Effect of habitat fragmentation on grassland-nesting birds in southwestern Missouri

• Main Author: Winter, Maiken
• Format: Dissertation
• Language: English
• Subjects: biology, ecology; biology, zoology
• ISBN: 9780599245020; 0599245026

Habitat loss and fragmentation on the breeding grounds appears to be a major cause of the apparent decline of many North American bird species. Habitat fragmentation can decrease bird populations through decreases of patch size, increase of edge habitat, and isolation of habitat fragments. These effects of habitat fragmentation have been well documented in forest-nesting birds. However, grassland-nesting birds are experiencing even greater and more consistent population declines, and the reasons for these declines are still poorly understood. Factors that can potentially influence density and nesting success of grassland-nesting birds on their breeding grounds can act at three spatial scales: within-patch scale (vegetation structure and management regime), local scale (patch size and proximity to edge habitat), and landscape scale (habitat surrounding the patch).
Between 1995 and 1997 I investigated how factors on each of these three spatial scales affected density and nesting success of grassland-nesting birds in 13 fragments of native tallgrass prairie in southwestern Missouri, focusing on two severely declining passerines, the Henslow's Sparrow (Ammodramus henslowii) and the Dickcissel (Spiza americana). The goal of this thesis was not merely to show that environmental factors at different scales affect density and nesting success of grassland birds; the simultaneous measurement of landscape at multiple scales has additionally allowed me to demonstrate an interaction between the factors measured at these different scales, which has not previously been investigated. Further, most studies on the effects of habitat fragmentation on grassland-nesting birds have based their conclusions on census data only, although census data do not reliably indicate how a species is affected by habitat fragmentation. Low litter depth (particularly the result of haying), small patch size, close proximity (<50m) to edge, and landscapes with little grasslands all decreased density and/or nesting success of my study species. The three habitat-specific scales also interacted such that the effect of vegetation structure on density and nesting success is altered by the size of the prairie fragment and the structure of the surrounding landscape. Further, the effect of fragment size on density and nesting success can differ in different landscapes.