L02 Ecosystems Function


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L02 Ecosystems Function

  1. 1. Ecosystems – how do they function? <ul><li>Ecosystems are the basic functional units of ecology </li></ul><ul><li>Ecosystems are living, dynamic systems encompassing all organisms and communities, their biotic and abiotic components and exchanges within and between each of these </li></ul><ul><li>How do these exchanges take place ? </li></ul>
  2. 2. Trophic relationships in ecosystems <ul><li>Food chain - pathway along which food (and energy and materials) is transferred from one trophic level to the next </li></ul><ul><li>a trophic level is a feeding level </li></ul>Simple food chain Miller Ch 4 Fig 4.14
  3. 3. <ul><li>Most organisms have more than one food type, so food chains are linked into more complex food webs </li></ul><ul><li>Primary productivity - the energy base for the ecosystem </li></ul><ul><li>Food webs - how the ecosystem is structured by energy flow </li></ul>Miller Ch 4 Fig 4.15
  4. 4. Food chains <ul><li>Grazing food chain - directly dependent on green plants (important in rangelands, grasslands) </li></ul><ul><li>Detritus food chain - primary food base is detritus (e.g. breakdown of leaf litter via soil arthropods) (important in forest ecosystems). </li></ul>
  5. 5. Detritus pathway in food webs <ul><li>Detritus - dead and decaying matter (Miller Ch 4 Fig 4.12) </li></ul><ul><li>Detritus food chains </li></ul><ul><ul><li>occur in all ecosystems, in parallel with the grazing food chain </li></ul></ul><ul><ul><li>much of the primary production (plant material) is not consumed directly but dies and enters the detritus pool </li></ul></ul><ul><ul><li>contribute to recycling of materials as well as to flow of energy </li></ul></ul>
  6. 6. Lengths of food chains <ul><li>Food chains are short, typically 3 or 4 trophic levels (rarely more than 5) </li></ul><ul><li>Insect and detritivore-dominated food chains may be longer </li></ul><ul><li>Why so short? </li></ul><ul><li>Reduction of energy as one moves up the chain - but then would expect different lengths in ecosystems with different productivity? </li></ul>
  7. 7. <ul><li>Materials cycles involve both </li></ul><ul><ul><li>the biotic component of ecosystems (producers, consumers, decomposers) </li></ul></ul><ul><ul><li>the abiotic component (gases, water, soil etc) </li></ul></ul><ul><li>These are called biogeochemical cycles </li></ul><ul><li>global cycles - those involving gases in atmosphere (e.g. CO 2 ,SO 2 , N) (Miller Ch4 Fig 4.23, 4.26) </li></ul><ul><li>local cycles - involving less mobile elements (e.g. P, K, Ca, Mg) (Miller Ch4 Fig 4.25) </li></ul>
  8. 8. Water cycling (hydrological cycle) in ecosystems <ul><li>Active pools (Miller Ch 4 Fig 4.22) </li></ul><ul><ul><li>Atmosphere </li></ul></ul><ul><ul><li>Soil moisture </li></ul></ul><ul><ul><li>Stream channels </li></ul></ul><ul><ul><li>Freshwater lakes </li></ul></ul><ul><ul><li>Saline lakes </li></ul></ul><ul><ul><li>Ocean </li></ul></ul><ul><li>Storage </li></ul><ul><ul><li>Icecaps and glaciers </li></ul></ul><ul><ul><li>Ground water </li></ul></ul>
  9. 9. The global water cycle ( Refer to Miller Pg.76)
  10. 10. Cycling of water <ul><li>Bulk of the water in the oceans (97%) </li></ul><ul><li>Major circulation between oceans and atmosphere through evaporation and precipitation </li></ul><ul><li>Soil moisture, ground water are two substantial pools </li></ul><ul><ul><li>deep drainage from soil moisture to ground water </li></ul></ul><ul><ul><li>direct evaporation from soil surface to atmosphere </li></ul></ul><ul><ul><li>transpiration through plants into atmosphere </li></ul></ul><ul><li>Eventually, most water precipitated over land returns to oceans - via run-off and stream flow </li></ul>
  11. 11. <ul><li>Water not equally available in all ecosystems </li></ul><ul><li>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 </li></ul>
  12. 12. Tropical Areas – eg. SE Asia
  13. 13. <ul><li>Australia particularly dry by world standards </li></ul><ul><ul><li>two-thirds desert </li></ul></ul><ul><ul><li>variability of rainfall high </li></ul></ul><ul><li>Because water is a limiting factor, it is a key influence on primary productivity </li></ul><ul><li>Infrequent, unpredictable rainfall (in deserts) means productivity is ‘pulsed’ rather than regular or seasonal </li></ul>
  14. 14. Deforestation and changes to water cycles <ul><li>Deep-rooted perennial plants - major users of soil moisture </li></ul><ul><li>Taken up by roots, transpired to atmosphere from foliage </li></ul><ul><li>Clearing - more moisture can drain into ground water </li></ul><ul><li>Ground water ‘recharged’ and rises </li></ul><ul><li>Ground water discharges as surface seeps, or drains into streams </li></ul><ul><li>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) </li></ul>
  15. 15. Carbon Cycling ( Refer to Miller Pg. 78) <ul><li>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) </li></ul><ul><li>Carbon in the atmosphere - only a tiny amount </li></ul><ul><li>BUT the atmospheric pool is most active </li></ul>
  16. 16. Carbon cycle <ul><li>Carbon dioxide withdrawn from the atmosphere during photosynthesis (rate determined by primary productivity of the ecosystem) </li></ul><ul><li>Carbon dioxide returned by cellular respiration (ie use of sugars/carbohydrates for energy and decomposition) </li></ul><ul><li>Atmospheric carbon dioxide also dissolves in the oceans, the largest active pool, and is available to marine plants </li></ul>
  17. 17. Exchange between active and storage pools of carbon <ul><li>Until recently, exchanges between storage pools (rocks, fossil fuels) and active pool (atmosphere) was very low - eg. weathering of carbonate rocks </li></ul><ul><li>BUT increased use of fossil fuels has greatly increased the return to the atmosphere </li></ul><ul><li>Carbon dioxide returned to atmosphere faster than it can be cycled - net increase in CO 2 in atmosphere, and implications for climate change </li></ul>