Plant Species Invade Walden Pond

Climate change has converted Henry David Thoreau’s beloved Walden Pond into a battle site between native and non-native plant species, and it seems as though the non-natives—particularly invasives—are emerging victorious, according to a study published by Harvard researchers last month.

The implications of the findings may have a "devastating" impact on biodiversity in Concord, according to Assistant Professor of Organismic and Evolutionary Biology Charles C. Davis, one of the study’s lead researchers.

The scientists discovered that over the past 100 years, invasive plant species have adjusted the times at which they flower to be about 11 days earlier than the flowering times of native species.

The adaptation has resulted in an increase of invasive species, which tend to disrupt ecosystems and require expensive management when they interfere with agriculture.

Davis said he believes that the knowledge offered by the study will help scientists to keep a cautious eye on non-native species that may eventually become invasive.

The current study built upon work performed over the past decade by Boston University professor Richard B. Primack ’72 and Abraham J. Miller-Rushing of the USA National Phenology Network and the Wildlife Society. The pair hunted for botanical records from Concord’s past to track changes in plant traits over the last 150 years—and fortunately, eastern Massachusetts proved to be “a treasure trove of historical records,” Primack said.

Primack and Miller-Rushing proceeded to record observations of about 500 species of plants at Walden Pond over a period of six years. Combining old and new data sets, the duo concluded that seasonal plant traits were shifting as climate change pushed springtime earlier.

"What really piqued my interest was that it wasn’t across the board—some species were shifting and others weren’t," Davis said. "For a person that thinks a lot about life on earth, this immediately struck me as interesting. I wondered, 'Is there a pattern?'"

To find the answer, Davis and his colleagues began to paint a unified evolutionary picture of plant species inhabiting Walden Pond.

The work proved challenging because many of the plants described by Thoreau, a Harvard alum, have been renamed or regrouped over the past century, according to graduate student Brad R. Ruhfel. But when Davis suggested cutting Thoreau’s data, Primack was in firm opposition to the idea.

"I said 'Don’t even think about that,'" Primack recalled. "[The study] connects a literary and philosophical source to one of the most important scientific issues of our time."

Graduate student Charles G. Willis, one of the study’s researchers, emphasized that the findings in Concord are just “one data point in the whole scheme of things,” but that to expand the research to other regions may prove challenging.

"There weren’t Thoreaus everywhere in the world, so we have to look around and find other data sets that can tell us the same story," Willis said.

Interestingly, the researchers’ botanical work has become associated with hot-button issues like climate change and historical figures like Thoreau, according to Ruhfel.

"The science on its own and the results are really stunning, but to have the connection with Thoreau amplifies the excitement all the more given his legacy as a conservationist," Davis said. "This is a very effective vehicle for delivering this message in a timely way."

—Staff writer Julie R. Barzilay can be reached at jbarzilay13@college.harvard.edu.

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