Bird composition as an ecological indicator of forest disturbance levels
Ecological indicators are needed to evaluate natural biodiversity and to estimate environmental change. I used bird community composition as an environmental indicator to evaluate forest disturbance (at the community level) in areas of Western Mexico. Surveyed bird and vegetation composition data at 79 sites within regions of the Sierra Madre del Sur and Transvolcanic Belt Provinces of Central Mexico were used. Bird species were classified into four habitat assemblages (groupings) according to their habitat preference (the habitat characteristic for their presence). Birds found primarily in late successional forests are sensitive to forest disturbance and a predominant presence of these species would indicate an undisturbed mature forest habitat. Birds tolerant to forest disturbance are characteristic of early-successional, clearcut, brushy and fragmented landscapes. Presence of predominately disturbance tolerant species would suggest a disturbed habitat. The number of species in each habitat assemblage were counted for each site and used to calculate bird community index values. The bird community index provides a value to assess the habitat disturbance on bird communities. Bird species were also classified into nesting, foraging and feeding guilds. In order to evaluate the relationship between individual species response and responses of other species of that same functional group, correlation coefficients between the presence or absence of each individual species and the number of co-occurring species from the same functional group or habitat assemblage were calculated. Binary logistic regressions were used to predict the probability of occurrence (for the dominant 4 co-occurring birds within each functional group and habitat assemblage) relative to percent canopy cover. Correlations between functional group species richness and average percent canopy cover and average stand basal area were calculated. Habitat assemblage species richness and total species richness for each site were also correlated to those same values. Bird communities on the majority of the surveyed areas were dominated by disturbance sensitive species. Only one site was dominated by disturbance tolerant species. Habitat assemblage and guild members showed internal cohesion (consistent responses to different disturbance levels) but did not show strong correlations to the canopy cover and basal area. The probability of occurrence versus canopy cover for the four top birds in each guild and habitat did not show consistent trends, indicating different responses toward vegetation metrics within the grouped species. The bird community index also did not show strong correlations to the percent cover and basal area, suggesting that the index should not be used independently in assessing bird community and forest condition. Because data on forestry and avian communities in Mexico are not readily available, bird community surveys could still serve as a good tool for assessing forest condition at the community level in this region.