The results were published April 27 in the journal Proceedings of the National Academy of Sciences.
When any species goes extinct, the web of relationships among surviving species often shifts in complex and cascading ways. When predators vanish, their prey can multiply unchecked, triggering a chain of downstream consequences. Senior author Lydia Beaudrot, an assistant professor of integrative biology and member of the Ecology, Evolution and Behavior program at Michigan State University, had suspected based on prior research that the extinction of large mammals tens of thousands of years ago could have produced long-lasting food web effects. "But there weren't that many data points," she said, prompting her team to develop new methods for synthesizing data at broader spatial scales.
For the new study, Beaudrot and first author Chia Hsieh led a team that analyzed predator-prey relationship data at 389 sites spanning tropical and subtropical regions of the Americas, Africa and Asia. The dataset covered more than 440 mammal species, including bears, wolves, elephants and lions. Food webs everywhere share the same basic trophic structure -- animals that eat and are in turn eaten -- but the number and types of species filling those roles differ greatly across regions.
The researchers found that food webs in the Americas today contain fewer and smaller prey animals compared with those in Africa and Asia. Predators in the Americas also tended to focus on prey with a narrower range of body sizes and activity patterns, with less overlap among them. Those differences could not be explained by current environmental factors such as climate or seasonality alone.
Instead, the team found that the severity of past extinctions played a significant role in shaping present-day food web structure. Each region suffered losses, but the Americas were hit hardest. More than three-quarters of all mammals weighing over 100 pounds were wiped out across North and South America during the last 50,000 years. South America, for instance, was once home to several species of giant deer. Their extinction left fewer prey options for large predators such as saber-toothed cats and dire wolves, essentially flattening and thinning out the lower tier of the food web. "A lot of the lower part of the food web was lost," Hsieh said.
Hsieh, who is a Presidential Postdoctoral Fellow in MSU's Ecology, Evolution, and Behavior program, noted that the differences between regions reflect these historical extinction pulses rather than present-day conditions alone.
The underlying causes of the ancient die-offs remain debated. Some researchers point to climate and environmental stress as primary drivers of the loss of mammoths and other megafauna. Others argue that the expansion of human populations out of Africa into new continents drove the extinctions. Whatever the cause, the new study confirms that the disappearance of these giants set off ripple effects that continue to shape ecological communities today.
The findings carry direct relevance for conservation. Nearly half of all mammals weighing more than 20 pounds are currently listed as vulnerable, endangered or critically endangered by the International Union for the Conservation of Nature. Understanding how past extinction events reshaped ecosystems can help scientists anticipate the long-term consequences of species losses happening now. As a next step, Beaudrot said the team plans to investigate whether the legacy of historical extinctions makes certain present-day communities more vulnerable to future disruption. "By studying the past, we can also try to understand what to expect in the future," Hsieh said.
Research Report:Historical legacies shape continental variation in contemporary mammal food webs
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