Effects of habitat edges and trampling on the distribution of ground beetles (Coleoptera, Carabidae) in urban forests

Abstract

Urban forest patches are generally small with highly contrasting edges bordering non-forest habitat, landscape features that increase with urbanisation. These forest patches are also subject to high human foot traffic resulting in trampling and other user disturbances that affect their quality as habitat for invertebrates. We studied the effects of these factors on carabid beetles in urban forests in the cities of Helsinki (Finland) and Edmonton (Canada). In both cities, the structure of carabid assemblages was affected by trampling intensity and distance to the forest edge. Moderate intensity of trail use was associated with increased beetle captures, especially in Edmonton. The effects on individual species were apparent in Edmonton but harder to demonstrate in Helsinki where forest specialist species may have been largely extirpated already. We suggest that these differences result because there has been a long history of fragmentation of urban forest patches in Helsinki, species loss from such patches is gradual, and understorey vegetation structure constrains human foot traffic in forest patches more in Edmonton than it does in Helsinki.