THOREAU'S CONCORD AS LIVING LABORATORY TO INVESTIGATE THE ECOLOGICAL IMPACTS OF CLIMATE CHANGED

Ecologists are increasingly combining historical observations of naturalists with modern observations to detect the ecological effects of climate change. These effects include shifting phenology, altered abundance of species, and ecological mismatches. This use of historical observations raises the question: How do we know that historical data are appropriate to use and accurate enough to answer current ecological questions? Here we address this question for environmental philosopher Henry David Thoreau, author of Walden, and use it as an example for how researchers can assess other historical natural history data sets. We and others have used Thoreau’s observations from around Walden Pond and elsewhere in Concord, Massachusetts, to assess shifts in phenology, abundance, and ecological interactions in plants and birds driven by climate change and other factors. But why should we trust his observations and later Concord naturalists? He has been praised for his natural history skills but has also been criticized as an unreliable observer. We evaluated Thoreau’s observations using a 3-step framework: we assessed the 1) rigor, 2)accuracy, and 3) appropriateness of his observations to investigate changes in plants and animals over time. We found that he kept good notes, had good natural history knowledge, and described his methods relatively well. The sequence of his spring phenological observations (flowering, leafing out, and bird arrival) were highly correlated with later observers and modern observers of these events. The regularity of his observations, number of species, and number of years made his observations effective for analyzing change over time and changes in relation to a warming climate. Moreover, the findings reached by analyzing his observations have been repeated in analyses of independent data sets in other locations. We conclude (unsurprisingly) that Thoreau was an accurate observer of nature and a reliable scientist. More importantly, this simple 3-step approach could be used to assess the accuracy of other naturalists. We describe how the approach has been applied to other historical data sets, such as those of eminent historical ecologists, Joseph Grinnell and Robert Whittaker, as well as those of lesser-known amateur naturalists and museum collections. These historical sets of observations are proving to be a powerful method for investigating the effects of climate change on natural systems, and are being used to develop new lines of ecological research.