Overgrazed shrublands support high taxonomic, functional and temporal diversity of Mediterranean ground spider assemblages

摘要

The phryganic and maquis shrublands form the most typical vegetal formations in the Eastern Mediterranean that since thousands of years have been subject to various types of anthropogenic disturbance, including grazing. We studied the impact of sheep and goat grazing on 50 assemblages of ground spiders (Araneae: Gnaphosidae) in phryganic, maquis and forest habitats from zero to 2000 m elevation on Crete, Greece using pitfall traps for one year at each sampling site. In total, 58 gnaphosid species and 16,592 individuals were collected. Cretan endemic Gnaphosidae were negatively affected by intensive grazing and, contrary to findings on other taxa studied on the island, they were sparse and rare throughout the study area. The species composition of gnaphosid assemblages was primarily determined by elevation. Trachyzelotes lyonneti, Urozelotes rusticus, Zelotes scrutatus, Anagraphis pallens and Berinda amabilis proved to be significant indicators of overgrazing. The vast majority of spiders belonging to synanthropic and nationally red-listed species were found in overgrazed sites. Despite theoretical expectations, while the average activity density decreased, the number of species, evenness, rarity, taxonomic, temporal and functional diversity (FRic) indices increased with increasing grazing intensity. This was mainly due to the nature of disturbance itself, which probably allows equibalanced low abundances of indigenous or synanthropic species, rather than overdominance of few, thus partly refuting the opportunistic species hypothesis (OSH). Ground spider assemblages in overgrazed sites comprised sparse populations of mostly rare, opportunistic species with different combinations of life-history traits that partly replace other species more commonly found in the less degraded sites. Species of these assemblages shorten their phenologies to minimize temporal co-occurrence with each other due to resource limitation. These distinct community patterns highlight ground spiders as good indicators of grazing pressure on Crete, on species and assemblage level. To conserve the indigenous and potentially endangered invertebrate fauna, the spatiotemporal discontinuity of grazing including rested patches is important in order to maintain spatial variability and possibly lead to habitat restoration.