Extinction Thresholds for Species in Fractal Landscapes

Abstract

Predicting species' responses to habitat loss and fragmentation is one of the greatest challenges facing conservation biologists, particularly if extinction is a threshold phenomenon. Extinction thresholds are abrupt declines in the patch occupancy of a metapopulation across a narrow range of habitat loss. Metapopulation-type models have been used to predict extinction thresholds for endangered populations. These models often make simplifying assumptions about the distribution of habitat (random) and the search for suitable habitat sites (random dispersal). We relaxed these two assumptions in a modeling approach that combines a metapopulation model with neutral landscape models of fractal habitat distributions. Dispersal success for suitable, unoccupied sites was higher on fractal landscapes for nearest-neighbor dispersers (moving through adjacent cells of the landscape) than for dispersers searching at random (random distance and direction between steps) on random landscapes. Consequently, species either did not suffer extinction thresholds or extinction thresholds occurred later, at lower levels of habitat abundance, than predicted previously. The exception is for species with limited demographic potential, owing to low reproductive output (R Ј o ϭ 1.01), in which extinction thresholds occurred sooner than on random landscapes in all but the most clumped fractal landscapes (H ϭ 1.0). Furthermore, the threshold was more precipitous for these species. Many species of conservation concern have limited demographic potential, and these species may be at greater risk from habitat loss and fragmentation than previously suspected. Umbrales de Extinción para Especies en Paisajes Fraccionados Resumen: La predicción de las respuestas de especies a la pérdida del hábitat y su fragmentación es uno de los retos más grandes a los que se enfrentan los biólogos conservacionistas, particularmente si la extinción es un fenómeno de umbrales. Los umbrales de extinción son declinaciones abruptas en la ocupación de parche de una metapoblación a lo largo de un rango somero de pérdida de hábitat. Los modelos de tipo metapoblacional han sido usados para predecir umbrales de extinciones para poblaciones amenazadas. Estos modelos frecuentemente simplifican las suposiciones sobre la distribución del hábitat (al azar) y la busqueda de sitios de hábitat apropiado (dispersión al azar). Relajamos estas dos suposiciones en un modelo que combina un modelo de metapoblación con modelos de paisaje neutral de distribuciones de hábitat fracturados. El éxito de dispersión para sitios disponibles y desocupados fue mayor en paisajes fracturados para dispersores del vecino más cercano (moviéndose a través de celdas en el paisaje), que para dispersores en busquedas al azar en paisajes aleatorios (distancia al azar y dirección entre pasos). Consecuentemente, las especies no sufrieron umbrales de extinción o los umbrales ocurrieron más tarde, a niveles aún mas bajos de abundancia del hábitat que los predecidos anteriormente. La excepción es para especies co...

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