Ecosystems – how do they function?

• Ecosystems are the basic functional units of ecology

• Ecosystems are living, dynamic systems encompassing all organisms and communities, their biotic and abiotic components and exchanges within and between each of these

• How do these exchanges take place ?

Trophic relationships in ecosystems• Food chain - pathway along which food (and energy and materials) is transferred from one trophic level to the next

• a trophic level is a feeding level

Simple food chain Miller Ch 4 Fig 4.14

Most organisms have more than one food type, so food chains are linked into more complex food webs

Primary productivity - the energy base for the ecosystem

Food webs - how the ecosystem is structured by energy flow

Miller Ch 4 Fig 4.15

Food chains

• Grazing food chain - directly dependent on green plants(important in rangelands, grasslands)

• Detritus food chain - primary food base is detritus (e.g. breakdown of leaf litter via soil arthropods) (important in forest ecosystems).

Detritus pathway in food webs• Detritus - dead and decaying matter (Miller Ch

4 Fig 4.12)

• Detritus food chains– occur in all ecosystems, in parallel with the

grazing food chain– much of the primary production (plant

material) is not consumed directly but dies and enters the detritus pool

– contribute to recycling of materials as well as to flow of energy

Lengths of food chains• Food chains are short, typically 3 or 4 trophic

levels (rarely more than 5)

• Insect and detritivore-dominated food chains may be longer

• Why so short?

• Reduction of energy as one moves up the chain - but then would expect different lengths in ecosystems with different productivity?

• Materials cycles involve both– the biotic component of ecosystems (producers,

consumers, decomposers)– the abiotic component (gases, water, soil etc)

• These are called biogeochemical cycles• global cycles - those involving gases in atmosphere

(e.g. CO2,SO2, N) (Miller Ch4 Fig 4.23, 4.26)

• local cycles - involving less mobile elements (e.g. P, K, Ca, Mg) (Miller Ch4 Fig 4.25)

Water cycling (hydrological cycle) in ecosystems

Active pools (Miller Ch 4 Fig 4.22)

AtmosphereSoil moistureStream channelsFreshwater lakesSaline lakesOcean

StorageIcecaps and glaciersGround water

The global water cycle(Refer to Miller Pg.76)

Cycling of water Bulk of the water in the oceans (97%)

Major circulation between oceans and atmosphere through evaporation and precipitation

Soil moisture, ground water are two substantial pools– deep drainage from soil moisture to ground water– direct evaporation from soil surface to atmosphere– transpiration through plants into atmosphere

Eventually, most water precipitated over land returns

to oceans - via run-off and stream flow

• Water not equally available in all ecosystems

• At local level, water effectively flows through the system (like energy) rather than being recycled - replenished only by new input ie. most lost from local ecosystems through run-off, evaporation, transpiration

Tropical Areas – eg. SE Asia

Australia particularly dry by world standards

– two-thirds desert– variability of rainfall high

Because water is a limiting factor, it is a key influence on primary productivity

Infrequent, unpredictable rainfall (in deserts) means productivity is ‘pulsed’ rather than regular or seasonal

Deforestation and changes to water cycles

• Deep-rooted perennial plants - major users of soil moisture

• Taken up by roots, transpired to atmosphere from foliage

• Clearing - more moisture can drain into ground water

• Ground water ‘recharged’ and rises

• Ground water discharges as surface seeps, or drains into streams

• PROBLEMS - soil can become saturated resulting in mud slides (tropical areas) or rising water-tables can bring salt from subsoil to soil surface (arid areas)

Carbon Cycling(Refer to Miller Pg. 78)

Most of the world’s carbon (C) exists in relatively inaccessible storage pools- carbonate in rocks (e.g. chalk, limestone, marble)- fossil fuels (coal, oil, natural gas) (Miller Ch 4 Fig 4.23)

Carbon in the atmosphere - only a tiny amount BUT the atmospheric pool is most active

Carbon cycle• Carbon dioxide withdrawn from the

atmosphere during photosynthesis (rate determined by primary productivity of the ecosystem)

• Carbon dioxide returned by cellular respiration (ie use of sugars/carbohydrates for energy and decomposition)

• Atmospheric carbon dioxide also dissolves in the oceans, the largest active pool, and is available to marine plants

Exchange between active and storage pools of carbon

• Until recently, exchanges between storage pools (rocks, fossil fuels) and active pool (atmosphere) was very low - eg. weathering of carbonate rocks

• BUT increased use of fossil fuels has greatly increased the return to the atmosphere

• Carbon dioxide returned to atmosphere faster than it can be cycled - net increase in CO2 in atmosphere, and implications for climate change