Fig. 2.
Colonization-extinction dynamics as mediated by shifting climates lead to changes in position of occupied range centroids relative to the climatic niche. (A) Four sites (circles labeled 1–4) within a geographic area (black ellipse) experience shifts in climate over time (dotted arrows), moving sites from a prior climatic position (blue circles) to a current climatic position (red circles). For a hypothetical species with a certain climatic niche defined by temperature and precipitation (light blue ellipse), a site can be unoccupied in both time periods if it remains outside the climatic niche (site 1), go extinct if the site shifts out of the climatic niche (site 2), stay occupied in both time periods if it remains inside the climatic niche (site 3), or be colonized if the site enters the climatic niche (site 4). (B) The centroids of the observed occupied ranges for a species in each time period (asterisks: blue for historic and red for current) can provide evidence of niche tracking when compared with the centroid of a species' climatic niche (gray cross). If the temperature or precipitation components of the vector from the historic range centroid to the climatic niche centroid (HT and HP, respectively) agree in sign with the corresponding climatic components of the vector from the historic range centroid to the current range centroid (RT and RP), then there is evidence for tracking for that component. (C) Individual sites can be defined by vector components describing the position of a site (e.g., site 4) either historically (hT and hP) or currently (mT and mP) relative to the climatic niche centroid. These site-specific vectors are used in combinations as covariates of colonization and extinction in occupancy models. Examples of movements of range centroids for three species show different levels of climatic niche tracking (for further details and other species, see Table S1). (D) Lazuli Bunting showed niche tracking of both temperature and precipitation, shifting to a cooler and wetter occupied range. The light blue circle is a 95% density ellipse around the full range of historic specimens that defined the climatic niche centroid. (E) Townsend's Solitaire showed niche tracking of temperature, but not precipitation. (F) Nuttall's Woodpecker tracked neither temperature nor precipitation components of the climatic niche.