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Succession Syllabus 2.3.5 – Describe the concept and processes of succession in a named habitat. 2.3.6 – Explain the changes in energy flows, gross and net productivity, diversity and mineral cycling in different stages of succession. 2.3.7 – Describe factors affecting the nature of climax communities. Ecological succession The gradual process by which the species population of a community changes is called ecological succession. A forest following a disturbance such as a fire. Succession takes places as a result of complex interactions of biotic and abiotic factors. Early communities modify the physical environment causing it to change. This in turn alters the biotic community which further alters the physical environment and so on. Succession – What happens? Each successive community makes the environment more favourable for the establishment of new species. A succession (or sere) proceeds in seral stages, until the formation of a climax community is reached. Primary Succession Refers to colonization of regions where there is no pre-existing community. Can you think of examples where this would occur? Primary Succession Community changes on a glacial moraines After a volcanic eruption Questions – Glacial Moraines During succession there is a change in species composition of a community. There are also changes in species diversity, stability of the ecosystem, and in gross and net production until a climax community is reached. 1. Explain what is meant by a climax community. Explain each of the following changes which occur during succession: Species diversity increases Gross production increases Stability of the ecosystem increases Give two reasons why farmland in the UK does not reach a climax community. 2. a) b) c) 3. Primary and Secondary Succession Primary Succession – occurs on newly formed habitats that have not previously supported a community. Examples? Secondary Succession – occurs on sites that have been disturbed. Examples? Primary Succession – Bare Rock Lichens, bryophytes and annual herbs Bare Rock Mosses, Grasses and small shrubs After 100-200 years Slower growing broadleaf species e.g. oak Fast growing trees e.g. Ash Complex Community Example for a Northern Hemisphere lithosere: a succession on bare rock In Summary - the 1st Invaders! These are usually fast growing plants that photosynthesize well in full sunlight. We call these pioneer species making up the pioneer community Examples = lichens, grasses, herbs As these species begin to grow well, they produce shade. Their own seedlings grow more poorly than shade-adapted plants. Plants that grow well under full sun are replaced by plants that germinate and grow better in deeper shade. Secondary Succession This type of succession takes place after a land clearance (e.g. from fire or landslide). These events do not involve loss of the soil. Secondary succession therefore occurs more rapidly than primary succession. Humans may deflect the natural course of succession in these circumstances (e.g. by mowing or farming). This leads to the development of a different climax community than would otherwise develop naturally. Secondary Succession – Cleared Land Primary Bare Earth Open pioneer community (annual grasses) Time to develop: Years Grasses and low growing perennials 1-2 3-5 Young broad leaved woodland 31-150 Mature woodland: mainly oak 150+ = climax community Scrub: shrubs and small trees 16-30 Succession Continues As the plant community changes, the soil will also undergo changes (abiotic factors will change). Decomposers will join the community as well as animal species. Animal species have a profound affect on the plant species occurring within a habitat. Changing conditions in the present community allows for new species to become established (the future community). Succession continues until the climax community is reached. Wetland Succession Wetland areas present a special case of ecological succession. Wetlands are constantly changing: Open water Plant invasion Siltation and Infilling • Wetland ecosystem may develop in a variety of ways: Wetland Succession • In well drained areas, pasture or heath may develop as a result of succession from fresh water to dry land. In non-acidic, poorly drained areas, a swamp will eventually develop into a fen. In special circumstances, a an acid peat bog may develop. (may take 5000+ years). Productivity Think back to the work on food webs/chains It is often useful to know how much energy is passing through a trophic level over a period of time. This is called productivity Productivity is a measure of the amount of energy incorporated into the organisms in a trophic level, in an area, over a certain period of time. Productivity The area is normally one square metre and the time is usually one year. It is therefore measured in units of kilojoules per square metre per year (kJ m- year ) 2 -1 The rate at which producers convert light energy into chemical energy is called primary productivity. Gross Productivity Gross Productivity (GP) – is the total gain in energy or biomass per unit time. This is sometimes shown as GPP – Gross Primary Productivity It is related to the total amount of chemical energy incorporated into the producers. The producers use some of this energy during respiration and energy needs which is eventually lost to the environment as heat. The remaining energy is available to the herbivores and is known as net primary productivity (NPP) Remember NPP = GPP – R Net Primary Productivity = Gross Primary Productivity - Respiration NPP and GPP in Succession The NPP and GPP of any ecosystem is going to fluctuate. This is especially the case during each seral stage. As ecosystems become more diverse, the overall GPP is also going to increase. This is because climax communities are better adapted to an efficient rate of utilisation of their resources. The Early Stages Gross Productivity = Low This is due to the initial conditions and the relatively low density of producers. Net Productivity = High This is due to low respiration rates of the initial producers and therefore a lot of energy available to be passed on. This allows the system to grow and biomass to accumulate. The Later Stages Gross Productivity = High This is due to an increase in the consumer community who can synthesise a lot of energy from the food they eat. Net Productivity = Low Increased rates of respiration and other energy sapping activities by consumers means that NP will begin approaching zero. Climax community The P:R ratio approaches zero. The productivity : respiration ratio approaches zero Think about it: NPP = GPP – R “0” = high value – high value The Climax Succession comes to an end with the establishment of a mature, relatively stable community – the climax Climax communities are more stable that the seral stages that preceded them. Ultimately, the climate will be responsible for affecting the nature of the climax community unless human or other factors maintain an equilibrium at a sub-climax community. Climax Communities The major factor affecting the formation of the climax community is the climate: temperature and humidity. Edaphic Factors, such as components of the soil or the presence of toxic substances also affect the formation of the climax community.