Urban influence on diversity of avifauna in the Edwards Plateau of Texas: effect of property sizes on rural landscape structure

The urban Influence on diversity of avifauna in the Edwards Plateau ecoregion and surrounding area was studied using spatial analysis. Indices and metrics of urban influence, ownership property sizes, landscape structure, and avian diversity were calculated for 31 North American Breeding Bird Survey (BBS) transects, 12 located within the Edwards Plateau ecoregion and 18 in contiguous ecoregions. Spatial correlations were calculated between each pair of these indices. The spatial analysis identified an emergent property at the landscape level: A ??????threshold of habitat fragmentation?????? at an ownership property size of 500 acres, which is reached when urban influence increases to an intermediate level. Highly significant spatial correlations among variables showed that property sizes lower than 500 acres produce habitat fragmentation represented by a decrease in mean patch size (MN) and proximity among habitat patches (Index PROX). Consequently, avian ????-diversity (richness) decreases because both MN and Index PROX are landscape metrics related to availability of suitable habitat for avian populations. The spatial analysis also made possible the prioritization of ecological subregions of the Edwards Plateau for conservation or restoration with respect to the threshold of habitat fragmentation and avian ???? and ????-diversity. Balcones Canyon Lands showed a high percentage of land covered by farms smaller than 500 acres (64%), an ownership property average size above the threshold of fragmentation (1440 acres) and the highest avian ????- diversity; so, management policies should focus on habitat conservation. In contrast, Lampasas Cut Plains showed the highest percentage of land covered by farms smaller than 500 acres (71%), and ownership property average size was very close to the threshold of fragmentation (625 acres); there, urban bird species are dominant and avian ????-diversity is low because of the loss of native bird species. Management in this ecoregion should focus on habitat restoration. Finally, the Live Oak-Mesquite Savannah subregion showed the highest average ownership property size (7305 acres), and the highest values of patch richness and ????-diversity. Management in this ecoregion should focus on conservation of land mosaic diversity to assure native avian species turnover.