Global biome models have been used to evaluate the changes in vegetation distribution likely to occur with a changing climate. One limitation of the global models is that the output represents a vegetation distribution that is in equilibrium with climate, a condition that is unlikely to occur in the next century. The spread of tree species involves several factors, including dispersal, regeneration on a suitable site, maturation, and seed production. If climate changes faster than trees can disperse to new, more suitable areas, the composition of the forest may change and the survival of some species could be at risk. Global-scale models are also inadequate to evaluate the indirect effects of climate, such as disturbances from pests, disease, fire, flooding, and wind damage.
Many animals are able to respond to climate at a faster rate than plants.
For those animals that do not migrate, a distribution change in response to a warming trend would occur at the population level as a result of changes in the ratios of extinctions to colonizations at the northern and southern boundaries of the range. A northward range shift would thus be reflected in either a net extinction at the southern boundary or a net colonization at the northern boundary. Factors other than climate may limit the extent to which organisms can shift their ranges. Physical barriers such as mountain ranges or extensive human settlement may prevent some species from shifting to more suitable habitat. In the case of isolated mountain top species, there may be no new habitat at higher elevation to colonize.