Effect of pitfall trap type and diameter on vertebrate by-catches and ground beetle (Coleoptera: Carabidae) and spider (Araneae) sampling

To determine the diversity of epigaeic arthropods, pitfall traps are a suitable and widely accepted sampling method. Unfortunately small mammals such as shrews, voles and mice are accidentally caught by these traps, sometimes very frequently depending on trap type and size. Here we present an experiment examining the influence of trap types, conventional cup traps vs. funnel traps, and of trap diameter on catches of carabid beetles, spiders and unwanted by-catches of small mammals. The number of sampled carabid beetles increased significantly with diameter of the traps and presence of a funnel. The number of species was not affected by trap type or trap size. For spiders, significantly more individuals and more species were found in larger traps. After correcting for difference in sample size by rarefaction, spider species number was still higher in larger traps. The use of a funnel had no effect on spider assemblages. 5.Funnel traps and a smaller trap diameter strongly reduced catches of small mammals, on average by at least 2/3. While catches of arthropods increased with increasing trap size independent of trap type, the number of small mammals caught in the traps was not greater in larger traps when a funnel was used. 6.Based on our results and other studies on the efficiency of different pitfall trap types, we recommend to use funnel traps of larger diameter (approximately 9cm) to more efficiently sample epigaeic arthropod assemblages and to minimize unwanted catches of small mammals.