Competitive exclusion of pulmonate landsnails in an invasion context

Abstract

Invasive species are non-native organisms that become established in an ecologically naive environment to the detriment of the local environment and/or economy. However, some species become established but fail to expand their range or have a significant impact. The European native grove snail Cepaea nemoralis has been widely introduced to new areas through human activities, and now occurs in a wide variety of locations owing to its ecological generalism and broad native distribution. The species occurs in SW Essex County, where it is abundant in urbanized and disturbed areas of Windsor but scarce in native woodlands. We hypothesize that the native forest-inhabiting, similarly-sized, and fellow detritivorous Mesodon thyroidus competitively excludes the grove snail from this habitat type. In order to test this, a functional response (FR) or feeding rate framework was applied to individual snails collected from urban areas of Windsor (C. nemoralis) and from Kopegaron Conservation Area (C. nemoralis and M. thyroidus). Feeding trials were conducted under controlled conditions, whereby identically sized leaf squares, analogous to \u27prey items\u27 in traditional FR methods, were consumed over a fixed time period of 24 hours. Statistical analyses of FR curves (feeding rate as a function of food supplied) and their derived parameters were conducted in \u27R\u27, using maximum likelihood model fitting of nonlinear FR curves. Results showed significantly greater resource acquisition parameters for the native M. thyroidus in comparison to the non-native C. nemoralis, indicating that the native snail competitively excludes the invasive from the forest interior. FR methods can be used to test all aspects of invasion biology, but this study represents the first time that FR has been used to test the competitive exclusion of terrestrial invertebrates in an invasion ecology scenario