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ECOLOGY OCTOBER 13, 2011 CAPE BIOLOGY UNIT II MRS. HAUGHTON TOPICS TODAY 1. FOOD CHAINS/WEBS 2. TROPHIC LEVEL DIAGRAMS 3. FLOW / TRANSFER OF ENERGY 4. EFFICIENCY OF ENERGY TRANSFER 5. PYRAMIDS OF NUMBERS, BIOMASS AND ENERGY 6. CARBON, NITROGEN, HYDROLOGICAL CYCLES TODAY Syllabus objectives: Energy Flow and Nutrient Cycling 3.2 -3.7 Biological science pages 298-314 for extra information. Please see powerpoints on website. Syllabus Objective 3.2 Discuss the way in which energy flows in an ecosystem by way of food chains and food webs. Food webs to be emphasized. FOOD CHAINS The passage of energy in the form of food Arrows used to represent the energy flow Organisms feed on only one other organism at a time Illustration limited in showing what actually occurs in nature FOOD CHAINS TERMS IN FOOD CHAIN Producer Primary consumer (herbivore) Secondary consumer (carnivore) Tertiary consumer (carnivore) Quaternary consumer (carnivore) Complex consumer (carnivore) Detritivores and decomposers on every level Producers: Organisms which convert some of the energy from the sun into stored chemical energy (usually plants). Primary consumers: Organisms that obtain energy by consuming producers. They are herbivores. Secondary consumers: Organisms which obtain energy by consuming primary consumers. They are carnivores. Decomposers: These organisms form the end point of every food chain. They are bacteria or fungi that obtain their energy by breaking down dead organisms from the other trophic levels. DETRITIVORES Each description of a trophic level will describe an organisms role in the ecosystem. Organisms may occupy more than one trophic level, (e.g. when acting as omnivores). FOOD WEBS 1. 2. 3. More complex than food chains. Advantages include: Show a more true depiction of how organisms feed on more than one other organism Able to show omnivores. Provides the basis for a more quantitative study of energy flow and material exchange. FOOD WEBS FLOW OF ENERGY AND MATERIALS Figure 10.4 on page 302 shows that energy flows in a straight line and as food is eaten, some of the energy is gained and most of it is lost by the organism as respiration and heat. The figure also shows that simultaneously, the nutrients or materials in each organism is recycled at each trophic level by detritivores and decomposers. Syllabus Objective 3.3 Discuss the efficiency of energy transfer between trophic levels. TROPHIC LEVELS There is usually no more than four or five trophic levels because: The numbers of organisms decreases as the trophic levels increase Energy from the previous trophic level decreases as the number of levels increase Transfer of energy between trophic levels Transfer of energy between trophic levels is relatively inefficient. Energy is transferred from one trophic level to another as organisms are consumed. In primary producers the main energy input is from the solar energy. In a plant, not all of the solar energy available actually makes it into the leaf. Transfer of energy between trophic levels There is loss of energy by reflection from the leaf, transmission through the leaf, and because some of the energy is the incorrect wavelength. The energy that is taken up by the producer is then fixed by photosynthesis, although again a proportion of this energy is lost as it is used up during photosynthetic reactions. Transfer of energy between trophic levels Of the energy that is fixed in photosynthesis some will be used during respiration whilst the remaining energy is the portion that is incorporated into the biomass. It is the energy that is incorporated into the biomass that is available for the next trophic level. Transfer of energy between trophic levels In the consumer a further series of energy losses occur. The consumer will take in a certain amount of energy from the trophic level beneath it. Transfer of energy between trophic levels This energy intake does not equal the amount of energy available in the biomass of this organism since feeding is an inefficient process. There will be a loss of energy through the production of urine and faeces, as well as losses through respiration and heat loss. This leaves a proportion of the energy consumed to be incorporated into the biomass. Transfer of energy between trophic levels It is generally accepted that only around 10% of the energy gained from the previous trophic level is passed on to the next level. All other energy is lost as described above. This limits the number of trophic levels in any food chain. EFFICIENCY OF ENERGY FLOW Sun is the main source of energy 100 % energy shines from it 40 % is reflected 15 % is absorbed and converted to atmospheric heat 45 % alone penetrates to the earth’s surface Only 1-5 % of this is used in photosynthesis to make organic substances EFFICIENCY OF ENERGY FLOW Of this 1-5% converted to organic substances, only 50-80% of the organic material remains after being used by the plant for its life processes. On a global average, plants fix only 0.1% of the energy supplied by the sun This energy then passes on the herbivores, then subsequent carnivores during feeding. EFFICIENCY OF ENERGY FLOW Production ecology studies productivity Gross primary productivity (GPP) is the rate at which energy is stored in plants in the form of organic substances. 20-25% of the GPP is used by the plant and the remainder is called the net primary productivity (NPP). It is this energy that passes on the herbivores or omnivores as they feed on the producer. EFFICIENCY OF ENERGY FLOW Not all of the materials available in the food is used by the consumer organisms for production. Some energy is lost as heat in respiration and waste during excretion and egestion. Detritivores and decomposers feed on the waste and eventually dead organisms The energy remaining in the organisms afer they have respired and excreted are available for the next trophic level. EFFICIENCY OF ENERGY FLOW Production in heterotrophs is called secondary production. The average efficiency of transfer of energy from plants to herbivores is 10% The average efficiency of transfer of energy from animals to animals is 20% EFFICIENCY OF ENERGY FLOW Herbivores make less efficient use of their food than carnivores because plants contain a high proportion of indigestible cellulose and lignin. Energy lost through respiration cannot be used by another organism, but that lost through excreta can be. EFFICIENCY OF ENERGY FLOW In a stable ecosystem, the biomass at the start of the year will be the same at the end. All energy that went into primary production will pass through the trophic levels and none retained in net production A young ecosystem would retain some of the energy input in the form of increased biomass at the end of the year (growth) EFFICIENCY OF ENERGY FLOW It is therefore obvious that eating plants is the most efficient way of extracting energy from the ecosystem However, animal protein is a better source of essential amino acids Animal protein is also more easily digestible (See diagram page 308) EXAMPLES OF ENERGY EFFICIENCY FOOD CHAIN Cultivated plant crop humans Cultivated plant crop livestockhumans Intensive grassland livestockhumans Grassland and crops livestockhumans ENERGY YIELD (kJ x 103 ha-1) 7800 – 11,000 745- 1423 339 (from meat) 3813 (from milk) 1356 Syllabus Objective 3.4 Discuss the concept of biological pyramids. Include limitations of the pyramids of numbers, biomass and energy. Ecological pyramids are used as a tool to illustrate the feeding relationships of the organisms, which together make up a community. PYRAMIDS OF NUMBERS The organisms of a habitat are counted and then grouped into their trophic levels A progressive decrease in the number of organisms on each successive level is usually observed. The bars that make up the pyramid are proportional to the number of organisms on the trophic levels. Data is easy to collect Base may be large or small, depending on organisms being counted. PYRAMIDS OF NUMBERS PROBLEMS WITH PYRAMIDS OF NUMBERS 1. Producers vary greatly in size 2. Sometimes the numbers of organisms on each level vary so greatly it is difficult to determine a suitable scale for diagram 3. The trophic level of an organism may be difficult to ascertain. 4. The pyramid may be inverted. PYRAMIDS OF BIOLOGICAL MASS These pyramids can help overcome some of the pyramids of numbers The total biomass of the organisms is estimated for each trophic level Typically dry mass is preferred Base usually largest. PYRAMIDS OF BIOMASS PROBLEMS WITH PYRAMIDS OF BIOMASS A major problem is that a pyramid of biomass can be inverted and also it does not take account of changes over time. The sampling must all be carried out at one moment in time and therefore indicates the standing crop and not the productivity. PROBLEMS WITH PYRAMIDS OF BIOMASS 1. 2. The biomass at the time of sampling is called the standing biomass or standing crop biomass (no rates considered) This inability to show productivity can be a problem because: It does not give an idea of the amount of material being passed from level to level If the producers are small, they have a higher turnover rate than a larger more stable organism. PYRAMIDS OF NUMBERS AND BIOMASS PYRAMIDS OF ENERGY 1. 2. 3. 4. 5. The most ideal way of representing organisms on each trophic level. The advantages are: Rate of production considered. Weight for weight, two species may not have the same energy content Different ecosystems can be compared Inverted pyramids are not obtained Input of solar energy can be added as an extra bar at the base of the pyramid (sun contribution) PYRAMID OF ENERGY PROBLEMS WITH PYRAMIDS OF ENERGY 1. 2. Most difficult pyramid for which to obtain data and require more measurements. Samples must be combusted and the energy released is recorded. Question, how can the data from a pyramid of biomass be used to obtain a pyramid of energy? OVERALL PROBLEMS WITH ECOLOGICAL PYRAMIDS 1. 2. 3. Identifying organism’s trophic level Plant material difficult to measure because a lot of it is useable only for the plant and not for subsequent consumers Dead organic matter (DOM detritivores and decomposers) are usually left off the pyramids even though they are very important.