Should fragment area reduction be considered a stress for forest bird assemblages? Evidence from diversity/dominance diagrams

Breeding bird assemblages and species present in two 'archipelagos' of wood fragments, included in fragmented landscape of Central Italy, were studied in springs 2002 and 2003 with line transect method (1: Cornicolan hills study area: 20 fragments; 2: Anzio-Nettuno study area: 13 fragments). An area effect was shown in diversity/dominance analyses carried out by species rank/frequency diagrams obtained for the wood fragment assemblages of two 'archipelagos'. Smaller fragments showed a lower species richness, a higher relative frequencies of first dominant species and a higher value of angular coefficient of assemblage lines. When fragment area decreases, the assemblage tendency lines in diversity/dominance diagrams show a higher slope (i.e., higher angular coefficient). Simpson dominance index was inversely correlated to fragment area: smaller fragments concentrate dominance in less species compared to larger ones. This approach suggests that the reduction in area of wood fragments could be comparable to a stress on breeding bird assemblages induced by anthropogenic habitat conversion and fragmentation, here considered as a disturbance at landscape level.