**4. Dispersal: how to get there?**

The rapid colonisation of newly exposed ground indicated that arthropods have a high dispersal ability. On Iceland, springtails and oribatid mites easily colonised recently emerged nunataks, and isolation of a few kilometres did not affect the colonisation [16]. These results strongly indicate aerial dispersal, and our study supports this. Fallout traps and sticky traps collected nine species of springtails and four species of oribatid mites, as well as some Actinedida mites and spiders (**Figure 11**). Among other items were unwinged aphids, some flies, several chironomid midges, a few seeds, and many fragments and diaspores of pioneer mosses [28]. Most aerial transport occurred rather close to the ground, below 0.5-m height. Sand grains in sticky traps up to this level illustrated the mechanical force of wind transport.

Some of the trapped species were assumed not to be able to thrive on pioneer ground, but to depend on a thicker organic layer [28]. In that case, their dispersal ability is high, but the pioneer ground may act as a 'sink' for them, where they will die. A 'real' pioneer species must

**Figure 11.** Invertebrates taken in sticky traps, proving airborne transport. A = the springtail *Bourletiella hortensis*. B = the mite *Bryobia* sp. C = the mite *Tectocepheus velatus*. D = the spider *Erigone arctica*. From Ref. [28].

be able both to arrive, to tolerate the harsh environmental conditions, to manage competition, to find food and to reproduce. In other words, pioneers must pass two 'filters': a 'dispersion filter' to arrive and an 'ecological filter' to establish a population.
