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Unit 4: Ecology 4.2: Energy flow Friday, July 17, 2015 Keywords: • pyramid of energy Learning Objectives: We are learning…. • What percentage of energy is transferred from one trophic level to the next? • How is energy lost along the food chain? • How does energy loss restrict the length of food chains? Starter: Write down five ways in which energy is lost at each trophic level of a food chain. Complete the food web and name the organism. Obtaining energy 4 of 31 © Boardworks Ltd 2009 Energy availability in photoautotrophs 5 of 31 © Boardworks Ltd 2009 Energy flow through ecosystems The study of the flow of energy through the ecosystem is known as ecological energetics. All the energy utilised by living organisms is ultimately derived from the sun but as little as 2% of its total radiant energy is actually captured by green plants for distribution throughout the ecosystem. Why is this? Some light is reflected because it is of a wavelength the plant is unable to absorb. This relatively small amount is nonetheless sufficient to support all life on earth. Primary production 7 of 31 © Boardworks Ltd 2009 Ecosystem examples Net primary productivity (NPP) is a good way to compare different ecosystems. The availability of light, water and nutrients are different around the world and therefore the productivity of the world’s ecosystems also varies greatly. desert ecosystem NPP = 260 kJm-2 year-1 8 of 31 tropical rainforest ecosystem NPP = 400,000 kJm-2 year-1 © Boardworks Ltd 2009 Ordering NPP of ecosystems 9 of 31 © Boardworks Ltd 2009 Food chains – the producers Green plants are known as producers because..... .......they manufacture sugars from simple raw materials using solar energy. All primary producers are autotrophic. An autotroph is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light (by photosynthesis) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water. They are able to make their own food, and do not need a living energy or carbon source. Food chains – the primary consumers Organisms that are unable to utilise light energy for synthesis of food must obtain it by consuming other organisms. These are heterotrophs and include all animals as well as fungi and some bacteria. If they feed off the primary producers they are called primary consumers. These are typically herbivores but also include plant parasites. Food chains – the secondary consumers Some heterotrophs, the carnivores, feed on other heterotrophs. These are called secondary consumers if they feed on a herbivore and tertiary consumers if they feed on other carnivores. There is a feeding hierarchy with the primary producers at the bottom and the consumers at the top. The energy is therefore passed along a chain of organisms, known as a food chain. Each feeding level in the chain is known as a trophic level, with only a small portion of the available energy being transferred from one trophic level to the next. Each step represents a trophic level, from producers through to quaternary consumers. Pink = energy transferred Blue = energy lost Decomposers and the food chain On the death of producers and consumers, some energy remains locked up in the complex organic compounds of which they are made. This is utilised by organisms called decomposers and detritivores. These organisms break down complex materials into simple components again and in doing so contribute to the recycling of nutrients. Are decomposers autotrophs or heterotrophs? Decomposers and detritivores Decomposers – fungi, bacteria A dead organism contains not only a potential source of energy but also many valuable minerals. Decomposers are saprobiontic. Saprobionts are organisms that digest their food externally (secreting enzymes) and then absorb the products. By breaking down the organic compounds an organism is made of decomposers release valuable nutrients like carbon, nitrogen and phosphorus, which may then be recycled. Apart from dead organisms, they also decompose the organic chemicals in urine, faeces and other wastes. Decomposers and detritivores Detrivores - Worms, crabs, millipedes and dung flies Detritus is the organic debris from decomposing plants and animals (dead stuff) and is normally in the form of small fragments. It forms the diet of a group of animals called detrivores. They are usually larger than decomposers and they digest food internally rather than externally. Trophic efficiency This is the percentage of the energy at one trophic level which is incorporated into the next trophic level. The values differ from one ecosystem to another. Trophic efficiency figures If 10,000 Joules of (Odum, Silver Springs, Florida) sunlight reach Photosynthesis – 1.2% Primary consumers (herbivores) – 4.5% producers, calculate how many joules of Secondary consumers – 4.5% energy reaches Tertiary consumers – 6.7% each trophic level. Food webs With rare exceptions, the diet of an individual is not restricted to a single food. Most animals feed on may different types. In the same way, an individual is normally a potential meal for many different species. The idea of a food chain as a sequence of species which feeds exclusively off the individuals below them in the series is clearly oversimplified. Individual food chains interconnect in a complex way. A single species may form part of many different food chains, not always occupying the same trophic level in each chain. Activity: Complete the worksheet – Constructing a food web Success Criteria: What I’m looking for….. The total quantity of energy that the plants in a community convert to organic matter is called the gross production. However, plants use 20-50 per cent of this energy in respiration, leaving little to be stored. The rate at which they store energy is called the net production. Net production = gross production – respiratory loss Only about ten per cent of the energy stored in plants is used by primary consumers for growth. Secondary and tertiary consumers are more efficient, transferring about 20 per cent of the energy available from their prey into their bodies. How is energy lost? To begin with, the majority of the suns energy is reflected back into space by clouds or dust absorbed by the atmosphere. Much of the light that reaches plants is not absorbed because it is of an unsuitable wavelength. Of the energy stored in plants, much of it is lost throughout the food chain through…. Waste – metabolic waste releases energy (excretion/urine/faeces, etc.) Not all organisms are eaten Respiration – Heat given off (higher in mammals because of high body temperature) Respiration and energy release Organisms use energy from respiration to move and maintain body temperature. As a result, some energy is lost to the environment as heat. The assimilated energy remaining after respiration goes into building the antelope’s body and this energy becomes available to consumers o the next trophic level. The energy from respiration is also used: • to synthesise molecules like proteins, DNA and glycogen/ starch • to actively transport ions and other substances across membranes • to move substances within cells (like vesicles or proteins in muscle fibre) • during cell division The energy used during these activities is in the form of ATP. Loss of energy between trophic levels? A large percentage of the energy is lost between trophic levels. This is as a result of the following: 23 of 31 Some of the organism cannot be eaten, e.g. bones, fur, etc. Once eaten, some of the organism cannot be digested. Energy is lost in excretory materials, such as urine. Energy is lost in the form of heat from respiration and body heat. © Boardworks Ltd 2009 Why are there only four or five trophic levels in a food chain? The ratio of the dry biomass of animals to the dry biomass of seaweeds is always a lot less than one. Explain why. (2 marks) Seaweeds/plants are producers/lower/first trophic level/ animals are consumers/higher trophic level/feed on seaweeds; Accept relevant position in food chain as trophic level Loss of energy between trophic levels; Accept: energy transfer is inefficient As a result of respiration/as heat; Accept: description of trophic levels Accept: not all seaweed/eaten 2 max Energy input In natural ecosystems the only source of energy is the sun. Agricultural ecosystems require additional inputs of energy, e.g. to maintain ideal conditions. This energy is supplied in two different forms: Food – energy for the manual labour involved in farming is provided by food. Fossil fuels – mechanical energy needed for ploughing, harvesting, transport, etc. is often supplied by fossil fuels. Heat energy from fossil fuels can also be used to maintain an ideal temperature for photosynthesis, in greenhouses. 25 of 31 © Boardworks Ltd 2009 Pyramids of energy Here the bar is drawn in proportion to the total energy utilised at each trophic level (two organisms of the same biomass may store different amounts of energy e.g. 1 gram of fat stores twice as much energy as 1 gram of carbohydrate). The total productivity of the primary consumers of a given area (e.g. 1 square metre) can be measured for a given period (e.g. 1 year). From this, the proportion of energy utilised by primary producers can be calculated, and so on up the food chain. The pyramids produced do not show anomalies, but obtaining the necessary data can be a complex and difficult affair. Maximizing secondary productivity It is possible for humans to manipulate the energy transfer from producer to consumer to increase secondary productivity. Animals are often culled when they are young, as they have a higher secondary productivity rate than that of adults. Antibiotics are used to avoid any unnecessary loss of energy to pathogens. Selective breeding allows farmers to produce livestock with faster growing rates and increased production of milk, eggs, etc. Some animals are kept in confined spaces to reduce energy loss. 27 of 31 © Boardworks Ltd 2009 Intensive rearing of animals Keeping animals in small enclosures such as barns or cages can help to reduce energy loss. This practice is often referred to as ‘factory farming’. Animals are kept in confined spaces to restrict movement, decreasing the energy lost in muscle contraction. This also helps to produce a warm environment, reducing the amount of energy lost as heat from the body. Feeding is controlled to prevent wastage and provide the animals with the optimum amount of food. The enclosure also helps to exclude predators. 28 of 31 © Boardworks Ltd 2009 Issues surrounding factory farming There are advantages and disadvantages to the methods of increasing secondary productivity: disadvantages advantages efficient energy conversion animals vulnerable to rapid spread of disease produces low cost food uses less space use of drugs can lead to antibiotic resistance easier to prevent disease being introduced unnatural conditions may cause stress to the animal restricted movement may cause osteoporosis and joint pain easier to isolate ill animals 29 of 31 © Boardworks Ltd 2009 Plenary: Energy is transferred through an ecosystem. Describe how and explain why the efficiency of energy transfer is different at different stages in the transfer. (6 marks) 1 Some light energy fails to strike/is reflected/not of appropriate wavelength; 2 Efficiency of photosynthesis in plants is low/approximately 2% efficient; 3 Respiratory loss/excretion/faeces/not eaten; 4 Loss as heat; 5 Efficiency of transfer to consumers greater than transfer to producers/approximately 10%; 6 Efficiency lower in older animals/herbivores/primary consumers/ warm blooded animals/homoiotherms; 7 Carnivores use more of their food than herbivores; Q Accept figures below 5%. Accept figures over 5% but below 10% if clearly related to maximum efficiency. 6 max Plenary: 1. Explain what the arrows represent on a food chain. The transfer of nutrients and energy. 2. Describe what is meant by a food web. A series of interlinking food chains. 3. State the initial energy source for most food chains. The sun. 4. Define ‘trophic level’ The position of an organism in a food chain. 5. List the three ways energy is lost when moving from one trophic level to the next. Not consumed, heat, excretion. 6. The leaves of a tree store 20 000J m-2y-1 of energy. Estimate the amount of energy stored by the caterpillars that feed on the leaves. 2000 J m-2 y-1. How successful were we this lesson? Learning Objective We were learning…..