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Ecosystems Notes

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5.3.1 Ecosystems Context Organisms do not work in isolation but form complex interactions, not just with other organisms but also with their physical environment. The efficiency of energy transfer limits the number of organisms in a particular ecosystem. Ecosystems are dynamic entities tending towards some form of climax community. Candidates should be able to: Define the term ecosystem.

State that ecosystems are dynamic systems. In most ecosystems populations rise and fall. This is because the community of living things in an ecosystem interacts with one another and with their physical environment.

Define the terms biotic factor and abiotic factor, using named examples. Biotic factor: Any living or biological factor. e.g. - Plants
- Animals
- Fungi Abiotic: Any non-living or physical factor. e.g. - Temperature
- Light intensity
- Humidity

- Oxygen levels
- Phosphate/nitrate levels
- pH of soil

Define the terms producer, consumer, decomposer and trophic level. Producer: An organism that produces food from carbon dioxide and water using light energy or chemical energy. Can be plant protoctistan or prokaryote. Consumer: An animal that eats other organisms Decomposer (Saprotroph): A microbe (bacterium or fungus) that lives on detritus. Trophic level: The stage of a food chain at which an organism feeds.

Describe how energy is transferred through ecosystems. Energy flows through ecosystems. Producers, such as algal films on the reef, convert light energy from the sun into chemical energy. Primary consumers, such as herbivorous fish, feed on the algae, becoming food for secondary consumers, such as predatory fish. This simple feeding relationship is a food chain.

Outline how energy transfers between trophic levels can be measured. Energy transfers can be calculated by measuring the energy content of samples of organisms from each trophic level. Each sample is dried to a constant mass and then burned in oxygen in a bomb calorimeter. The heat energy produced by the oxidation passes to a known mass of water, and the temperature rise of the water is measured. Given that 4.12 J of heat energy raises the temperature of 1 g of water by 1 degree Celsius, the energy content of each sample can be calculated.

Discuss the efficiency of energy transfers between trophic levels. Only a small part of the energy entering a trophic level becomes available to the next trophic level. The percentage varies according to the food chain concerned and the efficiency with which energy is transferred, but is rarely greater than 10%. Not much energy reaches animals at the top of food chains. Energy is lost in food chains because animals:
- Never eat all the available food
- Cannot digest all the food they eat
- Use energy in their respiration so they can move, hunt, chew, reproduce etc.
- Lose heat energy to their surroundings
- Lose energy in urine and faeces.

Explain how human activities can manipulate the flow of energy through ecosystems. Human activities in farming, forestry and fishing manipulate the flow of energy through an ecosystem by altering the productivity of one or more trophic level. For example…

Replacing natural vegetation and fauna with crops and livestock Deflecting natural succession to maintain grassland Increasing productivity of producers through soil improvement, irrigation, fertilisers and removal of competing weeds and damaging pathogens and pests. Increasing productivity of producers and consumers through selective breeding or genetic engineering Sheltering organisms from damaging environmental factors.

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