Beetle abundance-environment relationships at the Subantarctic-Patagonian transition zone

The spatial variation in the abundance of individuals may be associated with the variation in environmental variables. The productivity hypothesis proposes that climate affects plant productivity, which may limit the abundance of beetles. The thermal-limitation hypothesis proposes the direct effect of ambient temperature may limit beetle abundance. We analysed the abundance of epigaeic beetles at the Subantarctic-Patagonian transition to test for these abundance-environment relationships. We collected beetles using 450 pitfall traps within a similar to 150 x 150 km area representative of the Subantarctic-Patagonian transition. We used path analysis to evaluate the relationships between beetle abundance and plant cover, litter biomass, averaged minimum and daily temperature range, and mean annual precipitation. We used principal coordinates of neighbour matrices to model the spatial autocorrelation of the data. The abundance of beetles increased strongly with tree canopy cover and less strongly with herb cover. The increase in shrub cover had a positive effect on beetle abundance in areas to the east of the transition, within the scrubland-steppe, but it has a negative effect on beetle abundance in areas to the west of the transition, within the forests. The association between beetle abundance and minimum daily temperature was negative or weak throughout. Increased temperature variation had a negative effect on beetle abundance. We suggest that indirect positive climatic effects mediated through plant cover are important to account for the variation in beetle abundance, which favours the productivity hypothesis. Thermal limitation may operate locally through variation in daily temperature range.