On climax communities and neutral theory

We started our second session of Critical Thinking with a discussion on Gleason’s 1927 paper on the succession concept. Similarly to the previous older papers we talked about, we noted the differences in writing style compared to how papers are written today. In opposing the previous ideas on succession by Clements, Gleason often uses rather harsh language. He does provide multiple examples as evidence for his theory. We compared both views and discussed the differences.

Clements sees vegetation as a whole organism that goes through different life stages – youth, maturity, death. He talks about a ‘climax’ community – the state to which the organism is ultimately headed and where equilibrium will be achieved. Here, we defined equilibrium as the state where birth rates and mortality are equal, competition is shaping the community and there is usually one dominating species. We discussed whether in todays dynamically changing world climax communities can exist and provided two possible examples – the remnant patch of old growth rainforest Daintree in Australia and boreal forests in places like Northern Canada and Alaska.

Gleason puts forward the idea that disturbance prevents ecosystems from ever reaching the climax community. We agreed that this is especially true today, when the consequences of human activities can dominate natural processes.

The next paper we focused on was Hubbell’s “Tree Dispersion, Abundance, and Diversity in a Tropical Dry Forest”. He examined tree distribution on the Barro Colorado Island in Panama. This is a man-made island in the Panama Canal famous for its research on rainforest ecology. Based on his observations at Barro Colorado, Hubbell went on to postulate his Neutral Theory. According to Neutral Theory, biodiversity arises at random, as each species fallows a random walk. At each trophic level species are considered equal, e.g. the differences between their birth and death rates are neutral. In effect, neutral theory is the null hypothesis to niche theory where species evolve to fill a certain niche and function.

Neutral theory is proposed to be most valid when the system is ‘saturated’, e.g. in high biodiversity areas such as coral reefs and tropical rainforests. However, the theory remains controversial as there has been evidence put forward by Dornelas et al. (2006) indicating that coral reef diversity refutes it.

To finish off, we talked about why there are so many species of trees in tropical forests. We discussed the following possibilities and in the end agreed that it is most likely a combination of factors acting simultaneously.

  • Neutral theory – thought to work in super high diversity areas like rainforests (‘saturated’)
  • Niche theory – finely split niches in rainforests, more species evolve to fill them
  • Stable benign climate – rainforests tend to be around the Equator where climate has remained relatively constant, e.g. no glaciations; low disturbance, low extinction rates, high speciation rates
  • Janzen-Connell hypothesis – density-dependent mortality, diseases and pests concentrate near the parent, so surviving sapling are spaced further apart, supposed to decrease clumping and increase diversity
  • Energy – rainforests are high energy zones, lots of sunlight and rain, increased photosynthesis; higher energy -> more biomass -> increased rates of evolution?
  • History and Dispersal – valid on a more local scale; relate to plate tectonics, continental drift; dispersal of species

By Gergana

Note from Isla: We also chatted about Ecological Societies and the Scientific Literature.  We talked about the different journals that the papers we are reading have been published in such as Science, Nature, Ecology, American Naturalist, Ecological Monographs, etc. And who publishes Science, Nature and the major ecology journals – Scientific societies or publishing companies.  We briefly mentioned the concept of a journal impact factor and how we decide which journals are cooler than others.  We discussed how the British Ecological Society is holding it’s annual meeting in Edinburgh this December, which will be an exciting opportunity to check out the latest in ecological research right in our own city!

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