Trophic levels and habitat specialization of beetles caught on experimentally added aspen wood: Does trap type really matter?

Saproxylic (wood living) beetles make up an important part of forest biodiversity and are of importance for conservation practices. The beetles are commonly surveyed on various dead wood resources by use of flight interception traps (window traps), catching both species depending on the resource and vagrant (irrelevant) species. Enclosed traps have been suggested as an alternative sampling method, as they sample only the beetles utilizing the dead wood resource. This is the first study to compare the two trap types based on catches from experimentally added identical wood units. Our main aim was to test whether enclosed window traps catch a higher proportion of specialized species and species of a lower trophic level than open window traps, and whether proportions of vagrant species decrease with habitat specialization and trophic level in both trap types. The study was carried out during two summers with traps mounted on replicated, identical units of fresh aspen dead wood in boreal forest in south-eastern Norway. Contrary to our expectations and earlier findings, the enclosed window traps did not catch a higher proportion of habitat specialists (aspen-associated species) or species at the lowest trophic level (wood feeding) compared to the open traps. Rather, the proportion of predators was higher and fungivores lower in enclosed versus open window traps, and no difference was found between different categories of habitat specialization. The proportion of vagrant species was lowest among the beetles with the strongest specialization (aspen-associated) whereas there was no difference with trophic levels. We suggest that keeping vagrants but selecting species based on host tree affinity is the best way to use catches from open window traps on dead wood units. Studies of predators and fungivores should be interpreted with care.