Extinction vortex

Abstract: Predicting the extinction of single populations or species requires ecological and evolutionary information. Primary demographic factors affecting population dynamics include social structure, life history variation caused by environmental fluctuation, dispersal in spatially heterogeneous environments, and local extinction and colonization. In small populations, inbreeding can greatly reduce the average individual fitness, and loss of genetic variability from random genetic drift can diminish future adaptability to a changing environment. Theory and empirical examples suggest that demography is usually of more immediate importance than population genetics in determining the minimum viable sizes of wild populations. The practical need in biological conservation for understanding the interaction of demographic and genetic factors in extinction may provide a focus for fundamental advances at the interface of ecology and evolution.

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 captivity3,4,5,6,7 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.

Abstract: to environmental perturbation, or are so small that random birth and death events and genetic drift threaten them. The number of species in this set of vulnerable populations will depend on many factors— especially the size of the reserve, because the larger the reserve, the smaller the proportion of species that will have marginal population sizes. If choices must be made among candidate species, then the ecological role of the species is perhaps the most important criterion. Large predators and other keystone or mutualistic species such as some trees (Chapter 15) and pollinators (Chapters 19 and 21) are automatic targets. If "Nature knows best" and Nature abhors close inbreeding (Chapter 3), then managers should too. Every rule has its exceptions, however, including this one—especially when a past history of inbreeding requires extreme remedies, even the purging of certain recessive deleterious genes from a group (Chapter 6). Chapters 4 and 5 suggest that any measurable increase in homozygosity is likely to extract a cost in the currencies of immediate fitness and long-term adaptability. But even more dangerous than inbreeding depression is the kind of "depression" that afflicts some agencies when it appears that a population is too small to save. The point is that even when numbers are very low recovery is possible, given the will and the resources. Theory indicates that most genetic variation (though not the rare alleles) can be saved from even a handful of individuals. In such cases, it is usually financial resources, not genetic resources, that is the limiting factor. Finally, genetic systems are too heterogeneous to allow generalizations about the dangers of outbreeding depression in specific cases. Managers of threatened or endangered species, whether the species are captive or managed in the wild, must be aware of the hazards discussed above and in the following chapters. The problems range from meiotic disturbances and sterility in the FI generation to quantitative reduction in developmental homeostasis, viability, local adaptation, and host recognition, to inappropriate timing of reproduction (Templeton, Chapter 6). Anticipating all the potential problems may be impossible, but chromosomal surveys and autecological studies could eliminate most sources of failure.