A functional relationship between species richness of spiders and lichens in spruce

Modern forestry has created stands with even age distribution of trees and fragmentation of the habitat. In boreal forests, the effects on biodiversity within many taxa need to be examined. We tested the hypothesis that species richness of foliose and fruticose lichens and spiders is positively related in the lower canopy of spruce (Picea abies) in forests with, or without, management in central Sweden. High species richness of lichens may increase the structural complexity of the microhabitat on spruce branches, and bring a higher species richness also in the spider community. In six areas, spruce branches were sampled in old-growth and managed boreal forest stands, respectively. Forest management did not affect the species richness of spiders or lichens, but an effect due to sampling area was found in the latter taxon. There was a significant covariation between species richness of lichens and spiders, and the hypothesised positive correlation was confirmed by separate analyses for each area and combining the probabilities. Moreover, regression analysis on mean values from each site revealed a positive relationship. We conclude that species richness of lichens and spiders covary on spruce branches for functional reasons, i.e. more lichen species promotes a more diverse spider community by increasing the structural heterogeneity. Our results might provide a 'shortcut' for assessing biodiversity in boreal forests.