Brown bears (Ursus arctos) as ecological engineers: the prospective role of trees damaged by bears in forest ecosystems

Abstract

Brown bears (Ursus arctos L., 1758) damage trees (mainly the cambium) during feeding and marking. It is thought that their stripping of bark and their subsequent foraging on sapwood can lead to substantial economic losses. However, the part played by this process in the ecosystem is still unknown. We hypothesize that brown bear foraging makes resources and habitats available to other species. We anticipated that wounds perpetrated by bears during foraging would attract saproxylic insects and consequently insectivorous birds. Bear wounds were searched for on trees in the Bieszczady Mountains (Polish Eastern Carpathians) from 2008 to 2011. We analyzed 278 wounds in silver firs (Abies alba Mill.) of different age classes: 43% of them had holes made by wood-boring insects and 33% had signs of woodpecker feeding. The presence of insect holes and woodpecker marks in the wound was found to depend on wound area, wound age, and tree circumference (insects only). In particular, the oldest wound age class (>5 years old) was associated with a high probability of occurrence of insects and woodpeckers. Insects were attracted by wounds of smaller area than woodpeckers. The density of insect holes (number of wood-boring insects) depended on the wound age and tree circumference. Insect densities in fresh wounds (<1 year old) were significantly lower than in the older age classes. The results show that bear-made wounds provide breeding and feeding sites for both insects and birds. Especially for woodpeckers (forest indicator species) and for saproxylic insects (mainly endangered species), brown bears may play an important role as niche constructors and ecological engineers. Therefore, it would be advisable to leave a considerable number of bear-wounded trees in the forest ecosystems for conservation purposes.