Patch Area Cannot Predict Species Richness of Grassland Birds in Colorado’s Front Range

Abstract

Birds breeding in grassland have declined steeply over the last 50 years, and green-space systems in Colorado’s urbanizing Front Range have not maintained all grassland bird species the area originally supported. Patch area affects the species richness of urban green spaces, and researchers have suggested that protecting or enlarging green spaces should be effective ways to maximize richness and mitigate species loss. In the Front Range, protection of urban green space is expensive, conservation budgets are limited, and tools are needed to guide strategic protection decisions. Front Range planners use patch area as a criterion to prioritize grassland conservation, but the explanatory and predictive powers of patch area have not been comprehensively assessed. Using eBird community science data, I found that log-transformed grassland patch area was positively associated with the species richness of grassland birds and explained a large portion of its variance. However, 95% simultaneous prediction intervals for species richness were wide, and those of the smallest and largest patches examined overlapped. Thus the model cannot precisely predict a number of species, and it should not be used to quantitatively evaluate the expected return on investment from financial allocations to protect or enlarge grassland patches. Nonetheless, the model’s explanatory power supports the use of grassland patch area as a general principle guiding conservation of grassland birds. Planners should consider it among a suite of other habitat characteristics and prioritize large, regularly shaped grassland patches situated close to other grassland patches and with limited nearby forest cover and urban development.