Ecosystem Stability‣An ecosystem’s stability refers toits apparently unchanging natureover time.‣Components of ecosystemstability include inertia (the abilityto resist disturbance) andresilience (the ability to recoverfrom external disturbance). The diversity of ecosystems at low latitudes (nearer the equator) is generally higher than at higher latitudes (nearer the poles). This photograph shows a forest in Hawaii.
Stability and Species Loss• Ecological theory suggests that all species in an ecosystem contribute to ecosystem function.• Species loss past a certain point is likely to be detrimental to the functioning of the ecosystem and on its ability to resist change (its stability).• Ecosystem stability is closely linked with biodiversity but it is not clear what level of biodiversity is required to guard against ecosystem dysfunction.
Key SpeciesSpecies whose influences onecological communities are greaterthan would be expected on the basisof their abundance are called key(keystone) species. They are more influential in ecosystem stability than other species because of their pivotal role in some ecosystem function such as nutrient cycling.Elephants are a key species, and canalter the entire structure of thevegetation in those areas into whichthey migrate. Their pattern of grazing on taller plant species promotes a predominance of lower growing grasses with small leaves.
Community Patterns• Communities typically show patterns in both space and time. These include: – Zonation: Changes in the composition of a community which occur in response to an environmental gradient, e.g. with altitude or on a shoreline. Altitudinal zonation – Stratification: Layering of different plant species into distinct strata. – Succession: Changes in the species composition of a community over time. Succession on Maui, Hawaii
Community Change With AltitudeBoth vegetation and soil type may change with increasing altitude. On Mount Kosciusko, Australia, low altitude soils have low levels of organic matter supporting dry tussock grassland vegetation. The high altitude alpine soils are rich in organic matter, largely due to slow decay rates.
StratificationStratification describes a patternof vertical layering where the layers(or strata) comprise differentvegetation types.Stratification is a feature of bothtemperate and tropical forestcommunities throughout the world.Species composition variesaccording to local conditions(altitude, soil type, temperature,precipitation) and vegetationhistory.
Tropical Rainforest Structure Tropical rainforests are Canopy complex and can be divided into four distinct strata representing zones of different vegetation. The strata are: Subcanopy Canopy Subcanopy Understorey Ground layer. In addition, epiphytes Understorey (perching plants) and lianes (climbing vines) occupy Ground layer several strata in the forest.
Ecological SuccessionEcological succession is the process by whichcommunities in a particular area change over time.Succession takes place as a result of complex interactionsof biotic and abiotic factors. Community composition changes with time Past Present Future community community community Some species in the The present community Changing conditions in the past community were modifies such abiotic factors as: present community will out-competed or did not • Light intensity and quality allow new species to tolerate altered abiotic become established. These • Wind speed and direction conditions. will make up the future • Air temperature and humidity community. • Soil composition and water content
Early Successional CommunitiesA succession proceeds in sevralstages, until the formation of a Pioneer community, Hawaiiclimax community, which is stableuntil further disturbance.Early successional communities arecharacterized by:—Simple structure, with a smallnumber of species interactions—Broad niches—Low species diversity Broad niches
Climax CommunitiesIn contrast to early successional communities, climaxcommunities typically show:Complex structure, with a large number of speciesinteractions.Narrow niches.High species diversity.
Primary SuccessionPrimary succession refers to colonizationof a region where there is no pre-existingcommunity. Examples include:—Newly emerged coral atolls, volcanic islands—Newly formed glacial moraines—Islands where the previous community has beenextinguished by a volcanic eruption Hawaii: Local plants are able to rapidly recolonize barren areas
Primary SuccessionA classical sequence of colonizationbegins with lichens, mosses, andliverworts, progresses to ferns, grasses,shrubs, and culminates in a climaxcommunity of mature forest.In reality, this scenario is rare. Mature, slow growing trees Shrubs and fast growing Grasses and trees herbaceous Mosses and plants liverworts Bare rock and lichens
Secondary SuccessionSecondary successionoccurs where an existingcommunity has been clearedby a disturbance that does Cyclonenot involve complete soilloss.Such disturbance eventsinclude cyclone damage,forest fires and hillside slips. Forest fire
Secondary SuccessionBecause there is still soil present, the ecosystemrecovery tends to be more rapid than primarysuccession, although thetime scale depends on the speciesinvolved and on climatic andedaphic (soil) factors. Mature forest Young fast Shrubs and growing trees small trees Grasses and herbaceous plants Pioneer community Bare land (annual grasses)
Deflected SuccessionsHumans may deflect the natural course of succession, e.g.through controlled burning, mowing, or grazing livestock. Theresulting climax community will differ from the natural (pre-existing) community.A relatively stable plant community arising from a deflected (orarrested) succession is called a plagioclimax. • Grassland and heathland in lowland Britain are plagioclimaxes.
Gap Regeneration‣The reduced sunlight beneath largecanopy trees impedes the growth ofthe saplings below. When a large treefalls, a crucial hole opens in thecanopy, allowing sunlight to reach thesaplings below.‣The forest regeneration followingthe loss of a predominant canopytree is called gap regeneration.‣Gap regeneration is an example ofsecondary succession.
AdaptationChoose an animals and write one page about its adaptation