Classic metapopulation models assume that changes to occupancy in local populations depend on environmental variables that occur at the landscape or patch scale, including patch size and isolation. Under this paradigm, variation in habitat quality at the local-scale is often overlooked, but these variables can be important factors in population dynamics. Metapopulation models should incorporate both local-scale and patch-scale variables to accurately identify and describe the environmental mechanisms that drive population dynamics. Beyond the inclusion of variables from multiple scales, the dynamic processes of colonization and extinction are also scale-dependent. Colonization and extinction can happen both within patches and across patches, and a single-scale analysis may conflate these two processes, each of which likely has its own drivers. We present a spatially hierarchical dynamic occupancy model that allows multi-scale analysis of within-patch and between-patch occupancy patterns. We illustrate this model using the Black-backed Woodpecker (Picoides arcticus), a fire-reliant species that specializes on resources within a patchy network of recently burned forests. Our hierarchical approach allows examination of why certain post-fire areas become colonized or remain occupied, while also accounting for the intra-fire occupancy changes that are expected to occur as habitats rapidly change following fire.
Results/Conclusions
Environmental covariates of colonization and persistence took different forms and levels of importance when comparing across the patch-scale to the local-scale. Elevation, latitude and fire-initiation date demonstrated the strongest associations with patch-level colonization and persistence, while local-level dynamics were strongly controlled by burn severity and snag density. Our findings indicate that decreasing occupancy in older fires is a result of declining local persistence combined with a decrease in patch-level colonization probability. Thus, local populations of black-backed woodpeckers decline to near-extinction in older fires because resident individuals leave the population (through emigration or death), and individuals in surrounding patches are unlikely to re-colonize, providing critical insight into landscape-level dynamics of this species. Metapopulation dynamics are conventionally viewed as arising at the patch-level, providing limited means for understanding the impacts of within-patch heterogeneity. However, conservation and management actions are typically aimed at improving habitat quality or availability at the local-level. A cross-scale examination of these processes allows managers to identify and target specific local characteristics to improve overall metapopulation persistence.