AQA A2 Unit 4 Energy in Ecosystems


Energy Transfer

-Energy enters most ecosystems as sunlight, which is transferred by photosynthesis into chemical energy in organic compounds.

-Food chains and food webs show the direction of energy flow from producers and through each level of consumers.

-Pryamids of numbers, biomass and energy represent the quantity of these parameters at each trophic level.

-Energy is lost as heat at and between each trophic level. The proportion of the energy that is passed on from one trophic level to the next is known as the efficiency of energy transfer. It is generally about 10%.

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-Primary productvity is the amount of enegy per unit area per unit time that is transferred from sunlight to chemical energy in plants. This is net primary productivity, NPP.

-All organisms use some of the chemical energy in thier organic molecules for thier own purposes. This energy is released by respiration. The energy that is still available to be passed on to the next trophic level is the gross primary productivity, GPP.

-GPP = NPP - Respiration

-Anything else that limits the rate of photosynthesis, such as a low carbon dioxide concentration, low temperature or low light levels, also limits primary productivity. 

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Using Fertilisers to Increase Productivity

-Farmers us fertilisers to ensure that productivity of crop plants is not limited by lack of inorganic ions such as nitrate, potassium or phosphate. The quantity of fertiiser to be applied must be judged carefully, as over a particular level the extra yield obtained may not outweigh the costs of the fertiliser.

-Natural fertilisers, such as farmyard manure, have some advantages and some disadvantages compared with inorganic fertilisers. They may improve soil structure, and are a good way of dealing with otherwise unused waste material. However, thier ion content is not usually known and cannot be controlled, so it is not possible to calcuate exactly how much should be applied for a particular cropgrowing in a particular soil type.

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Pest Control

-Insects and other pests cause immense amounts of damage to crops each year, greatly reducing yields. This can be a particular problem in developing counrties, where it contributes to food shortages.

-Pesticides are chemicals that kill pests. Insecticides and fungicides are widely used. They may be specific (killing only the pest species) or broad-spectrum (killing many different species). There are few specific pesticides, and they tend to be very expensive.

-In developed countries, there are strict regulations about the amount of pesticide traces that may be present on food offered for sale. Many pesticides used in the past have now been banned, as evidence emerged of thier possible harmful effects on human health.

-In the past, widespread use of persistent insetides such as DDT and dieldrin led to thier bioaccumulation, and subsequent damage to organisms at the top end of food chains. These are no banned in many countries, but are still used in some developing countries as the only practible and affordable method of controlling malaria and other insect-borne diseases.

-Natural selection may produce strains of the pest that are resisitant to a pesticide.

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Pest Control Continued...

-Biological control uses natural predators or parasites of the pest to keep its numbers at a low level. This avoids the use of potentially harmful pesticides, but care has to be taken that the control agent does not itself become a pest

-Integrated pest management uses a carefully worked out combination of biological control and pesticides to control the pest.

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Intensive Rearing of Domestic Livestock

-Intensive farming involves the use of high levels of inputs - such as fertilisers, processed feeds, medicines - in order to achieve high productivity and therefore high outputs. Extensive farming uses few inputs, and achieves low productivity and low outputs.

-Intensive farming may result in cheaper food, but this is often at a cost of damage to the environment, or poor living conditions for animals.

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