UNM researcher exploring influence of ancient humans on extinction and body size patterns of mammals finds size-selective extinction is a hallmark of human activities

Researchers including a UNM biology professor have demonstrated that mammal biodiversity loss, a major conservation concern today, is part of a long-term trend lasting at least 125,000 years. As archaic humans, Neanderthals and other hominin species migrated out of Africa, a wave of size biased extinction in mammals followed.

This occurred on all continents and intensified over time. The study, published in the prestigious journal Science is the first to quantitatively show that human effects on mammal body size predates their migration out of Africa and that size-selective extinction is a hallmark of human activities including hunting and habitat modification.

Dr. Felisa Smith of the University of New Mexico led the research, funded by a National Science Foundation grant, along with colleagues from University of California San Diego, University of Nebraska-Lincoln and Stanford University.

To document what happened to mammals as early humans left Africa, researchers compiled extensive data on climate and on mammal body size, extinction status and geographic location over the last 125,000 years, and used conservation status of modern mammals to model diversity and body size distributions for 200 years in the future. The researchers investigated the role of body size and diet on the likelihood of extinction. These data were evaluated in light of climate change and human migration patterns over the same time frame.

The study showed that body size downgrading – the loss of the largest species on each continent over time – is a hallmark of human activity, both in the past and present. If this trend continues into the future, the largest terrestrial mammal in 200 years will be the domestic cow.

“One of the most surprising finds was that 125 thousand years ago the average body size of mammals on Africa was already 50 percent smaller than on other continents, Smith said.

This was surprising to the researchers because Africa is a larger continent and typically larger land masses house larger mammals.

“We suspect that this means that archaic humans and other hominins had already influenced mammal diversity and body size in the late Pleistocene,” Smith said. Over time, as humans migrated around the globe, extinctions of the largest mammals followed. “This was fascinating because it only occurred after the arrival of early humans,” added Rosemary Elliott Smith, a co-author.

By comparing extinction events with the entire record of mammal turnover over the past 65 million years, the researchers demonstrated that body size and diet did not influence extinction risk for mammals for most of their evolutionary history.

“You just don’t see extreme size selectivity for mammals until the late Pleistocene” said Kate Lyons, a co-author on the study. “Past climate changes just don’t result in size-selective extinction.” This has changed with the spread of humans. “We’re seeing a shift in selectivity that suggests habitat modification and climate change are becoming more and more important,” added Jon Payne, a co-author.

These results highlight a startling point. The role of ancient and modern humans has been vastly underappreciated.  “If this trend continues and all the currently threatened animal are lost, energy flow and taxonomic composition will be entirely restructured. In fact, mammalian body size around the globe will revert to what the world looked like 40 million years ago.”