Extinction in Subdivided Habitats

: The effect of the spatial structure of populations on extinction rates is a central question in conservation biology, ecology, and evolution. We show that the effect of subdivision on the mean time to extinction in independently varying habitat patches depends upon the relationship between extinction probabilities and habitat area Models of extinction by demographic stochasticity alone predict that subdivision should decrease the mean time to extinction. If environmental stochasticity is incorporated into the model, subdivision may increase the mean time to extinction. Empirical species persistence times estimated from island bio-geography studies show no systematic relationship to the degree of subdivision. However, subdivision always increases the probability of survival over a sufficiently short time interval. These results suggest that over ecologically relevant timescales, subdivision into a number of independent sub-populations may frequently act to decrease the probability of overall extinction in rare species. To the degree that these considerations may be applied to rare and endangered species, maintaining sufficient numbers of nature reserves is likely to be crucial to effective conservation strategies. Resumen: El efecto de la estructura espacial de las poblaciones en las tasas de extincion es una interrogante fundamental en conservacion, ecologia y evolucion. Este trabajo de muestra que el efecto de la subdivision de poblaciones sobre el tiempo promedio de extincion, en habitats fragmentados que varian independientemente, depende de la relacion entre la probabilidad de extincion y la extension del habitat. Los modelos de extincion basados exclusivamente en demografia estocastica, predicen que la subdivision debe reducir el tiempo promedio de extincion. Si se incorporan eventos aleatorias del ambiente (medio ambiente estocastico) en el modelo, la subdivision puede inmentar el tiempo promedio de extincion. Las estimaciones empiricas del tiempo de persistencia de las especies, provenientes de los estudios sobre biogeografia de islas, no demuestran una relacion sistematica con el grado de subdivision. No obstante, la subdivision siempre aumenta la probabilidad de sobrevivencia en un periodo de tiempo suficientemente corto. Estos resultados sugieren que sobre escalas de tiempo ecologicamente pertinentes, la subdivision en varias subpoblaciones independientes frecuentemente puede producir una reduccion en la probabilidad de extincion de las especies raras. Sera crucial para la efectividad de las estrategias de conservacion, el grado en que estas consideraciones se apliquen a las especies raras y en peligro de extinction, manteniendo un numero suficiente de reservas naturales.

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