, the team said, the study underscores the importance of maintaining biodiversity and protecting the massive, keystone mammal species that are, in increasingly evident ways, irreplaceable.


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