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  1. 1. Energy Fixation <ul><li>Photosynthesis is fundamental to energy flow through an ecosystem. It is achieved by the autotrophic organisms that convert the energy of sunlight into chemical energy. Energy fixation is achieved by photosynthesis . </li></ul><ul><li>All organisms require energy to carry out cellular activity, growth and reproduction. They obtain that energy from the food they eat. </li></ul>
  2. 2. Nutrition <ul><li>AUTOTROPHS </li></ul><ul><li>The term autotrophic ( 'self-feeding' ) defines organisms which are able to use external sources of energy in the synthesis of their organic food materials. </li></ul><ul><li>PHOTOAUTOTROPHS : use light energy, via photosynthesis, to make their organic molecules </li></ul><ul><li>CHEMOAUTOTROPHS : use energy from breakdown of inorganic molecules to synthesis complex organic molecules </li></ul>
  3. 3. <ul><li>HETEROTROPHS </li></ul><ul><li>Heterotrophic ( 'other-feeding' ) organisms obtain their energy by breaking down substances obtained from the bodies of other organisms. </li></ul><ul><li>HERBIVORES [= Primary Consumers] : feed only on green plants </li></ul><ul><li>CARNIVORES [Secondary/tertiary Consumers] : feed only on other animals </li></ul><ul><li>OMNIVORES : feed on both plants and animals </li></ul>
  4. 4. <ul><li>SAPROTROPHS </li></ul><ul><li>Saprotropic ( ‘decay-feeding’ ) organisms obtain </li></ul><ul><li>their nutrients from non-living organic matter, </li></ul><ul><li>usually dead and decaying plant or animal matter, </li></ul><ul><li>by absorbing soluble organic compounds. Since </li></ul><ul><li>saprotrophs cannot make food for themselves, </li></ul><ul><li>they are considered a type of heterotroph. They </li></ul><ul><li>include most fungi (the rest being parasites); </li></ul><ul><li>many bacteria and protozoa; animals such as </li></ul><ul><li>dung beetles and vultures; and a few unusual </li></ul><ul><li>plants, including several orchids. </li></ul>
  5. 5. Productivity <ul><li>PRIMARY PRODUCTIVITY : The amount of light energy converted into chemical energy by autotrophs in an ecosystem during a given period of time. Measured by the rate of accumulation of BIOMASS (dry weight of vegetation) in the ecosystem [usually expressed as per unit area in a given time e.g. g/m 2 /yr] </li></ul><ul><li>GROSS PRIMARY PRODUCTIVITY (GPP): the total primary productivity </li></ul>HOWEVER: Remember that plants not only photosynthesise, they also respire . Therefore not all the material produced is stored (and available as food for the primary consumers). Some of it is used for cellular respiration and other metabolic activities by the plant itself.
  6. 6. Net Primary Productivity <ul><li>The energy available to the next level in a food chain or food web is the gross primary productivity (GPP) minus the energy used by the plant during respiration (R) and is called the NET PRIMARY PRODUCTIVITY (NPP) </li></ul><ul><li>This can be written as an equation: NPP = GPP - R </li></ul><ul><li>Net primary productivity (NPP) is of interest because it represents the chemical energy available to the primary consumers (herbivores) and is the beginning of the flow of energy through an ecosystem. </li></ul>
  7. 7. Factors affecting productivity <ul><li>The same as those affecting photosynthesis! </li></ul><ul><ul><li>Light Intensity </li></ul></ul><ul><ul><li>Temperature </li></ul></ul><ul><ul><li>Rainfall </li></ul></ul><ul><ul><li>Soil Water/Nutrients </li></ul></ul><ul><ul><li>CO 2 concentration </li></ul></ul>
  8. 8. Energy Flow <ul><li>Energy fixation and productivity are the basis of ecosystem productivity. In this section the aim is to discuss how that energy flows through an ecosystem, and to consider the efficiency of that energy flow. </li></ul>
  9. 9. Ecological Niches <ul><li>There are three basic niches (or feeding relationships) in the flow of energy through an ecosystem: producers (autotrophs) , consumers ( heterotrophs ) and decomposers (saprotrophs) . </li></ul>
  10. 10. Energy in an Ecosystem FLOWS from the SUN to Autotrophs (Producers) then to Heterotrophs (Herbivores) that eat the Autotrophs, then to Heterotrophs (Carnivores) that feed on other organisms.
  11. 11. Trophic Levels <ul><li>Feeding relationships and therefore the flow of energy can be represented as a food chain. Each link in the chain is called a trophic level ( ‘ trophic’ meaning ‘ feeding’ ) </li></ul><ul><li>Each trophic level is the same number of steps from the Sun: </li></ul><ul><li>- Producers (Autotrophs) are the 1st Trophic Level </li></ul><ul><li>- Herbivores are the 2nd Trophic Level </li></ul><ul><li>- Carnivores are the 3rd/4th/5th Trophic Levels </li></ul><ul><li>Most Animals (carnivores) feed at more than one Trophic Level </li></ul><ul><li>In most ecosystems the feeding relationships (and the transfer of energy) are represented more accurately by a food web </li></ul>
  12. 12. Food Web
  13. 13. Energy Transfer & Efficiency <ul><li>The leaves only use some of the light energy which shines on them for photosynthesis. </li></ul><ul><li>The rest is … </li></ul>Consider this simple food chain: REFLECTED off the leaves TRANSMITTED through the leaves or belongs to WAVELENGTHS that cannot be used .
  14. 14. Energy Transfer & Loss <ul><li>BUT , only about 1% of the light energy striking a plant is converted into net primary productivity and available to be eaten by the caterpillars. </li></ul><ul><li>Only a proportion of this energy (that contained in the leaves and shoots) is consumed by the caterpillars, where most of it is used in respiration and heat production, and some is egested as waste . </li></ul><ul><li>Only a small fraction (about 10% ) is used for growth and available to the next trophic level, the sparrow. </li></ul><ul><li>Similarly only about 10% of the energy the sparrow receives from the caterpillars is available to be passed on to the next trophic level (the eagle) </li></ul>
  15. 15. Ecological Efficiency <ul><li>The ecological efficiency is the ratio of net productivity (ie the amount of energy) at one trophic level to net productivity at the level below </li></ul><ul><li>NPP (trophic level x) : NPP (trophic level x -1) </li></ul><ul><li>Ecological efficiencies vary depending on the organisms involved but usually range from 5-20%. </li></ul><ul><li>This means that 80-95% of the energy at one level never transfers to the next. </li></ul><ul><li>Because energy diminishes at each successive trophic level, food webs rarely contain more than 4 or 5 trophic levels. </li></ul>This is the reason why, generally speaking, the biomass of and the number of organisms in each trophic level decrease as you move along a food chain
  16. 16. Pyramids of numbers, biomass and productivity <ul><li>In an ecosystem, productivity , biomass and numbers of organisms tend to DECREASE at each trophic level in a food web. </li></ul><ul><li>This information can be quantified and illustrated diagramatically as ecological pyramids </li></ul>
  17. 17. Pyramids of Number <ul><li>Numbers of organisms in a given area in a given time are counted and then grouped into trophic levels. </li></ul><ul><li>Pyramids of numbers typically show a broad base of producers and a successive decrease in number of animals at each level </li></ul><ul><li>There are several situations that show inverted pyramids of numbers. For example, a tree, as the primary producer, supports large numbers of insects, which in turn are the food source of large numbers of birds or other predators </li></ul>
  18. 18. Pyramids of Biomass <ul><li>Each block = total dry mass of organisms at that trophic level. </li></ul><ul><li>At each level the biomass decreases . Normally a pyramid of biomass would have a broad base, getting narrower at each succeeding level. </li></ul>Occasionally, however, inverted pyramids of biomass can be found where the primary consumers outweigh the primary producers. E.g. In aquatic ecosystems the primary producers are algae. They are very productive and have a high turnover rate . This means that they grow in numbers rapidly but are also eaten in large numbers by zooplankton and small fish. Thus at any given time the biomass of the producers will be less than that of the primary consumers.
  19. 19. Pyramid of Productivity <ul><li>Size of each block is proportional to the productivity of (or energy available at) each level. </li></ul><ul><li>The producers form the foundation of the pyramid </li></ul>Qu: Look at the inverted pyramid of mass found in a marine phytoplankton food chain. Would the pyramid of productivity for this food chain be inverted or upright?
  20. 20. Pyramid of Productivity <ul><li>Size of each block is proportional to the productivity of (or energy available at) each level. </li></ul><ul><li>The producers form the foundation of the pyramid </li></ul>Qu: Look at the inverted pyramid of mass found in a marine phytoplankton food chain. Would the pyramid of productivity for this food chain be inverted or upright?