INBREEDING AND EXTINCTION IN A BUTTERFLY METAPOPULATION

It has been proposed that inbreeding contributes to the decline and ev entual extinction of small and isolated populations(1,2), There is amp le evidence of fitness reduction due to inbreeding (inbreeding depress ion) in captivity(3-7) and from a few experimental(8,9) and observatio nal field studies(10,11), but no field studies on natural populations have been conducted to test the proposed effect on extinction, It has been argued that in natural populations the impact of inbreeding depre ssion on population survival will be insignificant in comparison to th at of demographic and environmental stochasticity(12,13). We have now studied the effect of inbreeding on local extinction in a large metapo pulation(14) of the Glanville fritillary butterfly (Melitaea cinxia)(1 5). We found that extinction risk increased significantly with decreas ing heterozygosity, an indication of inbreeding(6), even after account ing for the effects of the relevant ecological factors, Larval surviva l, adult longevity and egg-hatching rate were found to be adversely af fected by inbreeding and appear to be the fitness components underlyin g the relationship between inbreeding and extinction. To our knowledge , this is the first demonstration of an effect of inbreeding on the ex tinction of natural populations. Our results are particularly relevant to the increasing number of species with small local populations due to habitat loss and fragmentation(16).