Energy transfer between trophic levels

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Energy losses in food chains

> The sun is the source of energy for ecosystems

> Green plants may only capture 1% of this energy

>Most of the Sun's energy is not converted to organic matter by photosynthesis because:

  > Over 90% of the Sun's energy is reflected back to space / absorbed by atmosphere

  > Not all wavelengths of light can be absorbed & used for photosynthesis

  > Light may not fall on a chlorophyll molecule

  > Factors such as CO2 may limit rate of photosynthesis

> Gross production = total quantity of energy that plants in a community convert to organic matter

> Net production = the rate at which plants store energy (GP - energy used in respiration) 


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Energy losses continued...

> Only about 10% of the food stored in plants is used by primary consumers for growth

> Secondary and tertiary consumers transfer about 20-30% of energy to their bodies

> Low percentage of energy transferred at each stage due to;

  > Some of the organism is not eaten

  > Some parts are eaten but cannot be digested > lost in faeces

  > Some of the energy is lost in excretory materials > eg urine

  > Energy lost as heat from respiration or directly from their body to the environment

> Relative inefficiency of energy transfer between trophic levels, explains why;

  > Most food chains have only 4 or 5 trophic levels - insufficient energy available to support a large enough breeding population at trophic levels higher than these

  > Biomass is less at higher trophic levels

  > Total amount of energy stored is less at each level (up the food chain)

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Energy transfer and ecological pyramids

> Energy available at different trophic levels is usually measured in KJ per square metre per year

Energy Transfer = energy available after the transfer / energy available before the transfer x 100


> Pyramids of number - number of organisms at lower levels usually higher than those at higher levels. 

> Drawbacks include: Does not account for size - often not a pyramid at all (inverted) - Number of individuals can be so great they cannot be displayed on the same scale as other species

> Pyramids of biomass - total mass of plants/animals in a particular place at each trophic level. Measured in grams per square metre or cubic metre

> Drawbacks: dry mass must be used so small sample may be killed (not representative). Taken at a particular time (seasonal differences not taken into account)

> Pyramids of energy - energy stored in organisms - reliable because two organisms of the same size could store different amount of energy - measured in KJ per square metre per year

> Drawbacks: difficult to collect this data

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Agricultural Ecosystems

> Agriculture tries to ensure that as much of the available energy from the sun as possible is transferred to humans. Increases the productivity of the human food chain as energy is directed away from other food chains.

> Productivity = rate at which something is produced

> The rate at which plants assimilate chemical energy = gross productivity, usually measured in KJ per square metre per year

> Net productivity is affected by 2 main factors:

  > Efficiency of the crop to carry out photosynthesis (improve by having all necessary conditions)

  > Area of the ground covered by the leaves of the crop

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Agricultural ecosystems continued...

> Natural ecosystems - only source of energy is the sun. To maintain an agricultural ecosystems we need to prevent the climax community developing - exclude most of the species in that community, leaving just the desired crop. This requires an additional input of energy (plough fields, sow crops, remove weeds, suppress pests & diseases, feed & house animals, transport stuff). This additional energy come in 2 forms:

  > Food - Farmworkers expend energy that comes from the food they eat

  > Fossil fuels - used to plough, harvest & transport crops, produce & apply fertilisers & pesticides and home, feed and transport livestock

> Productivity is relatively low in natural ecosystems. The additional energy input to agricultural ecosystems used to increase productivity of desired crop by reducing the effect of limiting factors. Fertilisers provide essential ions & pesticides are used to destroy pests & prevent disease

> Remember rate of photosynthesis is determined by the factor in shortest supply.

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Chemical & biological control of agricultural pest

> 'Pest' - an organism that competes with humans for food or space or is a danger to health.

> Pesticides - poisonous chemicals that kill pests. Effective pesticides should:

  > Be specific - only toxic to the organism at which it is targeted. Should be harmless to humans, other organisms and its natural predator.

  > Biodegrade - once applied breaks down into harmless substances in soil. Also needs shelf life

  > Cost - effective - high development costs for a pesticide that is only effective for short time due to genetic resistance that builds up in pests

  > Not accumulate - no build up in the organism or any part of the food chain

> Biological control - use predators or parasites of the pest. To CONTROL, not eradicate pest.

> Disadvantages: time delay, control organism may become a pest itself

> Advantages : very specific, once introduced the control organism reproduces itself, pests do not become resistant

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Pest control systems & productivity

> Integrated pest-control systems aim to integrate all forms of pest control rather than relying on one. Emphasis on deciding an acceptable level of pest rather than eradicate. It involves:

  > Choose animal/plant varieties suitable to the area and as pest-resistant as possible

  > Manage environment to provide suitable habitats, close to crops, for natural predators

  > Regularly monitor crop for signs of pest - can take early action

  > Remove pests mechanically if pest exceeds an acceptable level

  > Use biological agents if necessary / available

  > Use pesticides asa last resort - if pest population out of control

> Insect & fungal pests spread rapidly in monocultures

> Pests of domesticated animals may cause disease --> reduced productivity

> Important to limit the effect of pests on productivity and balance out cost. Farmer has to produce cheap food, make a living & conserve natural resources

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