1. Department of Ecology and Systematics, Division of Population Biology, PO Box 17, 00014 University of Helsinki, Finland
2. Tvärminne Zoological Station, University of Helsinki, 10900 Hanko, Finland

Correspondence to: Ilik Saccheri¹ Correspondence and requests for materials should be addressed to I.S. (e-mail: Email: ilik.saccheri@helsinki.fi).

Abstract

It has been proposed that inbreeding contributes to the decline and eventual extinction of small and isolated populations¹,². There is ample evidence of fitness reduction due to inbreeding (inbreeding depression) in captivity^{3, 4, 5, 6, 7} and from a few experimental⁸,⁹ and observational field studies¹⁰,¹¹, 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 depression on population survival will be insignificant in comparison to that of demographic and environmental stochasticity¹²,¹³. We have now studied the effect of inbreeding on local extinction in a large metapopulation¹⁴ of the Glanville fritillary butterfly (Melitaea cinxia)¹⁵. We found that extinction risk increased significantly with decreasing heterozygosity, an indication of inbreeding⁶, even after accounting for the effects of the relevant ecological factors. Larval survival, adult longevity and egg-hatching rate were found to be adversely affected by inbreeding and appear to be the fitness components underlying the relationship between inbreeding and extinction. To our knowledge, this is the first demonstration of an effect of inbreeding on the extinction of natural populations. Our results are particularly relevant to the increasing number of species with small local populations due to habitat loss and fragmentation¹⁶.