Influences of horizontal and vertical aspects of land cover and their interactions with regional factors on patterns of avian species-richness

We examined how both horizontal and vertical aspects of land-cover diversity influence patterns of avian species-richness across North America. Using count data from Breeding Bird Survey routes within the conterminous USA and land-cover data from the National Land Cover Data-set, we analyzed relationships between species-richness estimates, vegetative strata, landscape diversity and elevation and geographic position using both linear-regression models and a classification and regression tree. We found that latitude, the diversity of land-cover classes present, and the proportion of the landscape containing cover-classes representing 3 vegetative strata had the strongest influence on species richness. This illustrates that, while broad-scale biodiversity trends are strongly influenced by dominant regional factors, they are also sensitive to the structure of the intermediate-level landscape. Thus, factors at multiple scales must be considered when modeling spatial patterns of biodiversity such as avian species-richness.