During ecological succession, productivity, species diversity, and soil depth generally increase from the pioneer stages toward a mature community. However, productivity and diversity may eventually level off or decrease slightly, whereas soil usually continues developing.
Level and subtopic: SL and HL, 2.5 Zonation, succession and change in ecosystems. Detailed changes in gross productivity (GP) and net productivity (NP) are HL only.
The Mechanism
Succession is the replacement of one community by another over time as organisms modify biotic and abiotic conditions. These changes create new habitats and resources, allowing later seral communities to replace pioneer communities through competition.
| Variable | General change during succession | Reason |
|---|---|---|
| Productivity | Initially low, then rises rapidly; it may level off in later stages. | Pioneer communities have little biomass and limited photosynthesis. As plant cover and biomass increase, more light energy is converted into chemical energy. |
| Species diversity | Usually increases, then stabilizes; it may decline slightly in some mature communities. | Improving soil and more complex habitats create additional niches. Later, strong competitors may exclude some species. |
| Soil depth | Increases progressively. | Weathering, decomposition, and the accumulation of dead organic matter produce deeper, more structured soil. Roots also break up the substratum and reduce erosion. |
For HL, GP increases as producer biomass develops. NP is often high during intermediate stages because GP exceeds respiratory losses, but it may decrease toward maturity as the respiration of the larger community rises.
Primary succession at Glacier Bay, Alaska, illustrates these trends: newly exposed surfaces are colonized by pioneers, followed by increasingly complex vegetation as soil depth, biomass, and diversity develop.
A common misconception is that every variable increases continuously. Succession is a generalized model: disturbance, climate, grazing, and competition can alter the pattern, and mature ecosystems are not necessarily static.
Exam Technique
For an explain question, state each trend and link it to a cause. Distinguish succession, which occurs over time, from zonation, which describes community change along a spatial environmental gradient.