New Research Finds the ‘Ancient Past’ Matters as Much as Present Climate and Human Impacts in Explaining Global Biodiversity Patterns

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AMHERST, Mass. – While scientists have long sought to understand the factors that structure global biodiversity patterns, much of their research has focused on present-day climate, such as patterns of temperature or rainfall, or recent human impacts well-known to influence biodiversity, like urbanization and the destruction of wild lands.

What many of these studies overlook, however, is that present-day biodiversity patterns are the outcome of thousands of years of changes in Earth’s climate and, more recently, prehistoric human activity, say authors of a new study published today in Proceedings of the National Academy of Sciences. Their study reports, for the first time, that current patterns of mammal diversity across the world’s tropical and subtropical regions are structured by both past and present climate and human impacts. The findings imply that the present-day distribution of life’s diversity on Earth is not necessarily driven by recent climate or human impacts. Instead, the study shows that legacies of the ancient past continue to structure patterns of life on Earth today.

“We have long assumed that present-day biodiversity patterns reflect present-day factors, such as contemporary climate and human activity,” says study lead author John Rowan, Darwin fellow in organismic and evolutionary biology at the University of Massachusetts Amherst. “Our study shows that climate changes and human impacts that occurred over the last 100,000 years continue to shape tropical and subtropical mammal biodiversity today. In fact, we found that climatic and anthropogenic legacies of the ancient past can often override their present-day counterparts in terms of importance.”

Rowan and his colleagues analyzed a database of 515 mammal communities across the globe, which took several years to compile. For each mammal community they collected detailed data on the species present, their ecological traits (body size, diet, etc.), and their phylogenetic, or evolutionary relationships to one another. They then used this information to measure the ecological and phylogenetic structure of each community, and asked whether patterns of community structure were best explained by present-day climate (current temperature and rainfall), Quaternary paleoclimate changes (changes in temperature and rainfall from ~22,000 years ago to the present, and from ~6,000 years ago to the present), recent human activity (land-use transformation since the Industrial Revolution), or prehistoric human activity (human-driven mammal extinctions that occurred over the last ~100,000 years).

They found that across the world’s tropical and subtropical regions, past and present climate and human impacts co-determine mammal community structure, but that the specific effects vary by region. Changes in climate during the ‘Ice Ages’ have caused some species to shift their ranges over thousands of years as they tracked their preferred habitats, whereas prehistoric humans caused extinctions of many species as they spread out of Africa across the world during the last 100,000 years.

“In South and Central America, we found a very strong signal that mammals in this region have been shaped by prehistoric human-driven extinctions,” Rowan explains. “When humans arrived in the Americas in the last 20,000 years or so, we knew they decimated the region’s mammal faunas. What we didn’t know was that the effects of this event continue to linger in the surviving communities today.” Conversely, Africa was lightly impacted by these extinctions and the region’s present-day communities are mainly shaped by current and paleo- climates. Southeast Asia and Madagascar also have their own suite of past and present climatic and human factors that shape them.

The authors say this is one of the most important finding of their study: there is no one-size-fits-all explanation for what structures mammal biodiversity across the world. Each of the world’s major regions has a unique ecological and evolutionary history, and these histories continue to strongly influence the distribution and diversity of mammalian life on Earth. Both of these factors – ancient climate change and human impacts – are important for understanding why and how species became distributed the way we see them today.

Joining UMass Amherst’s Rowan as co-authors were: Lydia Beaudrot, assistant professor of biosciences at Rice University; Janet Franklin, distinguished professor of botany and plant science at the University of California, Riverside; Kaye Reed, president’s professor and director of the School of Human Evolution and Social Change at Arizona State University; Irene Smail, doctoral candidate in evolutionary anthropology at Arizona State University; and from UMass Amherst Andrew Zamora, doctoral candidate in anthropology, and Jason Kamilar, associate professor of anthropology.

Kamilar hopes the team’s findings will help guide future research. “As scientists continue to understand global patterns of biodiversity, we suggest they incorporate past climate and human impact factors into their studies,” he says. “This will result in a more holistic understanding of what drives biodiversity, and how it may respond to ongoing and future human-caused changes in the 21st century.”

The full study, “Geographically divergent evolutionary and ecological legacies shape mammal biodiversity in the global tropics and subtropics,” is available online at PNAS.org.