Litter hovels as havens for riparian spiders in an unregulated river

Abstract

Riparian spider abundance is influenced by the extent and architecture of litter, which is greatly influenced by flood regime. Flood-deposited clumps of intertwined plant material and inorganic debris (“litter hovels”) attached to trees and elevated above the ground by past high water events present a unique, persistent, and unstudied habitat type for spiders. We investigated spider use of litter hovels along an unregulated reach of West Clear Creek, Arizona, USA dominated by native riparian vegetation. Ninety two percent of litter hovels were occupied by at least one spider, with a mean occupancy of 4.5 spiders per hovel. Spider abundance and diversity at the family level were positively correlated with litter hovel size. Furthermore, spiders were non-randomly distributed among litter hovels in three areas of this riparian system: the vegetated floodplain, creek edges, and islands within the creek channel. Overall spider diversity was two-fold greater on creek islands than in the vegetated floodplain, and spider abundance also varied by predation guild among the three habitat types. Running and web-building spiders were most abundant on creek islands, while stalking and ambushing spiders were two-fold more abundant along creek edges in comparison with other habitats. For this riparian system, we estimate that spider densities could reach 270,000/ha in hovels alone, and therefore alteration of flood regimes through stream management may have important implications for arthropod dynamics. Current efforts to restore natural flood regimes and native vegetation in Southwestern streams are likely to benefit spider populations, their predators, and the regulation of herbivorous arthropods.