Ecological succession

An old story: bare rock to forest

Dig a pond, leave it alone, and come back in a hundred years. The pond may be a marsh. Come back in a few hundred years and you may find a forest. Take a bare rock instead. After a long time, you may find lichens, then mosses, then grass, then shrubs and trees.

This whole lesson is outside the NEET syllabus. Ecological succession was cut from the rationalised NCERT chapter, and the NTA text does not list it. A-Level, IB and AP ask it, so this paid course teaches it.

What succession means

Ecological succession is the gradual and fairly predictable change in the species composition of a given area. A set of communities replaces another, one after the other, until a stable community is reached.

• The first species to settle in an area are the pioneer species.
• Each community in the series is a seral stage (or seral community). The whole series is a sere.
• The last, stable community, which is in near equilibrium with its environment, is the climax community.

Succession is not blind. Each community changes the place. The pioneers make soil and shade. The new conditions suit the next community better than the pioneers, and the pioneers are replaced.

Primary and secondary succession

• Primary succession begins in a place where no living organism has ever lived, or where everything, even the soil, has gone. Examples are newly cooled lava, bare rock, and a newly created pond or reservoir. There is no soil, so the early stages must make soil. This takes a very long time, hundreds or thousands of years.
• Secondary succession begins in a place that has lost all its living organisms, but where soil or sediment is left. Examples are abandoned farm land, burnt or cut forest and flooded land. Seeds and spores are in the soil, so the process is much faster.

Two seres

On bare rock the sere is xerarch. A common order is: crustose lichens, foliose lichens, mosses, annual herbs and grasses, perennial herbs, shrubs and trees. The lichens give off acids that break the rock, and the dead lichens and mosses make the first thin soil.

In water the sere is hydrarch. A common order is: phytoplankton, submerged plants, floating plants, a reed swamp, a marsh meadow, shrubs and then trees. The dead plants and the mud settle, the water gets shallower, and the land slowly takes over. In both seres the climax is a forest, if the climate is wet enough for it.

What changes as succession goes on?

The usual trends, in a sere are these.

• The number of species increases.
• The biomass increases, and the community becomes more stable.
• The net primary productivity is high in the middle stages and falls towards the climax. The respiration of the large community nearly equals its gross production, so the ratio of gross production to respiration approaches 1.

Why does one stage give way to the next?

A model of three ideas is used at A-Level and IB (Connell and Slatyer, 1977). In facilitation, early species change the place and make it better for the next species. In tolerance, later species can grow in the early conditions but are slower. In inhibition, the early species keep the later ones out until they die or are damaged. All three can happen in one sere.

A model of the time it takes

Think of the sere as a line of stages. In each decade, the community has a small chance of moving on to the next stage. The mean waiting time in a stage is $1$ divided by its chance, in decades. The soil making stages have a small chance, so they wait long. The later stages are quicker. Slide the numbers in the model below.

You should find that secondary succession is several times faster. It skips the slow, soil making stages.

Try it in Python

The program plays the chain of six stages from bare rock to forest. It checks the sum of the waiting times against the mean of 20000 random runs.

The second program works out the production and respiration at three stages and the ratio between them.

Practise with new numbers

Common mistakes

• Saying that primary succession starts on soil. It starts where there is no soil, or no life at all.
• Giving the climax as a fixed list of species. It is a stable community in near equilibrium with its climate. Its species depend on the place.
• Saying that productivity rises until the end. The net primary productivity falls near the climax.
• Mixing the two seres. Xerarch starts on dry rock. Hydrarch starts in water.

Remember these

• Succession is a gradual and fairly predictable change in the species composition of an area.
• Pioneer species come first. The stages are seral stages. The last stage is the climax.
• Primary succession: no soil. Secondary succession: soil is left, so it is faster.
• Xerarch: rock to forest. Hydrarch: water to forest.
• Along a sere: more species, more biomass, a ratio of production to respiration near 1 at the climax.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Define ecological succession.

2. What is the difference between primary and secondary succession?

3. Name the first stage of a xerarch sere and the first stage of a hydrarch sere.

4. Why is secondary succession faster than primary succession?

5. Why does the net primary productivity fall near the climax?

References

• OpenStax Biology 2e (2018), chapter 45, Population and Community Ecology, section on community ecology.
• Campbell Biology, 12th edition (2021), Pearson, chapter 54, Community Ecology, section on disturbance and succession.
• Connell, J. H. and Slatyer, R. O. (1977). Mechanisms of succession in natural communities and their role in community stability and organization. The American Naturalist 111, 1119 to 1144.