Lack of homeward orientation and increased mobility result in high emigration rates from low-quality fragments in a dune wolf spider

Mobility and emigration were investigated for the wolf spider Pardosa monticola in grey dune fragments from two high-density and one low-density population, where population density was related to patch quality. Pitfall trapping was applied in combination with absolute density estimates by means of quadrat sampling. Orientation behaviour was additionally observed in the high- and low-density patches during two periods in the adult life-phase (mating and reproduction period). Dune grassland is the core habitat; moss dominated vegetation is the matrix in which dispersal occurs. Our field experiments confirm the hypothesis that increased activities of a dune wolf spider in a low-density habitat result in higher emigration rates. The diffusive emigration rates, which are higher close to the patch border, and mobility within the patch are higher in the low-density population. In the low-density patch, females are even more active than males and emigrate in the same proportions as males. Both males and females were not able to orientate and perform homeward movements during the spring period, in which vegetation height is more or less equal in the core habitat (grassland) and the matrix (moss dune). In June, no homeward orientation was observed in the habitat patch with low quality and low densities (low vegetation height). In the high-density patch, females but not males were able to perform homeward orientation behaviour at distances close to the pronounced border between grassland and moss dune. At distances of 3 m, females also orientated randomly. The pattern of differentiated homeward orientation behaviour indicates that it results from visual perception of high contrast borders by the females but not by the males or by sex-differentiated motivation to return to the core habitat. Increased emigration rates as a result of higher spider mobility together with the absence of visual orientation towards the patch border suggest the presence of an Allee effect in low-density patches