Small-scale habitat use and assemblage structure of ground-dwelling beetles in a Patagonian shrub steppe

摘要

Studies in arid systems have often focussed on the origin and maintenance of vegetation patchiness, but little is yet known about how microhabitat structure created by woody plants affects soil macroarthropods. We studied patterns of small-scale habitat use by darkling and ground beetles (Coleoptera: Tenebrionidae and Carabidae) in a two-phase shrub steppe mosaic, in NW Patagonia, Argentina. Beetles were sampled using pitfall traps during the early (December-January) and late (February-March) activity season (3675 trap-days). Traps were placed in three microsites: shrub centre, shrub periphery and off-shrub bare-soil areas. We also experimentally tested effects of aerial cover and litter accumulation, two key features of shrub patches, on beetle activity and assemblage structure. We captured 1415 individuals, the majority being omnivorous tenebrionids (67%) and predaceous carabids (28%). All recorded species (five tenebrionids plus two carabids) occurred in all three microsites and in both trapping seasons. Overall beetle activity, biomass and diversity were consistently greater in shrub microsites than in bare-soil areas. Mean diurnal soil temperatures were higher in bare soil (37.6℃) than beneath shrubs (23.6℃). Tenebrionid activity decreased markedly in the late season, coinciding with the increased frequency of daily maximum soil temperatures over 30℃ (shrub) and 50℃ (bare soil). Carabid activity did not vary with season, reflecting a shift in species dominance not found among tenebrionids. Addition of artificial shelters to bare-soil patches revealed differing perceptions of microhabitat cover by beetle groups, carabids being less selective than tenebrionids. In contrast, litter manipulation did not significantly alter microhabitat use by beetles. We conclude that shrubs act as 'keystone structures' providing sheltered habitat to ground-dwelling macroarthropods. Thus, disturbances and management actions altering shrub cover may have unwanted, community-wide impacts on biodiversity and ecosystem processes.