GRASSLAND BIRD RESPONSE TO RECENT LOSS AND DEGRADATION OF NATIVE PRAIRIE IN CENTRAL AND WESTERN SOUTH DAKOTA

摘要

Central and western South Dakota contain some of the largest intact blocks of mixed-grass prairie in North America, but conversion to row crops has accelerated sharply in recent years and existing grasslands have been further degraded by introduced plant species and the presence of anthropogenic woodlands. Our objective was to simultaneously investigate the effects of relatively recent habitat loss and degradation on grassland bird communities at the local, patch, and landscape scales. Specifically, to determine how 1) local- (vegetation structure, introduced plant coverage), 2) patch- (patch size, shape, and tree edge) and 3) landscape- (proportion of grassland surrounding surveyed sites) level habitat characteristics affect occurrence and density of six grassland bird species. We surveyed birds on 288 native grassland sites in 29 counties throughout central and western South Dakota. We calculated multiple metrics of habitat degradation and loss at the local, patch, and landscape scales and used logistic and zero-inflated negative binomial regression to model occurrence and density of six grassland bird species. All six species responded negatively to habitat degradation resulting from increased introduced plant coverage and/or percent wooded patch edge. Chestnut-collared Longspurs, Lark Buntings and Western Meadowlarks were negatively affected by degradation of native grasslands related to the increase of introduced plant species. Chestnut-collared Longspurs, Grasshopper Sparrows, Savannah Sparrows, and Western Meadowlarks were impacted negatively by increasing amounts of wooded edge surrounding a grassland patch. Contrary to studies conducted in more fragmented habitats, no species was associated with area of grassland patch, but three species were affected negatively by measures of loss of grassland habitat in the surrounding landscape. Our results indicate the need for conservation and management of grasslands at multiple scales. Habitat degradation resulting from invasion by exotic species and the inclusion of woody vegetation appear to be as detrimental to some grassland birds as habitat loss. Our results suggest there is an urgency to preserve large, intact native grasslands before habitat loss reaches levels that cause grassland bird species to shift from landscape scale associations to patch area measures of habitat, as significant population declines would likely accompany the habitat loss leading to such a shift.