Aerial dispersal plasticity under different wind velocities in a salt marsh wolf spider

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Abstract
Dispersal can be regarded as a process operating both between and within patches of suitable habitat. For uncontrolled dispersal processes, the risk of crossing the borders of the habitat patch and arriving in the unsuitable landscape matrix will increase with decreasing patch area, in particular when the distance between isolated habitat patches is larger than the species' average dispersal capacity. Ballooning dispersal in spiders can be considered as a passive dispersal process, in which dispersed distances depend on the prevalent wind velocity. We executed a reaction norm analysis to analyze how dispersal propensity of the salt marsh wolf spider Pardosa purbeckensis depended on population characteristics (patch size) and the environment (wind velocity). Dispersal propensity was affected by the interaction between wind velocity and maternal patch size. Ballooning propensities decreased with decreasing salt marsh size. Interestingly, genotypes from large salt marshes show higher ballooning propensities under higher wind velocities, whereas those from small habitat patches show their highest dispersal propensity under low wind velocities. Crossing reaction norms and subsequently strong genotype × environment interaction variation was observed in all populations but tended to be lower in genotypes from large salt marshes. It is likely that this pattern results from differences in wind velocity–related costs of within-habitat dispersal in salt marshes of different sizes.