Sky islands are isolated mountains surrounded by radically different lowland environments. The term originally referred to those found on the Mexican Plateau and has extended to similarly isolated high-elevation forests. The isolation has significant implications for these natural habitats. The American Southwest region began warming up between c. 20,000 and 10,000 years BP and atmospheric temperatures increased substantially, resulting in the formation of vast deserts that isolated the sky islands. Endemism, altitudinal migration, and relict populations are some of the natural phenomena to be found on sky islands.

In biogeography and paleontology, a relict is a population or taxon of organisms that was more widespread or more diverse in the past. A relictual population is a population currently inhabiting a restricted area whose range was far wider during a previous geologic epoch. Similarly, a relictual taxon is a taxon which is the sole surviving representative of a formerly diverse group.