Spatio-temporal variation in mortality rates of Mecodema spp. (Coleoptera: Carabidae) across a forest-grassland edge in New Zealand

1. Edge effects are a highly cited issue in conservation biology with a wealth of empirically described impacts on invertebrates, but the mechanisms underlying the responses of individual species to habitat edges have been seldom tested. One often-raised hypothesis is that individual species vary in their ability to survive along a gradient of changing microclimatic conditions across habitat edges. 2. I used a phylogenetically controlled, translocation experiment to investigate edge-driven variation in adult mortality rate for two closely related species that have contrasting abundance patterns with respect to forest-grassland edges; the habitat generalist Mecodema fulgidum Broun ( Coleoptera: Carabidae) and the forest-dwelling Mecodema rugiceps Sharp. 3. Adult individuals of both species were live-caught and translocated into containers at varying distances along an edge gradient extending from 1024 m inside a large forest fragment and 64 m into the surrounding grassland matrix. Experimental containers were connected to the above- and below-ground environments via mesh-covered panels, and beetle mortality over a 4-month period in the 2006/2007 austral summer was recorded. 4. Overall, mortality was higher for the habitat specialist M. rugiceps than for the generalist M. fulgidum. Neither species exhibited an edge-related trend in mortality, nor were mortality rates correlated with microclimatic gradients across the edge. 5. Weather records for the summer indicated that M. rugiceps was more sensitive than M. fulgidum to a soil moisture deficit that became more pronounced as summer progressed. Analysis of a 10-year climate record shows that soil moisture deficits occur annually, suggesting that seasonal variation in soil moisture may contribute to the variation in the distribution and abundance of the two species.