In ecology, a climax community is a biological association of flora and fauna which has attained an equilibrium condition. Such a circumstance comes about because the climax community is composed of species best adapted to conditions in that specific locale, and as an association this assembly of organisms has out-competed predecessory ecosystems in this microclimate and soils niche; in reality, there are numerous abiotic factors that comprise the set of niche parameters, including soil pH, nutrients available and topography. The attainment of a climax community arises through a process of ecological succession, e.g. the development of vegetation and fauna in an area over time; in fact, properly speaking the community includes lower lifeforms such as bacteria, algae and archaea. The climax designation is sometimes also applied in soil temporal development.
Origin of the concept
The idea of a single climax ecological state, which is alternatively called climatic climax community, originated with Frederic Clements in the early twentieth century. The first analysis of succession as leading to a climax phenomenon was actually articulated by Henry Cowles in 1899, but it was Clements who used the term climax to describe the idealized endpoint of ecological succession.
Clements described the successional development of an ecological communities as comparable to the ontogenetic development of individual organisms. Clements suggested only comparisons to very simple organisms. Later ecologists developed the concept that the ecological community is a superorganism, and some scientists even posited that communities could be homologous to complex organisms.
Clements's formalisms sought to define a single climax-type community for each geographic area. Arthur Tansley embellished this theory with the concept of polyclimax -- multiple steady-state end-points, determined by edaphic factors, in a given climatic zone. Clements had termed these end-points by other appellations, not climaxes, deeming them unstable, because by definition climax vegetation is intrinsically that plant community optimally-adapted to the climate of a given locale. Henry Gleason's early challenges to the Clements organism simile, and other of his strategies for describing vegetation, were largely disregarded for several decades until substantially vindicated by research in the 1950s and 1960s. Meanwhile, climax theory was deeply incorporated in both ntheoretical ecology and in vegetation management. Clements's terms such as '''pre-climax''', '''post-climax''', 'plagioclimax and '''disclimax''' continued to be used to describe the many communities which persist in states that diverge from the climax ideal for a particular area.
Clements did not assert that climax communities necessarily occur in any given area, or that the dominant cause of vegetation is climate, or that the component species in an ecological community are closely physiologically integrated, or that plant associations are delimited by crisp boundaries in time or space. In contrast, Clements employed the idea of a climax community--of the form of plantlife best adapted to some idealized set of environmental conditions--as entry portal for describing flora in a given area. There are good reasons to believe that the species best adapted to some conditions might appear there, when those conditions occur.
A considerable fraction of Clements's work was directed at characterizing outcomes for situations where such idealized conditions do not occur. In those cases, flora communities other than the ideal climax may occur. But such different vegetative assemblies can then be described as deviations from the climax ideal. Therefore Clements developed a very large vocabulary of theoretical terms describing the various possible causes of vegetation, and various non-climax states vegetation adopts as a consequence. His method of dealing with ecological complexity was to define an ideal form of vegetation--the climax community--and describe other forms of vegetation as deviations from that ideal.
Examples of climax community
Some examples of generally recognized climax community, which have achieved stability or equilibrium over a period of hundreds of thousands of years, and in some cases millions of years:
- Sonoran Desert, USA and northern Mexico
- Daintree Rainforest, Queensland, Australia
- Namib Desert, Namibia and southwestern Angola
- Mixed podocarp/broadleaf forest, Rakiura Island Ttemperate forests, Ulva Island, New Zealand
Counter to the concept
Support among ecologists for the climax theory declined, because some found the theory with its complex terminology difficult to apply, and because some scientists were dissatisfied how the theory compared to observations of individual organisms within the total community.
Although Clements recognized that vegetation follows abiotic gradients rather than being tightly bound, his rhetorical comparisons of ecological communities to organisms fostered the impression that communities, including the climax, have distinct boundary limits in space and time. However, Whittaker's work showed that plant species distribute themselves along nutrient and other abiotic gradients. Many ecologists saw this as a major reason to desist from employing the concept of climax community.
More recent palynological studies showed that modern species assemblages are ephemeral; vegetation in eastern North America since the last glacial maximum has consisted of several different species assemblages, many of which have no analogues in modern climax communities. That would mean, at least, that the climax types for those areas could not be stable to the degree Clements believed they were.
Ultimately, even if succession tends towards a steady state, the time required to achieve this state is quite long relative to time scales of external intrusion (e.g. in-migration of new or alien species) or disturbance regimes (volcanic eruptions, tsunamis) changing the landscape; in most cases, external disturbances and environmental change occur so frequently that the realization of a climax community is unlikely, and therefore it has come to be regarded as a less useful concept. Long-term vegetation dynamics are now more often characterized as resulting from the action of stochastic factors.
Continuing usage of climax
Despite partial abandonment of climax theory, during the 1990s use of climax concepts again became more popular among some theoretical ecologists such as Roughgarden. Many authors and wildlife observers continue to use the term climax in a diluted form to refer to what might otherwise be called mature or old-growth communities. The term "climax" has also been adopted as description for a late successional stage for marine macroinvertebrate communities.
- Frederic E.Clements,. 1916. Plant Succession: An Analysis of the Development of Vegetation. Washington D.C.: Carnegie Institution of Washington.
- James E.Cook. 1996. Implications of Modern Successional Theory for Habitat Typing: A Review. Forest Science 42(1): 67–75.
- Henry Chandler Cowles. 1899. The Ecological Relations of the Vegetation on the Sand Dunes of Lake Michigan. Botanical Gazette 27(2): 95-117; 27(3): 167-202; 27(4): 281-308; 27(5): 361-391.
- Christopher Eliot. Christopher. 2000. Method and Metaphysics in Clements’s and Gleason’s Ecological Explanations. Studies in History and Philosophy of Biological and Biomedical Sciences 38(1): 85–109.
- Joel B.Hagen. 1992. An Entangled Bank: The Origins of Ecosystem Ecology. New Brunswick: Rutgers University Press.
- R.Rosenberg, S.Agrenius, B.Hellman, H.C.Nilsson and K.Norling. 2002. Recovery of marine benthic habitats and fauna in a Swedish fjord following improved oxygen conditions. Marine Ecology Progress Series 234: 43-53.
- Jonathan Roughgarden, Robert M.May and Simon A.Levin, editors. 1989. Perspectives in Ecological Theory. Princeton: Princeton University Press.
- Ronald C.Tobey. 1981. Saving the prairies: the life cycle of the founding school of American plant ecology, 1895–1955. Berkeley: University of California Press.
- Robert H.Whittaker. 1953. A consideration of climax theory: the climax as a population and pattern. Ecological Monographs 23: 41–78.