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

Abstract

Abstract: 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.