Transcript Slide 1
Ecological Succession An Introduction… What is succession and why does it occur? What are the specific changes that take place during psammosere succession? What is succession? Succession is the orderly process of change over time in a community. Change occurs directionally and is not related to seasonal change. Changes in a community of organisms can cause changes in the physical environment that enable other communities to become established. The new community eventually replaces the existing community through competition. Therefore, later communities tend to be more complex. Succession often occurs in a number of identifiable stages, known as seral stages. Seral Stages Stage 1: Colonisation •Pioneer species (adapted to hostile conditions) colonise an area. •Soils – simple soils begin to develop from windblown dust & minerals Stage 2: Establishment •Increase in species diversity •Soils – increasing organic content, infiltration capacity & nutrients Stage 3: Competition •Colonisation of new species changes in microclimate •Larger plants increase & provide shelter establishment of K-strategists as earlier R-strategists are out-competed Stage 4: Stabilisation •Establishment of late colonisers fewer new species colonise •Development of more complex food webs Stage 5: Seral Climax •Final stage in succession: stable and self-perpetuating Succession may begin as a result of the creation of new land (e.g. sand deposition). Where no previous vegetation has existed, a process of primary succession takes place. Succession may also occur due to disturbance that removes existing communities (e.g. forest fire). Where vegetation has previously existed on a surface, recolonisation and secondary succession may occur. The first plants to colonise an area of land are known as pioneer plants. These plants often need to be resilient to relatively hostile conditions. If succession continues to occur (undisturbed), it will reach a climatic climax. This is the final stage of succession, characterised by a balance between vegetation and environmental (soil & climate) conditions. If succession is disturbed by human activity, a plagioclimax is reached. Climax Communities Succession occurs due to the complex interrelationship between climatic conditions, edaphic (soil) conditions and vegetation: Climate Soil Vegetation Climax communities are stable communities. Change no longer needs to take place as a balance between climate, soil and vegetation is achieved. Climate Soil Veg Plagioclimax Communities (a.k.a. Plagioseres) Plagioclimax communities are also stable communities where change no longer needs to take place as a balance between climate, soil and vegetation is achieved. Human factors that prevent a climatic climax community from developing include: • fires • agriculture/grazing • habitat destruction for roads, settlements, tourism & recreation etc Subclimax Communities A sere can be stopped/arrested at any stage by any biotic or abiotic factor: • waterlogging of soil • heavy grazing The arrested sere is known as a subclimax community – development can continue if the arresting (limiting) factor is removed. Succession vs Zonation Succession refers to temporal changes in vegetation. Zonation refers to spatial variations along an environmental gradient. Succession Zonation • temporal changes to an ecosystem • dynamic process • e.g. development of deciduous forest ecosystem • spatial variations along an environmental gradient (e.g. soil moisture content, salt %, altitude etc) • static process • e.g. clear vegetation zones on mountains Nb: Sand dune ecosystems demonstrate both dynamic succession and also zonation of seral stages. Psammosere Succession Land created by sand dune formation is very hostile to plants. Pioneer species must be adapted to survive in the following conditions: •High levels of salt •Exposure to strong winds •Abrasion by sand particles •High temperatures •Drought conditions •Occasional inundation by salt water Very few plants have adapted to survive these conditions. Sand Dune Formation A combination of coastal processes lead to sand dune formation: • transportation of sediment (longshore drift, onshore winds) • deposition of sediment (constructive waves) • macro-tidal environment A number of specific conditions are necessary: • small particle size (0.2-2.0mm) • large inter-tidal range • prevalence of onshore winds What are the specific changes that take place in psammoseres? •To vegetation (diversity, productivity, complexity) •To soil conditions (depth, nutrients, pH, texture, moisture content/infiltration rate) • To climatic conditions (wind, temperature, humidity) Changes during Succession GPP NPP •To dunes (slope gradient/dune size) biodiversity • Species diversity Early Stage •Low GPP •High & NPP (due to low respiration losses) •Little increase in biomass Time Middle Stage •GPP high •Increased photosynthesis •Increases in biomass as plant forms become bigger Late Stage • Trees reach maximum size •Ratio of NPP to R is roughly equal