The second ingredient is the resistant surface. Resistant surfaces have been increasingly used in landscape ecology as we move from the binary island biogeographic approach to a more nuanced approach that recognizes variation in habitat quality. To use a resistant surface, you need a map of the landscape. We used a 1999 land use coverage from the UMass Resource Mapping Lab, along with roads from the Massachusetts Highway Department.
We start by assigning a resistance value to each land cover type, to represent both the movement rate of an organism across that cover type, as well as its survival rate. Vernal pool amphibians are happiest in forests, so we assign a low resistance value of 1 to forests. These species move more slowly and face higher mortality in open fields, so we assign a higher resistance value of 10 to croplands. For really deadly cover types, such as highways, we assign a much higher value, such as 50.