| Summary: | The species composition and relative density of bats were compared in forests of various sizes occurring as “islands” in the agricultural landscape of central Poland. The following island categories were distinguished: very small (0.3–0.7 km2), small (1.0–1.5 km2), medium (2.0–3.5km2) and large (approx. 18 km2). Bats flying over lanes were caught at 34 mist net stations in 13 islands at the end of June and beginning of July (period I) and again at the end of July and beginning of August (period II). Twelve species of bats were recorded (Plecotus auritus, Eptesicus serotinus and Barbastella barbastellus were the dominant species), and the number of species in specific categories of islands ranged from 8–9, except in the very small islands, where only 4 species were confirmed. Species diversity rose with the size of the islands. Nyctalus leisleri and Myotis mystacinus were caught only in the large island. The frequency of B. barbastellus and Nyctalus noctula clearly increased with island size as opposed to E. serotinus and P. auritus. The relative density (mean numbers of individuals caught at one location on one night) during period I increased with island size from very small (1.8) to large (8.1), while during period II, the highest values were achieved in the medium-sized islands (13.4). The mean number of species for one location and night rose in a similar manner. Forest fragmentation to very small units of less than 1 km2 in size negatively influences bat ensembles.
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