By drilling into lake bottoms, researchers collect mud cores with fossil pollen that reveal the history of plants.
By Elizabeth Pennisi
Aug. 5, 2020 , 12:00 PM
Recent human activity, including agriculture, has had a greater impact on North America’s plants and animals than even the glaciers that retreated more than 10,000 years ago. Those findings, presented this week at the virtual annual meeting of the Ecological Society of America, reveal that more North American forests and grasslands have abruptly disappeared in the past 250 years than in the previous 14,000 years, likely as a result of human activity. The authors say the new work, based on hundreds of fossilized pollen samples, supports the establishment of a new epoch in geological history known as the Anthropocene, with a start date in the past 250 years.
“It’s hard to overemphasize how profound the effects of ending a glacial cycle are,” says Zak Ratajczak, an ecologist at Kansas State University, Manhattan, who was not involved with the work. “So for humans to have that kind of impact is pretty amazing.”
For more than 10 years, researchers have debated when humans started to make their mark on the planet. Some argue agriculture transformed landscapes thousands of years ago, disrupting previously stable interactions between plants and animals. Others argue the launch of large-scale mining and smelting operations—seen in glacial records going back thousands of years—means the Anthropocene predates the industrial revolution. For geologists, however, the epoch starts with a different signal: nuclear explosions and a sharp uptick in fossil fuel use in the mid–20th century.
But some skeptics suggest the ice ages have had an even greater effect on the world’s ecosystems. To test that idea, Stanford University paleoecologist M. Allison Stegner turned to Neotoma, a decade-old fossil database that combines records from thousands of sites around the world. Her question: When—and how abruptly—did ecosystems change in North America over the past 14,000 years? Climate-altering glaciers, which started their retreat roughly 20,000 years ago, pulsed back during a cold period called the Younger Dryas, from about 12,800 until 11,700 years ago. After that, North America abruptly warmed, marking the beginning of our current epoch, the Holocene.
To answer her question, Stegner and colleagues looked at how vegetation shifted in locations across North America, using fossilized pollen to determine which species of plants were present at any given time. From 1900 records of mud cores drilled from lake bottom, Stegner found 400 with enough fossil pollen—and accurate enough dating—to analyze.
She and her colleagues then tracked how the mix of pollen in each core changed over time, paying close attention to abrupt shifts. Such shifts can mark the transformation of an entire ecosystem, for example, when a grassland becomes a forest or when a spruce forest changes into an oak forest. Looking at 250-year intervals, the researchers ran two types of statistical analyses that separately picked out temporary and long-term disruptions. “Allison used some very creative and rigorous methods,” says Jennifer McGuire, a paleoecologist at the Georgia Institute of Technology who was not involved with the work.
When the last ice age ended, forests and grasslands regrew across North America, creating a landscape that remained stable for thousands of years. But humans have changed all that, Stegner reports this week. Her team found just 10 abrupt changes per 250 years for every 100 sites from 11,000 years ago to about 1700 C.E. But that number doubled, to 20 abrupt changes per 100 sites, in the 250-year interval between 1700 and 1950. When the ice sheets of the Younger Dryas retreated, starting about 12,000 years ago, that number was 15. This suggests, Stegner says, that human activity starting 250 years ago—from land use change to pollution and perhaps even climate change—had more of an impact on ecosystems than the last glaciers.
The researchers also analyzed whether some regions have changed more swiftly than others. Over the past 250 years the U.S. Midwest, Southwest, and Southeast have undergone massive shifts from forest, grassland, and desert ecosystems to agriculture and tree plantations, she says. In contrast, Alaska, northern Canada, and parts of the Pacific Northwest underwent more changes as the glaciers melted than in the past 250 years.
“We already know plenty about climate change,” says Kai Zhu, an ecologist at the University of California, Santa Cruz. “This study adds land use change, [which] might accelerate climate change in altering plants at a continental scale.”
That’s worrisome, McGuire adds, because plants are the foundation of an ecosystem. “This rapid turnover is a harbinger of the extinction risk and the overall ecosystem disruption that is impending,” she says. At another meeting session, she and student Yue Wang reported “very similar trends” after using pollen to examine how forests, tundra, deserts, and other biomes have bounced back from disruptions through time. Combined, the new work “eliminates any doubt” that humans have set off a new geologic epoch, Stegner says.
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