It is not a mere piece of the mainland: Fluvial islands contribute to regional spider diversity in a seasonally flooded Amazonian archipelago

Abstract

Abstract Islands and analogous insular habitats are found in several ecological contexts. With the exception of oceanic islands, however, the contribution of these insular habitats to regional biodiversity remains poorly investigated, especially in tropical regions. We investigated the understory spider assemblages in an Amazonian river archipelago to evaluate the contribution of fluvial islands to regional spider diversity. We collected spiders on 30 islands ranging from 1 to 1527 ha and at 23 sites in mainland habitats represented by continuous igapó flooded forests and terra firme forests. Spiders were collected by shaking the vegetation over a white sheet during a standardised time of 30 min. From 1404 adult individuals, we identified 290 spider species or morphospecies, with 150 species being recorded on the fluvial islands, 122 in the igapó flooded forests and 103 in the terra firme forests. Spider assemblages of fluvial islands exhibit lower alpha diversity and equitability compared with mainland habitats. The species composition of fluvial islands was significantly distinct from that of mainland habitats, even when compared with neighbouring igapó flooded forests. Several families and species of spider were significantly associated with a particular habitat, including 8 families and 10 species identified as indicators of fluvial islands. Fluvial islands effectively contribute to regional diversity since their spider fauna is not a mere impoverished subset of species from mainland habitats. Events that alter the flood pulse in Amazonian rivers (e.g., construction of hydroelectric dams) could threaten the unique biological assemblages of the floodplain habitats, especially the fluvial islands.