Urbanization and species-energy relationships

Abstract

Species-energy relationships (SER) have been used scarcely to predict responses of biodiversity to changes in land use. In this work, two hypothesis of the SER were tested on bird communities along urbanization gradients: the increasing population size (IPS) and the niche position (NP) hypotheses. The IPS hypothesis states that sites with more primary productivity have more individuals, decreasing the probability of extinction and increasing species richness. The HPN predicts that sites with greater primary productivity and environmental heterogeneity allow the availability of a greater variety of resources and the presence of specialist species, thus increasing species richness. The hypothesis of habitat heterogeneity was evaluated by which more complex habitats provide more variety of resources and greater species richness. Birds were counted along three urban-rural gradients in the Pampean region of Argentina during the breeding season. The productive energy was measured from the NDVI and the environmental heterogeneity through the Shannon-Wiener diversity index, calculated with the percentage coverage of different substrates. Bird richness and habitat diversity were higher at intermediate levels of urbanization, while primary productivity had a negative relationship with urbanization. Although there was a positive relationship between bird richness and abundance, there was no positive relationship between bird abundance and primary productivity. Bird richness showed a positive relationship with habitat diversity and primary productivity; although path analysis revealed that the habitat diversity increased bird richness through an increase of bird abundance. The NP hypothesis was the most consistent to predict the bird richness along urban-rural gradients.