Biophysical Factors, Land Use, and Species Viability in and around Nature Reserves

Many nature reserves are located in landscapes with extreme biophysical conditions. We examined the effects of interactions between biophysical factors and land use on bird population viability inside and outside of Yellowstone National Park. Our hypotheses were as follows: (1) biophysical factors constrain bird species richness and bird reproduction at higher elevations; (2) nature reserves are located at higher elevations, whereas private lands and more intense land use occur mostly at lower elevations with more mild climates and fertile soils; and (3) intense land use at lower elevations favors nest predators and brood parasites and thereby reduces reproductive output for some bird species. We used simulation models to evaluate whether favorable habitats outside reserves are population source areas and whether intense land use can convert these habitats to population sinks and reduce population viability within reserves. Bird species richness and abundance were high in small hotspots in productive, low-elevation habitats. Length of breeding season—and opportunity for renesting—was greatest at the lowest elevations for both American Robins ( Turdus migratorius ) and Yellow Warblers ( Dendroica petechia ). Nature reserves were higher in elevation than private lands, so hotspots for bird richness and abundance occurred primarily on or near private lands, where rural residential development was concentrated. Brown-headed Cowbirds ( Molothrus ater ) were significantly more abundant near rural homes, but nests of American Robins were not parasitized and their nest success did not differ with home density. Nests of Yellow Warblers were commonly parasitized by cowbirds, and their nest success was significantly lower near rural homes. Estimated intrinsic population growth ( ) for American Robins suggested that low-elevation hotspots were population source areas for this species. Estimated for the Yellow Warbler suggested that the entire study area was a population sink, likely due to the effects of intense land use at lower elevations and climate constraints at higher elevations. Removing the effect of land use from the simulations revealed that highelevation hotspots were population sinks, whereas low-elevation hotspots were source areas. Our results are consistent with the possibility that bird-population source areas outside nature reserves can be converted to population sinks by intense human use, thereby reducing the viability of subpopulations within reserves. Factores Biofísicos, Uso de Suelo y Viabilidad de Especies en Reservas Naturales y sus Alrededores Resumen: Muchas reservas naturales se encuentran en paisajes con condiciones biofísicas relativamente extremas. Examinamos las interacciones entre los factores biofísicos y el uso del suelo en cuanto a la viabilidad poblacional de aves dentro y fuera del Parque Nacional Yellowstone. Nuestras hipótesis fueron las siguientes: (1) los factores biofísicos limitan la riqueza y la reproducción de especies de aves en alturas mayores; (2) las reservas naturales están a mayores alturas, mientras que los terrenos privados y los usos más intensos del suelo están principalmente a menores alturas con climas más templados y tierras más fértiles y (3) el uso intensivo de suelo a menores alturas favorece a depredadores y parásitos de nidos y, por lo tanto, reduce el éxito reproductivo de algunas especies de aves. Utilizamos modelos de simulación para evaluar si los hábitat favorables fuera de las reservas son áreas fuente para las poblaciones y si el uso intensivo de suelo puede convertir a esos hábitat vertederos y reducir la viabilidad poblacional dentro de las reservas. La riqueza y abundancia de especies fueron altas en pequeñas áreas en hábitat productivos situados a elevaciones bajas. La duración de la época reproductiva (y la oportunidad para volver a anidar) fue mayor a menores elevaciones para petirrojos ( Turdus migratorius ) y chipes amarillos ( Dendroica petechia ). Las reservas naturales estuvieron a mayor altura * email [email protected] Paper submitted December 19, 2000; revised manuscript accepted August 15, 2001.