Hella Klosius


Species richness and composition of bird assemblages along an elevational gradient in the Eastern Alps

Most studies about changes of bird species richness along elevational gradients were carried out in tropical regions, whereas quantitative information is scarce for temperate mountain ranges such as the Alps. We conducted standardized bird surveys using point censuses along an elevation gradient in the Eastern Alps to study changes of species richness, species composition and relative abundance of feeding guilds. Additionally, we related altitudinal changes of the avifauna to changes of biotic (vegetation structure) and abiotic variables (area of elevation belts).
The investigated altitudinal gradient consisted of three different elevation transects in the National Park Gesäuse (Styria, Austria) on the southern slopes of the Buchstein massif covering an altitudinal range between >500 m (valley floor of Ennstal) to 2,035 m a.s.l. (summit of Tamischbachturm, 2 transects) and 1,928 m a.s.l. (rocky massif of Großer Buchstein, 1 transect), respectively. In total 87 census points were selected along the three transects. Birds were recorded visually and acoustically between 3rd April and 25th July at each census point during three 10 min-observation units. For each bird it was noted if it was recorded within or outside a radius of 50 m around the census point. The following habitat parameters were measured or estimated for each census point: altitude a.s.l., distribution of tree diameters, maximum height of woody vegetation, species richness of woody plants and tree density. Furthermore, for all 100 m-elevation belts their total area and the southern slope area [km²] were extracted from digitalized maps.
A total of 40 breeding bird species were recorded. Species richness did not decline linearly but showed a plateau between 500 and 1,200 m, where an average of 10 species per census point was recorded. From 1,200 m towards higher altitudes species numbers decreased continuously, reaching a minimum of 1-2 species per census point at the highest elevations. Beside the factors altitude and area per altitudinal zone, which accounted for most of the variation of species richness, the variables vegetation height [m], tree density, tree diameter [cm] and woody plant species diversity proved to influence richness of bird assemblages. A comparison of breeding bird species composition between census points based on Bray-Curtis similarity indices demonstrated significant differences between the forest belt and the zone above the timberline, and within the forest belt between mixed forest and coniferous woodlands. Species composition was significantly related to altitude, woody plant species richness and maximum height of woody vegetation. Differences of the relative abundance of feeding guilds between elevation belts mirrored changes of habitat structure, e.g. insectivorous stem climbers disappeared at the timberline. Regarding absolute numbers of individuals a significant decrease of omnivores, ground-dwelling insectivores and insectivores foraging within the vegetation could be observed. Insectivorous stem climbers reached their highest abundances at mid-elevations, perhaps indicating an intensive forest use until recently at lower elevations.

In cooperation with the National Park Gesäuse.