Ecological succession

dynamic meaning

• Ecosystems are dynamic, meaning that they are constantly changing

what is succession

a process by which communities of animals and plants colonies an area and then over periods of time, are replaced by other, usually more varied, communities.

what is primary succession

Primary succession is the process that occurs when newly formed or newly exposed land is inhabited by an increasing number of species

how new land forms

• Newly formed land can be created by e.g.

  • The magma from erupting volcanoes cooling and forming new rock surfaces or new rocky islands in the sea

• Newly exposed land can form by e.g.

  • A landslide that exposes bare rock

  • A glacier that retreats to reveal bare rock

how primary succession starts

• Primary succession can occur on any type of bare land, including sand dunes at the edge of the ocean, and on exposed rock

• The first stage of succession is colonization and the first organisms are called pioneer species or opportunists. They include organisms such as algae, mosses and fungi. Seeds and spores that are carried by the wind land on exposed rock and begin to grow

why pioneer species can grow in primary succession land

• Pioneer species can germinate easily and withstand harsh conditions such as low nutrient and water availability 

how pioneer species allow other organisms to colonise

• These organisms can penetrate the rock surface, some will secete an acid that will dissolve the rock. They penetrate small cracks in the rock with root hairs and hyphae

• Acid secreted by these organisms this causes breakdown of the rock, into small grains.It also traps organic material (dead pioneer organisms) which breaks down to form humus, the inorganic rock and the humus start to form soil.

• In the soil seeds of other species land can grow, grasses, ferns are able to establish roots. These are adapted to survive in shallow, nutrient-poor soil

• when the ferns and grasses die and decay adds to the soil.As these small plants die and decompose and more water retained, the soil becomes deeper and more nutrient-rich. This allows less resistant plant species to survive.

• Larger plants and shrubs, as well as small trees can now begin to grow

  • These larger plants and small trees also require more water, which can be stored in deeper soils

• Over time the soil becomes sufficiently deep, contains enough nutrients, and can hold enough water to support the growth of large trees and plant biodiversity increases

• The final species to colonise the new land become the dominant species of the now complex ecosystem

what is secondary succession?

• There is also a type of succession called secondary succession which takes place on previously occupied land e.g. after a wild fire or deforestation or river changes course

  • Secondary succession is very similar to primary succession except that soil is already present and contains seeds, roots and soil organisms so the number of plants and animals present right from the beginning of the succession are much higher

what is climax community?

• A climax community is reached, this is when the range of species is generally constant. Primary succession is key importance in the formation of the biosphere.

• A climax community is not always the most biodiverse stage of succession, but it is a stable community it is self-sustaining and is usually the most productive group of organisms that the environment can support

what is climatic climax community

• F.E Clements suggested that the type of climax community that forms depends on the location of the original bare land;/climate in the tropics the climax community would be a rain forest, while in temperate regions it might be deciduous woodland 

• He suggested that for each climate there was only one possible climax community known as climatic climax community

how humans can prevent succession

• Human activities often prevent or interrupt the process of succession

• This stops a climax community from developing e.g.

  • Regular mowing prevents woody plants from establishing themselves in a lawn

  • The grazing activity of livestock such as sheep and cattle prevent new plants from establishing

• produces plagioclimax communities

what is plagioclimax community?

these are constant and self sustaining communities but they are not completely natural as they are partly a result of human intervention.

how succession changes the biotic and abiotic condiitons, effect of newly arriving species on local environment

• At each stage in succession the newly arriving species change the local environment so that it becomes more suitable for other species that have not yet colonised the new land e.g.

  • Pioneer species such as lichens help to slowly break apart the top surface of bare rock; this fragmented rock, along with the dead organic matter left behind when the lichens die and are broken down, forms a basic soil 

  • Species such as grasses grow roots that stabilise the soil, enabling it to hold more moisture and nutrients

effect of newer species on previous species

• Often the new colonising species then change the environment in such a way that it becomes less suitable for the previous species e.g.

  • Lichens cannot grow on soil so they disappear from the ecosystem once soil begins to form; the new species change the environment in such a way that it becomes less suitable for the lichens

    • Pioneer species may not be found in a climax community as they will be out-competed for light and other resources by the species that arrive during the later stages of succession

    • Pioneer species are well adapted for harsh conditions but are often poor competitors

  • As soil deepens and trees are able to grow, they may block out the light to shrubs and other smaller plant, out-competing them and causing them to die