Effect of forestry on the abundance and diversity of arboreal spiders in the boreal spruce forest

Abstract

Spiders were sampled from spruce branches during late winter in northern Sweden, to investigate the effects of forestry on the community structure of arboreal spiders. Five lichen-rich, natural spruce Picea abies forests and adjacent mature, selectively-logged lichen-poor forests were selected as sample sites. Lichen-rich forests had over three times more spiders on the branches than the lichen-poor forests. The spider community was dominated by web spinners, i.e. the families Linyphiidae. Araneidae, Tetragnathidae and Theridiidae. Hunters, i.e. Philodromidae and Clubionidae. comprised 82% of all spiders were juveniles. Among the dominant species, only the orb-weaver Araneus nordmanni was found in all sites whereas the sheet-web spider Lepthyphantes suffusus was found in all five lichen-rich forests but only in two of the lichen-poor. Both species composition and dominance differed from spider communities in southern boreal spruce canopies. Small prey items, severe abiotic conditions and high predation pressure from birds are possible reasons why web spiders dominate the arboreal community in northern Sweden. These factors could also explain the observed shift in dominance from sheet-web spiders with large body size to those with a small body size, compared with arboreal spider communities in southern boreal forests. Diversity indices (jack-knifing of Simpson index and Q statistic) showed a higher diversity of spiders in lichen-rich than in lichen-poor forests. The lichen-rich forest had more species and less dominance, in both rank abundance and the Berger-Parker index of dominance. However, rarefaction plots indicated no differences besides lower abundance of spiders on sampled branches in lichen-poor forests. It is suggested, that habitat structure (branch size and epiphytic lichen abundance) could be an explanation for the greater number of spiders in old, lichen-rich spruce forests.