Ecological succession: What controls the climax community?

What is a climax community?

Climax community the final community in the process of ecological succession. The climax community is expected to be in equilibrium with local environmental conditions and is considered stable.

This stage of ecological succession has been the object of much scrutiny and attention from ecologists, merely because it is rare and so many different possibilities can theoretically arise, depending on the path of succession.

Features of climax communities

However, all our studies are shown that every climax community has a certain specific set of characteristics-

• They have high tolerance to ecological disturbances. They don’t go down easy.
• They have a “middle path”, ie. they have moderate conditions, also called mesic conditions.
• They have high species diversity, and transfer of energy is in the form of complex food webs, not simple food chains.
• The size of organisms in the community is large, and they all have their specific niches.
• Net community production is low, while biomass and organic matter is high.
• Mineral cycles and nutrient exchange in the community is slow.

But this is where the unanimous agreement of ecologists end. How does the climax community originate and what controls their composition? This has been a matter of great debate for much of the 20th century. As with any debate, certain arguments shine through. Three theories have taken the forefront, and the ecological community is split between them.

Monoclimax theory

This theory states that the generation of a climax community is solely the control of the climate of the region. Therefore, the climax community becomes the “index” of that region. This theory was put for by Clements, when he studied communities in the early part of the 20th century.

He argued that within the same type of climate, the adaptations and requirements of organisms remain the same. It also possible that two communities may have formed with the presence of the same pioneer species. Hence, the way they modify their surroundings (which eventually leads to change in communities) will also be largely the same. Consequently, each climactic region is characterized by climax communities where a few specific species are dominant in the climax community.

This theory recieved backlash for proposing that climate is the sole control of climax communities. According to Daubenmire (1968), believing in this theory meant that admitting all factors other than climate are secondary in nature.

What is the alternative? It is the possibility that all environmental factors are equally important, and accordingly, many climax communities can exist.

Polyclimax theory

This suggests just that. According to a controlling influence by each environmental factor (which becomes dominant is purely a matter of chance), there can be several climax communities. This was proposed by Tansley in 1935. According to this, there is a particular climax community for a region called climactic-climax community. However, other factors can prevent the community from reaching this stage.

For example, consider a community that has been affected by a road construction project. This area has an overriding influence enforced by humans; therefore, it cannot reach it’s climactic-climax. One of the earlier seral stages may get stabilized in such a situation and forms a “pre-climax” or a “sere-climax”.

It is also possible that an an entirely new community can get established, that is vastly different from the expected climactic-climax. Since the overriding influence was of humans, the climax is called anthropogenic climax.

Different climaxes, based on different factors having an overriding influence, have thus been proposed.

Climax-Pattern Hypothesis

This was an entirely new concept proposed by Whittaker in 1953. According to him, the climax community was controlled by all environmental factors equally. Therefore, no one climax is established in isolation. A series of climax communities are established, parallel to each other depending on the changing environmental gradients.

Which to believe?

I leave that to you. However, I would like to share what I feel about this debate. As I have repeatedly written before in my posts, nature is hopeless complex. It involves multiple dimensions and all resulting interactions cannot be perceived by humans. This is one of the reasons Whittaker’s climax-pattern hypothesis remains a hypothesis; it is yet to be proved on the field.

Considering this multidimensional aspect of nature, I agree with the polyclimax theory as far as saying that many different factors can influence the occurrence of the climax community and its composition. There will also be some factors more dominant than others in a given area. However, it is also important to understand that communities never occur in isolation. There is always a gradual change from one community to another. Therefore, in a given region with changing communities, the environmental factors will also change accordingly. This will also have an impact on the resulting community composition. Different community structures based on different environmental parameters can occur parallel to each other as these conditions gradually change from one to another.

For example, a community of trees at a particular elevation will gradually change into another community composition as the elevation changes. Along with this change in elevation, temperature and soil conditions will also change. Different permutations and combinations arise with just these three factors (3×3=9), resulting in the possibility that any number of climax communities out of the possible 9 can occur in that area simultaneously.

So, I believe in a combination of polyclimax theory and climax-pattern hypothesis (weird, I know).

What do you think? Comment below!