Tree microhabitats as indicators of bird and bat communities in Mediterranean forests

Tree microhabitats (e.g., canopy deadwood, cavities, and loose bark) may play an important role in forest biodiversity conservation. Indeed, many species depend on tree microhabitats during their life-cycles for food, shelter, and breeding habitat. Although recent studies have developed a set of definitions and descriptions for tree microhabitats, the relationships between tree microhabitat inventory data and biodiversity remain poorly understood. In this study, we identified relationships among tree microhabitat variables (i.e., the density and diversity of microhabitat) and bird and bat metrics using data from 59 plots in 3 Mediterranean forest ecosystems in France. In each plot, 9 types of tree microhabitats were inventoried: canopy deadwood; woodpecker cavities; non-woodpecker cavities (divided into lower, medium, and upper cavities); Cerambyx cavities; loose bark or cracks; conks of fungi; and ivy. We also assessed structural characteristics (e.g., basal area, stand height), the time since last cutting (i.e., the number of years since the stand was last cut), the number of forest habitats, and the distance to the nearest road. We performed bird and bat inventories in the same plots and we used abundances and a number of bird and bat community indices (e.g., species richness, mean forest specialization). We found that variations in tree microhabitat (more specifically, variations in microhabitat diversity) were a major factor in explaining the abundance and community response of birds and bats. Bird species, including cavity-nesting birds, were most strongly affected by tree microhabitat diversity, while bat species were positively affected by both the diversity of the tree microhabitat and the density of cavities created by Cerambyx spp. Tree microhabitats were better predictors of bird and bat responses than other stand characteristics. Tree microhabitat characteristics provide a reliable measure of ecological niches in forest ecosystems and we propose that these microhabitats be used as indicators for assessing biodiversity in forests.