Ecosystem Succession

An ecosystem succession is the changing phases of dominant plant species occupying a particular climatic habitat or ecosystem.

For instance, at the end of the last ice advance as the ice sheets melted and bare landscape was revealed, it did not take long for plants to start to colonise the exposed glacial till and rock surfaces.

The first plants to colonise are known as a pioneer community, and in Britain around 16000 years ago, mosses, lichens and tough tundra-adapted species of wiry grass and low shrubs were the first forms of vegetation to recolonise the landscape.

Each group of plants transforms the micro-habitat and soil conditions over time, adding humus, creating a soil layer and affecting the chemical and biological balance within it. They may alter the micro-climate by providing shade, which reduces soil evaporation and windspeed.

Conditions may change sufficiently for a new species to arrive, take root, and dominate the landscape.

Each of these phases is known as sere (as in part of a ‘series’) where a dominant species characterises the local habitat for a particular time (often thousands of years).

In the case of the British Isles, following the retreat of the ice sheets, low birch and willow species became established after mosses and grasses.

Over time, as conditions change, new seres occur. Ultimately a dominant species is widespread and in harmony with the prevailing climate, soil and environmental habitat conditions, and no substantial further change takes place.

In much of Britain this is mixed oak woodland and it becomes the "climatic climax community" where conditions allow.

In mountainous parts of Britain, the soil is still too thin and the climate too cool for this sere to occur and so the climax vegetation remains at a sere level below that found in lowland and more southerly parts of the British Isles.

This is known as a sub-climax.

On chalk uplands, such as the Chiltern Hills, the soil remains too thin and dry for oak to dominate, but beech forest is the climatic climax community on this geology.

The existence of different climatic climax communities within the same climatic zone due to differences in relief, drainage and geology is referred to as a polyclimax (many-climax).

Different types of "new surface" give rise to different types of plant succession:

Lithosere

When a new rock surface begins to be colonised and the habitat changes.

This may be from a volcanic ash fall or lava flow, where glacial retreat exposes a bare rock or till surface, or where a raised beach occurs as a result of tectonic uplift.

Hydrosere

A wet fresh-water environment that gradually becomes drier as plants grow, die and decay in lakes or on a river bank, raising the level of the bed and trapping more sediment.

Halosere

Occurs in salt-water – as in tidal mudflats or lagoons and saltmarshes. .

Psammosere

A beach where sand dunes start to be stabilised as plants colonise the surface and change the nature of the dune ecosystem.

Human activity can interrupt a succession and prevent the climatic climax being achieved.

By clearing woodland, deliberately burning grasslands and moorlands or farming an area, the species available and the conditions that occur are modified.

What emerges in human-influenced environment is known as a plagioclimax.

An example of this is the heather moorland covering uplands, such as in the North Yorkshire Moors, which is repeatedly burned to reinvigorate new shoots from heather bushes (fed on by grouse), which shooting estates capitalise on as a profitable leisure activity.

If farmland is abandoned, as it has been in much of the Appalachian mountains of the USA, re-colonisation by plants may occur. Here, a sere succession may take place more rapidly and is known as a secondary succession.