An Environment‐metapopulation Approach to Population Viability Analysis for a Threatened Invertebrate

: Most previous population viability analyses of endangered species have focused on large vertebrates: long-lived species with low rates of population increase, long generation times, and comparatively low habitat specificity. Habitat fragmentation not only reduces the distribution of such species, but reduces population densities to levels at which genetic and demographic constraints threaten population persistence. Many other endangered species, in contrast, are characterized by small body size, high rates of population increase, short generation times, and high habitat specificity. Habitat fragmentation reduces distributions of such species, but within remnant habitats population densities may continue to be high. Population viability analyses for these species — which include many small vertebrates, invertebrates, and plants — must focus on the environmental factors and metapopulation characteristics that determine population persistence. Population viability analysis for the threatened Bay checkerspot butterfly (Euphydryas editha bayensis) exemplifies the environment-metapopulation approach. Variation in thermal conditions and rainfall through time (macroclimate) and across local topography (topoclimate) drives the population dynamics of this insect. Because of the great sensitivity of larvae and host plants to thermal differences between slope exposures and to annual variation in rainfall, extinction of local butterfly populations is common. Both habitat patch quality (size, topographic diversity, and resource abundance) and distance from a reservoir population affect the likelihood of patch occupancy. An understanding of metapopulation dynamics, therefore, is necessary to explain the regional distribution of the butterfly at any given time. Although parameterization of key life stages of the Bay checkerspot butterfly is not yet complete, insights from previous studies can be used to help make decisions in reserve design and management of the species. Resumen: Los analisis previamente realizados sobre la viabilidad de las poblaciones de especies en peligro de extincion, se han concentrado en vertebrados grandes: especies longevas, con tasas bajas de incremento poblacionul, periodos generacionales largos y, comparativamente, baja especificidad de requerimientos de habitat. La fragmentacion del habitat no solamente reduce la distribucion de dichas especies, sino tambien, reduce las densidades poblacionales a niveles en los cuales las limitaciones geneticas y demograficas amenazan la persistencia de las poblaciones En contraste, muchas otras especies en peligro de extineion se caracterizan por ser pequenas de tamano, tener tasas altas de incremento poblacionul, periodos generacionales cortos y una alta especifcidad de habitat. La fragmentation del habitat reduce la distribucion de dichas especies, pero su densidad poblacional puede continuar siendo alta dentro del habitat remanente. Los anaisis de viabilidad poblacional para estas especies, que incluyen a muchos vertebrados pequenos, invertebrados y plantas, deben enfocarse en factores ambientales y caracteristicas metapoblacionales que determinan la persistencia de sus poblaciones. El analisis de viabilidad poblacional de la mariposa Bay checkerspot (Euphydryas editha bayensis) ejemplifica el enfoque ambiental y metapoblacional. Las variaciones de las condiciones termicas y pluviales, a travez del tiempo (macroclitma), al igual que a traves de la topografia local (topoclima), determina la dinamica de poblaciones de este insecto. La extincion de poblaciones locales es comun, debido a la gran sensibilidad de las larvas y las planta anfitrionas a las diferencias temicas entre pendientes de diferente exposicion y a las variaciones anuales de la pluviosidad. La calidad de los segmatos de habitat (tamano, diver-suad topografica, y abunduncia de recursos) y su distancia de “reservorios de poblacion” afecta la posibilidad de ocupacion de un segmento de habitat individual. El enten-dimiento de la dinamicade una metapoblacion es, entonces, necesario para explicar la distribucion regional de la mariposa en cualquier momento dado en el tiempo. Aunque laparametrikacion de las etapas claves en la vida de la mariposa Bay checkerspot no ha sido aun completada, conocimientos generados por estudios previos pueden ser utilizados para asistir en la toma de desciciones para el disetno de reservas y el manejo de especies.

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