Ecological succession is the replacement of one ecological community by another in the same area over time because biotic and abiotic conditions change. In IB ESS subtopic 2.5, this is core content for both SL and HL.
How Ecological Succession Works
Succession usually proceeds through a sequence of seral communities, with each stage called a sere. Early pioneer species, such as lichens on bare rock, modify environmental conditions by contributing organic matter, retaining water and supporting soil formation.
These changes allow new species to colonize. Competition then causes some species to replace others, while soil depth, nutrient cycling, biomass and species diversity generally increase. The sequence may eventually produce a relatively stable climax community, although disturbances can interrupt or redirect it.
| Type | Starting conditions | Example |
|---|---|---|
| Primary succession | Begins on newly formed substrate without soil or a pre-existing community | Colonization of volcanic surfaces after the 1883 Krakatau eruption |
| Secondary succession | Begins where soil remains after the removal or disturbance of an existing community | Regrowth of vegetation on abandoned farmland or after a forest fire |
A common misconception is that succession is simply seasonal change. It is not: succession involves the long-term replacement of communities. It must also be distinguished from zonation, which describes spatial changes in a community along an environmental gradient, rather than changes at one location over time.
Succession can increase ecosystem resilience by developing larger storages, more complex food webs and greater diversity. However, human activities such as grazing, burning or repeated cultivation may prevent the expected climax community from forming and create a plagioclimax. This term and challenges to the climax-community model are HL-only extensions.
IB Exam Technique
For an explain question, give a causal sequence: pioneer colonization, environmental modification, competition, community replacement and increasing system complexity. Name a real example and distinguish primary from secondary succession. In data-based questions, connect changes in soil depth, productivity or diversity to the relevant seral stage rather than merely describing the graph.