Impacts of habitat change and protected areas on alpha and beta diversity of Mexican birds

Aims To investigate how habitat change and different levels of protection interact to determine variation in the (alpha and beta) diversity of bird communities in three bioclimatic zones, considering the impacts of non-native species, and the contribution of these effects to global (gamma) diversity. Location Sierra Gorda Biosphere Reserve, central Mexico. Methods We carried out bird surveys in a number of locations which varied according to their underlying vegetation type, their level of protection and the degree to which they had undergone modification by humans. We conducted a range of analyses to determine the impacts of protection and modification on the richness and mean global population trends of the species found in each location. We compared community composition to assess the homogenization effect of habitat modification. Results Human-modified environments in each of three bioclimatic zones held significantly greater numbers of species (alpha diversity) than unmodified habitats. Human-mediated changes to local bird communities altered patterns of beta diversity in opposite directions; communities across bioclimatic zones were more similar to one another in modified areas than in unmodified areas but, on a local scale, modification of vegetation increased community dissimilarity. The changes are likely to contribute to a decline in global (gamma) diversity, given that globally vulnerable (declining) birds were mostly associated with unmodified, strictly protected areas. Main conclusions We highlight that assessment of human impacts and conservation need depends on the metric of biodiversity used and scale considered: small-scale habitat change increased local and regional avian diversity but strictly protected areas are still required to protect globally vulnerable species.