Comparing the efficacy and cost-effectiveness of two sampling methods for monitoring carabid beetle diversity, species assemblages and conservation status in an alpine grassland

Abstract

Among the methods aimed at assessing carabid beetle diversity, pitfall trapping (PFT) is currently much more widely used than hand searching (HS). However, few studies have evaluated the effectiveness of these two methods based on the same sampling effort, none in mountain areas. Here, we compared PFT and HS in terms of accuracy of the species richness estimate, detection of threatened species and cost-effectiveness, based on equal time spent in the field. The study was conducted along an elevational gradient in the alpine zone of the French Pyrenees. Our results showed that recorded species richness was significantly greater when sampling was performed using HS, suggesting that PFT only detected a subset of the ground beetle assemblage existing at each sampling plot. HS was more effective at characterizing rare species, especially microendemic species threatened by climate change. HS also enabled better detection of small species and of winged species. Additionally, HS involves fewer logistical constraints in performing field work and prevents overkilling in fragile alpine populations. However, some of the benefits we have highlighted in favour of HS may be specific to the studied habitat type, calling for more studies in different mountain habitats.

Implications for insect conservation

Our results show that PFT alone does not allow for an accurate assessment of the conservation status of highly specialised alpine species, if they are small in size. Moreover, HS has a lower lethal impact on local insect populations. HS should therefore be recommended, whenever it is practically feasible, in the most sensitive and threatened high-alpine environments.