Least-cost path length versus accumulated-cost as connectivity measures

Least-cost modelling has become a popular method for measuring connectivity. By representing the landscape as a cost-surface, least-cost paths can be calculated that represent the route of maximum efficiency between two locations as a function of the distance travelled and the costs traversed. Both the length and the accumulated-cost of a least-cost path have been used as measures of connectivity between pairs of locations. However, we are concerned that in some situations the length of a least-cost path may provide a misleading measure of connectivity as it only accounts for the distance travelled while ignoring the costs traversed, and results in a measure that may be little better than Euclidean distance. Through simulations using fractal landscapes we demonstrate that least-cost path length is often highly correlated with Euclidean distance. This indicates that least-cost path length provides a poor measure of connectivity in many situations, as it does not capture sufficient information about the ecological costs to movement represented by the cost-surface. We recommend that in most situations the accumulated-cost of a least-cost path provides a more appropriate measure of connectivity between locations as it accounts for both the distance travelled and costs traversed, and that the generation of vector least-cost paths should be reserved for visualisation purposes.