Fig. 2. Representative modes of population and species-range response to environmental changes since the last glacial maximum, documented for selected North American conifer trees and Eurasian cervids. Populations of many species have persisted in situ at individual sites since the last glacial maximum (toleration) and many have undergone habitat shifts, moving short distances (1 to 10 km) to sites with different aspects, slopes, elevations, and other attributes as the environment changed. Migrations of 100 to 1000 km are well documented for many species. Both migration and habitat shift are forms of environment tracking, in which species adjust their geographic locations to track suitable environments. At least a few species have undergone universal extinction (e.g., Megaloceros giganteus) owing to environmental
change; others have experienced loss of genetic diversity, usually associated with severe population bottlenecks (near-extinction episodes) (e.g., Picea martinezii). Species’ responses to climate change may consist of multiple modes. For example, since the last glacial maximum, populations of Juniperus osteosperma and Alces alces have persisted at some sites (toleration), undergone habitat shifts (usually elevational or topographic) within some regions, and colonized extensive new territory while disappearing from previously occupied territory (migration). Alces alces has also undergone a severe genetic bottleneck. Differences among modes within and among species depend on rates, magnitudes, and geographic patterns of climatic change, the capacity of species populations to adapt (via phenotypic plasticity, evolution, and/or dispersal), and other factors (e.g., geographic barriers, and other stressors and interactions). References, additional examples, and detailed discussion are provided in the supporting online material.