Plasticity in dispersal: Increases in density and relatedness increase dispersal distance in the two-spotted spider mite

Background/Question/Methods

Dispersal is a multi-phase process that influences the genetic demography of populations, the geographic distribution of species, and the success of species attempting to expand their ranges. The distances individuals disperse, and the environmental factors influencing these decisions, deserve the utmost attention in a world where habitat fragmentation is increasing and the ability of an organism to reach a new habitat can determine the fate of the species. In this study, we investigated the effects of kin competition and population density on dispersal distance. Theoretically, kin competition and density have been predicted to increase dispersal distances, yet empirical data are lacking. Using Tetranychus urticae, the two-spotted spider mite, we manipulated the conditions on a starting patch of bean-leaf linearly connected to 24 other patches. We used four different density treatments (2.5ind cm^-2, 12.5ind cm^-2, 25ind cm^-2, 37.5ind cm^-2 with four replicates of each) and five different relatedness treatments (0, 0.2, 0.48, 0.71, and 1 with seven replicates of each). Each day for seven days, the distribution of the mites along the 25 patch system was counted. We then analyzed mean dispersal distance and the skew and kurtosis of the dispersal kernel as a function of density and relatedness.

Results/Conclusions

Increases in density significantly increased the mean dispersal distance by a factor of 1.9 between the lowest and highest density (F_1,30=21.25, p<0.001). Mean dispersal distance also significantly increased by a factor of 1.5 between the lowest and highest relatedness (F_1,29=11.82, p=0.002). While density did not change the shape of the dispersal distribution, increased genetic relatedness significantly increased the skew and kurtosis of the dispersal kernel. Higher related populations had a dispersal kernel with a positive skew and a fat-tail. Our results support theoretical predictions by clearly indicating that population density and relatedness have a positive relationship with dispersal distance. Our experiments provide a basis for which further experiments examining the costs of dispersal and the fitness of individuals along the dispersal kernel can be examined. Incorporating our results into future theoretical work on the range margins of expanding species would provide more accurate predictive power and be more useful in an applied context.