Walden and a Warming World

Henry David Thoreau ventured into the woods near Concord, Mass., to “see if I could not learn what it had to teach me.” More than 150 years later, a Boston University scientist finds that Thoreau’s meticulous environmental observations continue to instruct.

Richard Primack, a College of Arts and Sciences professor of biology, has been using Thoreau’s notations of plant flowering cycles and bird migration patterns as a basis for research into the local effects of global climate change. He has been working with Charles Davis, an evolutionary biologist at Harvard, to investigate how climate-sensitive traits that are shared by certain plant species, such as flowering times, might help predict the patterns of species loss and resilience in the face of climate change. Their findings appear in this week’s Proceedings of the National Academy of Sciences.

Their research would not have been possible without the long historical record of species abundance and behavior in and around Concord, a century and a half of methodical record-keeping that began with Thoreau and was continued by generations of amateur naturalists who kept notes on several of the same species over the years. Beginning in 2004, those journals, along with musty herbarium specimens, old farmers’ diaries, and sepia-toned photographs, provided Primack and Abe Miller-Rushing (GRS’07), a postdoc researcher at the University of Maryland, with historical climate data they then compared with present-day observations in the field.

In articles published in a 2006 issue of the American Journal of Botany and a 2008 issue of Ecology, they reported that, on average, “spring events” such as bird migrations and plant flowerings are happening a week earlier than they did in Thoreau’s time.

In the study published this week, Primack, Davis, and a team of researchers tracked the relative abundance of the nearly 500 species observed by Thoreau. They noted that the mean annual temperature in Concord has risen by about four degrees Fahrenheit over the past century and that since Thoreau’s time “a striking number of species” (about 27 percent) have become extinct locally; 36 percent more have dwindled so much that they may soon die off as well.

These losses weren’t random. They were “highly correlated,” the researchers report, with the climate sensitivity of a plant’s flowering time, an evolutionary trait that distinguishes large groups of species. In general, they found that species whose flowering times did not shift in response to climate suffered the sharpest declines in population — possibly because they are now flowering in a time of fewer pollinators or more abundant seed predators. These species include varieties of orchids, mints, dogwoods, lilies, and iris. Meanwhile, species whose flowering times have adjusted to rising temperatures are faring better.

The findings are important for models that attempt to predict how species will react to climate change, both in terms of population and of range shifts.

The results also suggest that rising temperatures could accelerate declines in the planet’s biodiversity, according to the researchers, because “groups of closely related species are being selectively trimmed from the Tree of Life, rather than individual species being randomly pruned from its tips.”

Chris Berdik can be reached at cberdik@bu.edu.