MODELING DISPERSAL DISTANCES IN A SOIL GRADIENT - THE INFLUENCE OF METAL RESISTANCE, COMPETITION, AND EXPERIENCE

A discrete probability model was developed to describe the distributions of dispersal distances of two soil-living collembolan species, Folsomia fimetarioides and Isotomiella minor, in an experimental gradient with metal polluted soil. The model assumes that animals move stepwise between adjacent habitats with a probability that is influenced by soil metal concentration, metal induced mortality and resistance, interspecific competition and the capability of individuals to perceive concentration differences. Theoretical and observed distributions were compared to determine the contribution of each factor to the dispersal pattern. Mor soils from a contaminated area were used to construct gradients consisting of three connected 20-cm(3) PVC rings filled with soil of different zinc and copper concentrations. Adult collembolans were added to each ring, and survivors and newly hatched juveniles were counted after 2 and 10 weeks of incubation. The probability that individuals remained in the soil where they were released was generally < 0.5. They responded to increasing metal concentrations by an exponentially decreasing probability to settle. The model suggested that dispersal distances of F. fimetarioides were largely affected by resistance to metals, which was supported by independent observations of enhanced reproduction by individuals collected in the polluted field soil compared with individuals from an unpolluted soil in the same area. Both species preferred to settle in the unpolluted part of the gradient, but F. fimetarioides was slower due to a poorer dispersal ability or a slower response to high metal concentrations. Significantly more F. fimetarioides remained in the most polluted soil during the first 2 weeks of incubation when I. minor was also present in the gradient, but the contribution of competition to the model description of dispersal distances was weak. The closest fit to the experimental data was attained when individuals were assumed to perceive differences in metal concentrations as they moved from one soil to another.