Ecology and Ecosystems

ORGANISMSO AND THEIR ENVIRONMENT 4.1 Ecology and ecosystems OBJECTIVES living organisms – and some are abiotic – the ■ To understand that living organisms require non-living components of the habitat. Ecology certain conditions for their survival is the study of living organisms in relation to ■ To understand that living organisms interact with one their environment. The interactions between the another, and with their non-living environment organism and its environment are summarised ■ To define population, community and ecosystem below. ■ To realise that available resources change through the year Changing with the seasons The ability of the habitat to supply living organisms with their requirements may vary Environmental survival kit at different times of year. The ecosystem in the All living organisms depend upon their photograph opposite will only exist for a certain environment for three ‘survival essentials’. period of time – as food or water becomes These are a supply of food, shelter from exhausted some animals may leave. These will undesirable physical conditions and a breeding then be followed by the predators which feed on site. The living organism interacts with its them. The great animal migrations seen in East environment – for example, a living plant: Africa result from the changing conditions in the ■ removes carbon dioxide, water and light animals’ environment, for example: energy from its habitat ■ poor rain means little growth of grass ■ may be eaten by an animal or a parasite ■ herbivores leave for areas of fresh growth ■ depends upon soil for support. ■ carnivores follow herbivores Factors in the environment affect the growth of ■ (then scavengers follow carnivores!). the plant. Some of these factors are biotic – other Living together Carbon dioxide Living organisms normally exist in groups. The names given to these groups, and the way they interact with the abiotic environment, are explained opposite. A living organism interacts with its environment Abiotic Biotic (‘living’) ‘Fertiliser’ Food (‘non-living’) factors e.g. in faeces factors predators food Water and mineral mates ions from soil A giraffe feeds on a thorn tree. The tree requires water, mineral ions, carbon dioxide and light to grow. The giraffe Physical e.g. Climatic e.g. may provide carbon dioxide from respiration, and ions oxygen and carbon temperature from decomposition of its faeces. dioxide concentration humidity light intensity water availability 222 ORGANISMS AND THEIR ENVIRONMENT A population is all of the members of the same species (e.g. wildebeest) in a particular area. Air, water and soil A community is all of the make up the abiotic populations of living organisms environment. in one area (e.g. acacia trees, zebra, wildebeest and grass). The community is the biotic environment. An ecosystem is all the living organisms and A habitat is a part of the environment that the non-living factors interacting together can provide food, shelter and a breeding site in a particular part of the environment. for a living organism (e.g. a patch of grassland). Organisms exist in groups within an ecosystem Q 1 Define the terms population, community and The students measured the amount of sunlight ecosystem. reaching each layer at different times in the year. 2 Name two abiotic factors that might determine Their results are shown on the graph. whether or not a habitat is suitable for a living organism. 3 Suggest two ways in which a plant and an animal in the same habitat may interact. 4 What must a habitat provide? Light reaching 5 How are the following observations related? tree layer ■ Very few flying insects are found in Britain during Light intensity the winter. Light reaching ground layer ■ Swallows migrate to Africa when it is winter in the UK. ■ Hobbies (small bird-eating falcons) leave Britain in Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec late autumn. Month 6 What is meant by the term ecology? 7 a A group of students were studying a forest.They i During which month did most light reach the noticed that the plants grew in two main layers. They tree layer? called these the tree layer and the ground layer. ii During which month did most light reach the ground layer? iii Suggest why the amount of sunlight reaching tree layer the ground layer is lower in mid-summer than in the spring. b The pupils found bluebells growing in the ground layer. Bluebells grow rapidly from bulbs. They flower in April and by June their leaves have died. i Suggest why bluebells grow rapidly in April. ii Suggest why the bluebell leaves have died by June. ground layer 223 ORGANISMSO AND THEIR ENVIRONMENT 4.2 Flow of energy: food chains and food webs OBJECTIVES lost but is converted into some other form. ■ To know that the feeding relationships in an ecosystem During respiration, some energy is transferred can be expressed as food chains to the environment as heat. The flow of energy ■ To understand why energy transfer through an through a food chain, and the heat losses to ecosystem is inefficient the environment, are illustrated in the diagram ■ To understand why complex food webs are the most stable opposite. Food webs Food chains Since so little energy is transferred from the base The most obvious interaction between different to the top of a food chain, a top carnivore must eat organisms in an ecosystem is feeding. During many herbivores. These herbivores are probably feeding, one organism is obtaining food – energy not all of the same species. In turn, each herbivore and raw materials – from another one. Usually one is likely to feed on many different plant species. All organism eats another, but then may itself be food these interconnected food chains in one part of an for a third species. The flow of energy between ecosystem can be shown in a food web. different organisms in the ecosystem can be The more complicated a food web, the more stable shown in a food chain, as in the diagram below. the community is. For example, in the forest food web shown opposite, if the number of squirrels Energy transfer is inefficient fell, the owls could eat more worms, mice and rats. The amount of energy that is passed on in a The mice and rats would have less competition for food chain is reduced at every step. Since energy food from squirrels, and so might reproduce more can be neither created nor destroyed, it is not successfully. Sunlight provides the Producers, usually green plants, Consumers are organisms that obtain Secondary consumers energy to drive the make their own organic nutrients, food energy by feeding on other organisms. are carnivores. They obtain food chain. usually using energy from sunlight their energy by eating Primary consumers are herbivores. through photosynthesis. other animals. They obtain their energy in food compounds obtained from producers (i.e. plants). Arrows point in the direction of Secondary consumers may be energy flow along eaten by tertiary consumers. the food chain. These longer food chains are Decomposers, fungi and many bacteria, more common in aquatic obtain their energy and raw materials habitats. The final consumer from the wastes (e.g. faeces) and remains in the food chain is called the (e.g. dead bodies) of other organisms. top carnivore. Food chains show energy flow through an ecosystem. The position of each organism in the food chain represents a different trophic (feeding) level. 224 ORGANISMS AND THEIR ENVIRONMENT Energy transfer Less than 1% of the Respiration losses occur from each trophic energy released from level. Respiration is not 100% efficient and the Sun falls onto leaves. eventually all of this energy is lost as heat. Because of this, food chains rarely have more than 4 or 5 trophic levels. R Energy transfer to Energy transfer to R R primary consumer R secondary consumer is only 5–10%: is between 10% and 20%: much of plant body animal material has a higher Energy ‘fixed’ by producers is indigestible energy value is only 5–8% of the energy consumer rarely eats animal material is more that falls onto their leaves, whole plant – roots digestible. because: or stems may be some is transmitted left behind. (passes right through) some is reflected some is not the correct wavelength (only red or blue light is absorbed – see page 44). Key R respiration Note that energy flow is not cyclic! chemical sunlight bond heat energy Energy transfer to decomposers is very As a result there must be a variable, but eventually the entire energy content continuous input of light energy to of the animal and plant remains will be released ‘drive’ life in an ecosystem. as heat from inefficient respiration. It is more efficient for humans to eat plants than to feed plants to animals, and then to eat the animals. Every step in the chain loses 80–90% of the available energy as heat, so THE FEWER ‘STEPS’ THE BETTER! A simple forest food web Q Trophic level 1 Define the terms producer, Tertiary Hawk consumer consumer and decomposer. Which of these could be omitted from an ecosystem? Explain your answer. Secondary Starling Owl consumers 2 Write out a food chain from a named ecosystem which you have studied. Primary 3 Why are food chains usually Snails Woodlice Worms Rats Mice Squirrels consumers restricted to three or four trophic levels? Producers Fallen leaves Living leaves Grasses Seeds 225 4.2 Flow of energy: food chains and food webs More examples of feeding relationships Food chains and food webs in aquatic (watery) environments can be longer than those on the land. This is because this type of environment has space and ideal growth conditions for many producers. Even with energy losses at every stage there is enough ‘trapped’ energy for more steps in the chain.

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