Recent human activity, including agriculture, has had a greater impact on North American plants and animals than even glaciers that retreated more than 10,000 years ago. These findings, presented this week at the Ecological Society of America’s annual virtual meeting, reveal that more North American forests and grasslands have suddenly disappeared over the past 250 years than the previous 14,000 years, possibly due to 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 date beginning in the last 250 years.
“It is difficult to over-emphasize how profound the effects of the end of a glacial cycle are,” says Zak Ratajczak, an ecologist at Kansas State University, Manhattan, who was not involved in the work. “So for humans, having that kind of impact is pretty amazing.”
For more than 10 years, researchers have debated when humans started making their mark on the planet. Some argue that agriculture transformed landscapes thousands of years ago, disrupting previously stable interactions between plants and animals. Others argue that the launch of large-scale mining and smelting operations – seen in glacial records dating back thousands of years – means the Anthropocene precedes the industrial revolution. For geologists, however, the era begins with a different signal: nuclear explosions and a sharp increase in the use of fossil fuels in the mid-20th century.
But some skeptics suggest that ice ages have had an even greater effect on the world’s ecosystems. To test this idea, Stanford University paleoecologist M. Allison Stegner turned to Neotoma, a ten-year-old fossil database that combines records from thousands of sites around the world. His question: When, and how quickly, have ecosystems changed in North America over the past 14,000 years? Climate-altering glaciers, which began their retreat about 20,000 years ago, reverted back during a cold period called the Younger Dryas, from about 12,800 to 11,700 years ago. After that, North America suddenly warmed up, marking the beginning of our current age, the Holocene.
To answer his question, Stegner and colleagues looked at how vegetation moved to places in North America, using fossilized pollen to determine which plant species were present at any given time. By 1900 records of mud cores drilled from the lake bottom, Stegner found 400 with enough fossil pollen – and sufficiently accurate dating – to analyze.
She and her colleagues then monitored how the pollen mix in each nucleus changed over time, paying close attention to sudden changes. Such changes can mark the transformation of an entire ecosystem, for example, when a grassland becomes a forest or when a spruce forest transforms into an oak forest. Looking at 250-year intervals, the researchers performed two types of statistical analyzes that separately selected temporary and long-term outages. “Allison used very creative and rigorous methods,” says Jennifer McGuire, a paleoecologist at the Georgia Institute of Technology who was not involved in the work.
As the last ice age ended, forests and grasslands regrown across North America, creating a landscape that has remained stable for thousands of years. But humans have changed all that, Stegner reports this week. His team found only 10 abrupt changes every 250 years for every 100 sites from 11,000 years ago to around 1700 CE, but that number doubled, to 20 abrupt changes per 100 sites, in the 250-year interval between 1700 and 1950. When the Younger Dryas ice sheets retreated, starting about 12,000 years ago, that number was 15. This suggests, says Stegner, that human activity began 250 years ago – from land use change to pollution and perhaps even climate change – has had a greater impact on the ecosystems of the last glaciers.
The researchers also analyzed whether some regions changed faster than others. Over the past 250 years, the Midwest, Southwest, and Southeast of the United States have undergone tremendous changes from forest, grassland and desert ecosystems to agriculture and tree planting, he says. In contrast, Alaska, northern Canada, and parts of the Pacific Northwest have undergone more changes with melting glaciers than in the past 250 years.
“We already know a lot about climate change,” says Kai Zhu, an ecologist at the University of California, Santa Cruz. “This study adds changes in land use, [which] could accelerate climate change by altering plants on a continental scale “.
This is worrying, adds McGuire, because plants are the foundation of an ecosystem. “This rapid turnover is a harbinger of the risk of extinction and the general disruption of the ecosystem that is imminent,” he says. In 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 recovered from disruptions over time. Together, the new work “removes all doubt” that humans have ushered in a new geological epoch, says Stegner.