Maximizing the effectiveness of grassland management for a grasshopper sparrow (Ammodramus savannarum) metapopulation

Abstract

Changes in hayfield management associated with agricultural intensification, including earlier and more frequent harvests, have a particularly severe impact on grassland birds. Government-administered conservation incentive programs benefit grassland birds by delaying harvest dates on enrolled land to allow nesting pairs to successfully fledge at least one brood during the breeding season. In contrast, hayfields that are mowed during the breeding season support sink populations and may even function as ecological traps. We examined the effect of increasing levels of hayfield enrollment on grasshopper sparrow population viability using a spatially-explicit, stage-structured, stochastic model of a grasshopper sparrow metapopulation in an urbanizing region of New Jersey. The probability of metapopulation extinction (POE) decreased quickly with increasing proportion of enrolled hayfields. We identified a threshold at 31 to 48% enrollment after which POE decreased minimally with an increase in enrollment. POE also decreased with increasing numbers of enrolled hayfields most likely because hayfield enrollment removes a sink population from the landscape in addition to creating a source population. This effect diminished with increasing enrollment. Our results are encouraging as they demonstrate that extinction risk can be reduced without having to protect or manage all remaining grassland habitat in the landscape.