Drought-mediated extinction of an arid-land amphibian: insights from a spatially explicit dynamic occupancy model.

Understanding how natural and anthropogenic processes affect population dynamics of species with patchy distributions is critical to predicting their responses to environmental changes. Despite considerable evidence that demographic rates and dispersal patterns vary temporally in response to an array of biotic and abiotic processes, few applications of metapopulation theory have sought to explore factors that explain spatiotemporal variation in extinction or colonization rates. To facilitate exploring these factors, we extended a spatially explicit model of metapopulation dynamics to create a framework that requires only binary presence-absence data, makes few assumptions about the dispersal process, and accounts for imperfect detection. We apply this framework to 22 yr of biannual survey data for lowland leopard frogs, Lithobates yavapaiensis, an amphibian that inhabits arid stream systems in the southwestern United States and northern Mexico. Our results highlight the importance of accounting for factors that govern temporal variation in transition probabilities, as both extinction and colonization rates varied with hydrologic conditions. Specifically, local extinctions were more frequent during drought periods, particularly at sites without reliable surface water. Colonization rates increased when larval and dispersal periods were wetter than normal, which increased the probability that potential emigrants metamorphosed and reached neighboring sites. Extirpation of frogs from all sites in one watershed during a period of severe drought demonstrated the influence of site-level features, as frogs persisted only in areas where most sites held water consistently and where the amount of sediment deposited from high-elevation wildfires was low. Application of our model provided novel insights into how climate-related processes affected the distribution and population dynamics of an arid-land amphibian. The approach we describe has application to a wide array of species that inhabit patchy environments, can improve our understanding of factors that govern metapopulation dynamics, and can inform strategies for conservation of imperiled species.

Importance of environmental and spatial gradients on patterns and consequences of resource selection.

Strategies to conserve rare species require identifying resources that function as important habitat elements and that promote high demographic performance. Assessing the relative importance of resources, however, can be confounded by natural variation in resource availability and by the hierarchical spatial structure in which resources are organized. Because availability and relative importance of resources often vary across environmental and spatial gradients, we used gradients together with resource selection functions and variance decomposition to assess the relative importance of resources to nest site selection and reproductive performance of Ferruginous Pygmy-Owls (Glaucidium brasilianum). We measured habitat characteristics of 106 nests and paired available sites at five spatial scales across a 220-km gradient of precipitation and vegetation in northwest Mexico, in a region adjacent to the southwestern United States where pygmy-owls have declined to near extinction. Resources explained 76-85% of variation in nest site selection and 21-31% of variation in reproductive performance across all spatial scales combined. Although we found evidence of resource selection at each scale, the magnitude of selection and influence of resources on reproductive performance were greatest where availability of selected resources were low and where temperature extremes and predation risk likely increased the relative importance of these resources. At larger scales, geographic changes in resource use corresponded with changes in availability, whereas at smaller scales, resource use varied little despite changes in availability, suggesting higher specificity and importance of resources at smaller scales. At the smallest scale, owls selected nest cavities with smaller entrances, larger volume, greater height, and orientations that produced cooler microclimates in the hottest regions of the study area; these choices promoted higher reproductive performance. Cavity resources explained more variation in selection and reproductive performance than resources at larger scales, highlighting their importance as conservation targets. High correlation of resource characteristics among spatial scales, however, indicated that selection of resources at small scales depended on characteristics of resources at larger scales. Assessing how resource selection changes in response to underlying variation in resource availability can help prioritize resources most important for conservation and management.