Humans have altered North America's ecosystems more than melting glaciers - Science Magazine

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.

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.

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.

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.

When the last ice age ended, forests and grasslands regrew across North America, creating a landscape that remained stable for thousands of years.

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.

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.

Over the past 250 years the U.S.

In contrast, Alaska, northern Canada, and parts of the Pacific Northwest underwent more changes as the glaciers melted than in the past 250 years.

Combined, the new work “eliminates any doubt” that humans have set off a new geologic epoch, Stegner says.