Henry David Thoreau famously catalogued the plants around Walden Pond more than 150 years ago, and the information he gathered then is helping to illustrate the effects of invasive species and global warming on the area today.

According to a paper published January 26 in the journal PLoS ONE, climate change has given invasive and nonnative species a leg up in the Walden Pond area, and native species are the losers. (A nonnative species is considered invasive if it has the potential to disperse widely and rapidly, especially within habitats that are minimally maintained by humans. Some nonnative species do not spread quickly enough to qualify as invasive.)

Researchers compared Thoreau's information with data on temperature and plant populations from this century as well as full information on plant phenology (flowering time, germination, migration and other seasonal activities). They found that the average temperature in Concord, Mass., has increased 2.4 degrees Celsius since Thoreau's time, and that some nonnative plants have adapted by flowering as much as three weeks earlier than they used to. Some native species, by contrast, were less flexible and have not been able to adjust their flowering times. As a result, their populations have dropped.

"These results demonstrate for the first time that climate change likely plays a direct role in promoting nonnative species success," senior author Charles C. Davis, an assistant professor of organismic and evolutionary biology at Harvard University, said in a prepared statement. "Our research suggests quite decisively that nonnative and invasive species have been the climate change winners. Climate change will lead to an as yet unknown shuffling of species, and it appears that invasive species will become more dominant."

The team didn't just count the plants to come up with this observation. They also examined "habit, plant height at maturity, leaf mass per area, flower diameter, pollination syndrome and seed weight" to come up with a full picture of the health of the species in the area.

This isn't the first time that Davis and his colleagues have studied Walden's plants. Two years ago, they found that 27 percent of the plant species Thoreau recorded from 1851 to 1858 are now locally extinct. Another 36 percent "are so sparse that extinction may be imminent," they wrote in a previous paper. At that time, species like lilies, orchids, violets, roses and dogwoods were shown to have seen the biggest losses.

Image: Cover to Walden by Henry David Thoreau, via Wikipedia