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Density limits and survival of local populations in 64 carabid species with different powers of dispersal

Patterns of density fluctuations and survival times were estimated for the 64 most abundant carabid species, sampled continuously over 23 years with pitfalls in 89 sites in Drenthe (The Netherlands). I show that for most carabid populations density fluctuated between years, either randomly or between wider bounds than expected with random fluctuations. This was true for all groups, not just those occupying temporary habitats. I discuss the selective processes connected with dispersal (flight) abilities inside and outside populations of species occupying different kinds of habitat, and conclude that under natural conditions the powers of dispersal usually favour an optimal chance of survival of the species; this fits Wright's shifting balance model. Under cultivation, stable habitats have been drastically reduced and fragmented, so that local populations have become highly isolated and the risk of extinction is no longer spread over local groups. This has accelerated selection against dispersal features in isolated populations, so that species with low powers of dispersal apparently can no longer compensate for population extinctions by (re)foundings. Without adequate measures such species are doomed in these areas. Our work leads us to the conclusion that the current ideas on regulation of numbers and on group selection do not adequately describe the situation.