Effects of cropland encroachment on prairie pothole wetlands: numbers, density, size, shape, and structural connectivity

Abstract

Agricultural expansion is the greatest source of wetland loss in the Prairie Pothole Region (PPR) of North and South Dakota, a critical waterfowl production area in North America. It is unknown how wetland losses from grassland conversion may alter structural connectivity in the prairie pothole network, however. We examined how agricultural expansion over the period 2001–2011 altered the number, size, shape, and structural connectivity of PPR wetlands. We hypothesized that the loss of wetlands or wetland area would decrease structural connectivity on the landscape. We analyzed a published raster database that quantified 2001–2011 agricultural conversion of wetlands in the Dakotas. A suite of structural connectivity metrics was computed using the igraph R package. Wetland area decreased by 25% within the study area, density decreased by 16%, and average size decreased from 2.41 to 2.16 ha with no increase in perimeter:area ratios, thus indicating changes more from the splitting of larger wetlands (accounting for 23% of area lost) and “nibbling” at patch area (38%) than from complete wetland elimination (39%). Despite loss of wetlands and wetland area to cropland, however, the network did not display constrained structural connectivity. Structural connectivity has not been significantly affected by wetland losses because of the large number of remaining wetlands, but wetlands will continue to be lost with ongoing grassland conversion and climate shifts. It is unknown where the tipping point of wetland losses lies in the PPR that will incur ecological costs.