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This lesson builds directly on 2.5.1 to 2.5.8. You already understand what succession is, the difference between primary and secondary succession, and how energy flow, species diversity, soil depth and nutrient cycling change during succession. This lesson asks: what determines the type of community that actually develops, and what happens when something prevents or diverts the process?

What Shapes the Final Community?

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2.5.9 The type of community that develops in a succession is influenced by climatic factors, the properties of the local bedrock and soil, geomorphology, together with fire and weather-related events that can occur. There can also be top-down influences from primary consumers or higher trophic levels.

Include factors such as steep slopes restricting soil development, lack of drainage causing waterlogging, or underlying/parent rock causing ultra-basic or other extreme soil types to develop. Living organisms, such as wolves in Yellowstone Park or elephants in savannahs, influence the final community.

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The climax community is often described as the community "in equilibrium with the prevailing climate." But climate alone does not determine what develops. A range of abiotic and biotic factors can prevent succession from reaching the expected climax, or divert it towards a different community entirely.

Abiotic factors

Climatic factors

Temperature, precipitation and insolation set the broad limits. A location with the same bedrock and slope in northern Finland and central France will develop very different communities because the climate determines which species can survive.

Bedrock and soil properties

The parent rock determines the chemistry of the soil that forms. Some rock types produce soils so chemically extreme that most plant species cannot grow in them.

Example: The serpentine soils of the Troodos Mountains, Cyprus

The Troodos massif is composed of oceanic crust that was pushed above sea level. Its ultra-basic rocks weather to produce soils with very high concentrations of nickel and chromium and very low levels of essential nutrients such as calcium and potassium. Most plant species cannot tolerate these conditions. Instead of the mixed Mediterranean woodland predicted by the climate, the serpentine areas support a sparse community of specialist metallophyte plants, many of which are endemic to Cyprus. Succession is effectively halted by soil chemistry.

Geomorphology (slope and drainage)

Steep slopes restrict soil development because weathered material is removed by gravity before it can accumulate. Scree slopes and cliff faces may remain at an early seral stage indefinitely.

Poor drainage causes waterlogging, which creates anaerobic conditions in the soil. This slows decomposition, prevents nutrient cycling and halts succession.

Example: The Flow Country, Caithness and Sutherland, Scotland

This is one of the largest blanket bog areas in the world (approximately 4,000 km²). The flat topography, impermeable bedrock and high rainfall mean the soil is permanently saturated. Instead of the boreal or temperate forest predicted by latitude and temperature, the community is dominated by Sphagnum mosses, cotton grass and sundews. Waterlogging prevents tree establishment and halts succession in a bog community.

Fire and weather-related events

Example: Fynbos, Western Cape, South Africa

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The fynbos biome depends on fire. Many fynbos species have fire-adapted traits: hard seed coats that crack in heat, underground storage organs that resprout, and serotinous cones that release seeds only after burning. Without regular fire (every 10 to 30 years), the fynbos is gradually replaced by taller Afrotemperate forest. Fire resets the succession and maintains the shrubland community. This is a natural fire-dependent community, not a plagioclimax.

  1. State three categories of abiotic factor that influence the type of community that develops during succession.

  2. The climate of the Troodos Mountains in Cyprus would normally support Mediterranean woodland. Explain why this does not develop on the serpentine soils.

  3. Explain how poor drainage in the Flow Country prevents succession from reaching a forested climax community. In your answer, refer to the effects of waterlogging on decomposition and nutrient cycling.

  4. The fynbos of South Africa depends on regular fire. Explain why this is considered a natural process rather than a human-caused plagioclimax.

Top-down biotic influences

The syllabus also requires us to consider top-down influences from primary consumers or higher trophic levels. These are cases where animals shape the plant community through grazing, browsing, trampling or predation.

Wolves in Yellowstone National Park

Grey wolves were extirpated from Yellowstone by the 1920s as part of a government predator control programme. Without wolves, elk populations grew and browsed heavily on willow, aspen and cottonwood along riverbanks. Streamside vegetation declined, riverbanks eroded and beaver populations collapsed (beavers depend on willow).

When 31 wolves were reintroduced in 1995 and 1996, elk changed their behaviour, spending less time in exposed riparian areas. Willow and aspen regenerated, beavers returned, and the riparian community shifted towards a more complex, multi-layered structure. The presence of a top predator changed the entire trajectory of community development.

This is an example of a trophic cascade: a top-down effect that ripples through multiple trophic levels.

Elephants in savannah ecosystems

African elephants feed on saplings and small trees, push over mature trees and trample ground vegetation. By reducing tree density, elephants maintain the open grassland characteristic of the savannah biome. Without elephants, many savannah areas would succeed to dense woodland. Elephants are therefore a top-down influence that shapes the final community.

Sea otters in the North Pacific

Sea otters feed on sea urchins, which graze on kelp. Where sea otters are present, urchin numbers are controlled and dense kelp forests develop. Where otters have been removed (historically through the fur trade), sea urchin populations explode, overgrazing the kelp and creating "urchin barrens", a rocky seabed with almost no macroalgae. The community that develops depends entirely on the presence or absence of a single predator species.

  1. Define the term "top-down influence" in the context of succession.

  2. Explain how the removal and reintroduction of wolves in Yellowstone demonstrates that top-down influences can change the type of community that develops.

  3. Elephants are sometimes described as "ecosystem engineers." Using the savannah example, explain why this term is appropriate.

  4. The sea otter example shows that removing a single species can shift an ecosystem between two very different states. What term from Topic 1.2 describes this kind of shift?