THE SURVIVAL VALUE OF DISPERSAL IN TERRESTRIAL ARTHROPODS

Dispersal in terrestrial arthropods cannot merely be a means to 'escape from adverse conditions'. During the 'dispersal phase' aeronautic behaviour of insects and spiders is made possible by suitable weather, often resulting in airborne 'plankton', the washing ashore of large numbers of winged insects, regular catches on 'window' traps, and in the rapid colonization of new polders. Therefore, the primary biological significance of dispersal will be the maintenance and extension of the presence of a species in space by (re)founding and supplementing local populations. Accordingly, dispersing individuals of some species show a higher egg production than those which do not disperse. Depending on the spatial relationships between habitats natural selection will 'manipulate' the powers of dispersal of many species. In a cultivated area this will endanger poorly dispersing species by increasingly isolating them in remnants of the 'old landscape'. In carabids mean survival times of such local populations were estimated to be some decades, but not to exceed a few centuries. Therefore, for a species to survive, a complete turnover (extinction/(re)founding) of local groups must be maintained by sufficient dispersal. Carabids adapted to extensive, stable habitats usually live there in heterogeneously structured metapopulations. Spreading the risk of extinction over interconnected and differently fluctuating local groups will then result in almost unrestricted survival of the species. The reclamation of such areas breaks up these metapopulations into isolated fragments, which will usually no longer be favoured by this kind of risk spreading. The local populations which remain are confined to small localities and without replacement are eventually likely to become extinct. This process also applies to other organisms, such as poorly dispersing bird species which became extinct on Barro Colorado Island following the creation of the Panama Canal. © 1990.