Chapter 7 Ecological succession Hint Pioneer communities put some organic material into the soil when they die. This allows recycling to start and increases mineral ions in the soil. amount oorganic matter available rom the death othe se plants, a thicker layer osoil is built up. This then supports small owering plants such as grasses and, in turn, shrubs and trees. In the UK the ultimate community is most likely to be deciduous oak woodland. The stable state thus ormed comprises a balanced equilibrium ospecies with ew, iany, new species replacing those that have become established. This is called the climax community. This is a stable state, with many species ourishing. This community consists oanimals as well as plants. The animals have undergone a similar series osuccessional changes, which have been largely determined by the plant types available or ood and as habitats. Within the climax community there is normally a dominant plant and animal species. During any succession there are a number ocommon eatures that emerge: the non-living environment becomes less hostile, e.g. soil orms, nutrients are more plentiul and plants provide shelter rom the wind. This leads to a greater number and variety ohabitats that in turn produce increased biodiversityas dierent species occupy these habitats. This is especially evident in the early stages, reaching a peak in mid-succession, but decreasing as the climax community is reached. The decrease is due to dominant species out-competing pioneer and other species, leading to their elimination rom the community. With increased biodiversity comes more complex ood webs, leading to increased biomass, especially during mid-succession. Climax communities are in a stable equilibrium with the prevailingclimate. It is this climate that determines the dominant species othe community. In the lowlands othe UK, the climax community is deciduous woodland. In other climates othe world it may be tundra, steppe or rain orest. Secondary succession Secondary succession occurs when land that has already sustained lie is suddenly altered. This may be the result oland clearance or agriculture or a orest fre. The process by which the ecosystem returns to its climaxcommunity is the same as or primary succession, except that it normallyoccurs more rapidly. This is because spores and seeds oten remain alive in the soil, and there is an inux oanimals and plants through dispersal and migration rom the surrounding area. Secondary succession thereore does not begin with pioneer species, but with organisms rom subsequent successional stages. Because the land has been altered in some way, e.g. by fre, some othe species in the climax community will be dierent. Figure 3 summarises the events oecological succession on land. Succession Learning objectives: What changes occur in the variety ospecies that occupy an area over time? What are meant by the terms primary succession, secondary succession and climax community? How can managing succession help to conserve habitats? Specication reerence: .4.7 We have seen that ecosystems are made up oall the interactingbioticand abiotic actors in a particular area, within which there are a number ocommunities oorganisms. As we look around at natural ecosystems such as moorland or orest, we may get the impression that they have been there orever. This is ar rom the case. Ecosystems constantlychange, sometimes slowly and sometimes very rapidly. Succession is the term used to describe the changes in the species that occupy a particular area, over time. Primary succession Primary succession occurs when bare rock or other barren land is frst colonised. This may occur as a result o: a glacier retreating and depositing rocksand being piled into dunes by wind or sea volcanoes erupting and depositing lava lakes or ponds being created by land subsidingsilt and mud being deposited at river estuaries. The frst stage oprimary succession is the colonisation oan inhospitable environment by organisms called pioneer species. Pioneer species oten have eatures that suit them to colonisation. They may: produce vast quantities owind-dispersed seeds or spores and so easilyreach isolated situations such as volcanic islands not require a period odormancy and so germinate quickly on arrival be able to photosynthesise, as light is normally available but other ‘ood’ is not – they are thereore not dependent on animal species oten fx nitrogen rom the atmosphere because, even ithere is soil, it has ew or no nutrients be able to tolerate extreme conditions. Succession takes place in a series ostages. At each stage, certain species can be identifed which change the environment, especially the soil, so that it becomes more suitable or other species. These other species then out-compete the species in the existing community and so a new community is ormed. Imagine an area obare rock. One othe ew kinds oorganism capable osurviving on such an inhospitable area is lichen. Lichen is thereore a pioneer species. Lichen can survive considerable drying out. In time, weathering othis base rock produces sand or soil, although in itselthis cannot support other plants. However, as the lichens die and decompose they release sufcient nutrients to support a community osmall plants. Mosses are typically th e next stage in succession, ollowed by erns. With the continuing erosion othe rock and the increasingHint The climax community is determined by the main abiotic actor. For example, trees may not develop on very high mountains because it is too windy or the soil layer is too thin. 1Ecological succession 1 7 7.1 Succession Figure 1 Lichens, with their ability to withstand dry conditions and to colonise bare rock, are requently the rst pioneerspecies on barren terrain Figure Deciduous woodland is normally the climax community in lowlandBritain
