IB Environmental systems and societies 2.5B Succession Notes

Succession: How an Ecosystem Builds Itself Over Time

1. Succession is the process by which a community develops and changes over time, from a few pioneer species to a complex, more stable ecosystem with high biodiversity.
2. It is a temporal change, which is what distinguishes it from zonation, the change in a community across space.
3. It is driven by the interaction of biotic and abiotic factors, because each community changes the conditions for the next.

Seral Stages: From Pioneers to a Climax Community

1. Each stage of succession is a sere (or seral stage), and the community living in it is a seral community.
2. Each seral community changes the environment, for example by building soil, which paves the way for the one after it.

1. Pioneer stage

1. The first colonisers are hardy pioneer species such as lichens, mosses, and algae.
2. They tolerate harsh conditions like temperature extremes, drying out, and low nutrients.
3. As they weather rock and then die, they add organic matter and build the first thin layer of soil.

2. Intermediate stage

1. As soil deepens, grasses and small plants take hold, improving water retention and nutrient cycling.
2. Roots stabilise the soil and microorganisms enrich it with nitrogen and organic matter.
3. Shrubs and small trees then outcompete the grasses, helped by taller growth and deeper roots.

3. Climax stage

1. The climax community is the final, relatively stable stage, in balance with the local climate and soil.
2. Species diversity, biomass, and nutrient cycling are all at their highest.
3. Soil is deep and rich, and complex food webs keep energy flow stable.
4. The type of climax depends on climate (climatic climax) and on local conditions such as soil (edaphic climax).
5. A single stable endpoint is the classical model; in practice a site may settle into one of several stable states, as covered in Ecosystem resilience.

Primary and Secondary Succession

Primary succession: starting from bare ground

1. Primary succession begins where no life existed before and there is no soil, such as new lava, bare rock, or land left by a retreating glacier.
2. Pioneer species colonise first and slowly build soil by breaking down rock.
3. As soil improves, grasses, shrubs, and trees replace the earlier species.
4. Surtsey, Iceland, formed by eruptions between 1963 and 1967, is a classic example on bare volcanic rock.

Secondary succession: starting from disturbed ground

1. Secondary succession happens where a community was disturbed but the soil and seed bank remain, for example after a wildfire, flood, or on abandoned farmland.
2. It is faster than primary succession because soil and seeds are already present.
3. Grasses and small plants appear first, followed by larger shrubs and trees.

What Changes as Succession Proceeds

1. As succession advances, productivity, diversity, soil, and nutrient cycling all shift in a predictable direction.

1. Early on, net primary productivity (NPP) is high because most captured energy goes into new growth.
2. At the climax, gross primary productivity (GPP) is balanced by respiration (R), so NPP approaches zero and biomass stops increasing.

Example

• In a sand dune succession, humus and nitrogen rise inland while soil pH falls, driving the shift from grasses to shrubs to trees.

Disturbance Resets Succession

1. Events such as fires, floods, or human activity can reset succession to an earlier stage.
2. Recovery afterwards is secondary succession, so it is faster because soil and seed banks survive.
3. Some ecosystems are fire-adapted, such as pine forests, and rely on periodic fires to regenerate.

Mapping and Monitoring Succession

1. Ecologists use GIS (Geographic Information Systems) and remote sensing to track changes in vegetation and land cover over time.
2. Satellite and aerial images let them identify seral stages and follow succession, for example vegetation recovery after wildfires or on abandoned farmland.
3. LIDAR (Light Detection and Ranging) builds 3D maps of canopy height, which help judge how mature a succession has become.