Walden Woods is a bit of an anomaly on the landscape - pitch pines dominate in pockets over exceptionally sandy soils left behind by the retreating ice at the end of the Wisconsin glaciation. Unusual environmental conditions can lead to unusual wildlife and plant species compositions, and there is perennial interest in understanding what is happening to wildlife populations.
In the late 1990s, Massachusetts began state-wide efforts to document biodiversity, and one-day bio-blitz events, dubbed "Biodiversity Days," were conducted at sites around the state. On July 4, 1998, local experts gathered in Concord for a major Biodiversity Day event where hundreds of species of plants, animals, fungi, lichen and moss, and all manner of life were cataloged. An extensive checklist was produced for the observations made on Biodiversity Day in 1998.
To celebrate E.O. Wilson's 80th birthday in 2009, the Walden Woods Project sponsored a second Biodiversity Day and brought together dozens of experts and amateur naturalists to reassess the biodiversity of Walden Woods. Over the 4th of July weekend, once again an extensive checklist was created.