Donald A. Falk
The progression of changes to Earth’s climate poses unprecedented challenges to the science and practice of ecosystem management. The viability of many populations, species, and even ecosystems is increasingly uncertain in their current form. Effects of climate change per se are compounded by multiple interacting stressors, including landscape modification and fragmentation, alerted disturbance regimes (particularly wildland fire), and the increasing presence of non-native invasive species. In framing a meaningful response to global environmental change, all of these interacting factors must be taken into account. For example, the ability of species to migrate in response to changing climate geography—as nearly all species have done during past eras of rapid climate change, such as the end of the last interglacial period—may be impaired by fragmented landscapes that pose barriers to movement; lack of co-evolved dispersal agents, pollinators, or other essential symbionts; the presence of vigorous and established populations of non-native species that can outcompete some natives under novel environmental conditions; and many other factors.
Ecological responses to climate variability and change are complex and notoriously difficult to model or predict. Part of this complexity arises because climate variability and change influence elements and processes at multiple levels of biological hierarchy: individuals, populations, species, communities, and entire ecosystems. As a consequence, scientists and land managers will have to find their way through the current period of rapid change with relatively few guideposts. For example, drought-tolerant genotypes—which could prove more adaptive under emerging climate conditions in many areas—occur within many widespread plant species, but where these occur and how rapidly these adaptations can spread among populations is generally unknown. Some species populations endemic to alpine and high mountain areas, or aquatic species, may become extirpated even as new suitable habitats develop elsewhere. At the community level, the ecological communities we see today will likely be reassembled due to the individualistic nature of species responses to change. At the highest level of organization, ecosystem processes such as landscape fire, soil formation, and hydrologic and biogeochemical cycling may be so altered under emerging conditions as to preclude the maintenance of some currently existing landscapes.