Assessing The Expansion of Beaver in Northeastern Coastal Watersheds Using High-Resolution Remotely-Sensed Data and the Implications for Biogeochemical Fluxes

Abstract:

Humans have considerably changed the landscape, leading to alterations to hydrological and biogeochemical regimes. However, while humans have had a substantial impact on watershed structure and function, they are not the only organism causing changes to landscapes. North American beaver (Castor canadensis) are also ecosystem engineers and were once ubiquitous in North America before being hunted to near extinction by the beginning of the 20^th century. Recently in the northeastern US, as in many other parts of the country, North American beaver populations are making a remarkable comeback. In particular, Massachusetts (MA) has seen an expansion of the beaver population stemming from conservation efforts and policy changes beginning in the late 1990s and continuing to present day. Although the evidence of their expansion is clear, comprehensive estimates of beaver populations in MA at watershed scales remain unknown owing to changes in beaver harvest reporting and the feasibility of performing colony density surveys. We mapped the areal expansion of beaver ponds in two watersheds in northeastern MA using land cover classification techniques implemented in Google Earth Engine. This classification uses publicly available high-resolution aerial imagery from five time periods starting before the beaver expansion in the mid-1990s and ending in the mid-2010s. Early results using data from a single year indicate a classification accuracy of nearly 70% for a model using a supervised Random Forest classification on segments generated through Simple Non-Iterative Clustering. With these maps, we gain a better understanding of when and how beaver have propagated throughout these study watersheds. This helps to explain how alterations to the landscape by both humans and beaver interact at the watershed scale to regulate biogeochemical processes and ultimately implications for river network-scale nutrient exports in a constantly changing environment.