Energy flow through Ecosystems

Key definitions/summaries

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  • Created by: Fatima
  • Created on: 15-11-13 21:03
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Secondary productivity= rate at which consumers accumulate energy in the form of cells or tissues
Biological productivity= the rate at which biomass accumulates in an ecosystem. Has two components
o Primary productivity= the production of new organic matter by green plants
o Secondary productivity= the production of new organic matter by consumers
Biomass= the dry weight of organic matter comprising of a group of organisms in a particular habitat
Energy flow through consumers
Consumers have a conversion efficiency of ~10% so...for every 100g of plant material ingested, only ~ 10g is
incorporated into herbivore biomass.
o Only part of the NPP of the ecosystem is transferred to primary consumers
For example...A cow feeding on grass in a field...
Some plant material is not eaten cattle eat grasses and edible weeds but not roots / woody parts
Cows feed on plant material which contains cellulose and can't be digested this passes out of the body
as faeces containing a high proportion of undigested matter.
o This energy is not wasted as it is available to decomposers
Some food material is eaten by other herbivores e.g. rabbits
Much of the energy in the food consumed by a cow is mostly lost by 2 processes
1) Respiration about 30% is lost as heat
2) Excretion about 60% is lost in urine/ faeces
To increase productivity farmers keep cattle indoors reduces heat loss!
Herbivores have a lower secondary productivity than carnivores carnivores are more efficient at
energy conversion that herbivores. This is because...
o Carnivores proteinrich diet is more readily and efficiently digested
o Only about 20% of the energy intake is lost in the faeces and urine compared to a loss of 60% in
o Carnivores absorb approx. twice as much energy per unit mass of food compared with herbivores
The loss of energy at each trophic level gives ecological pyramids their characteristic shapes
Pyramids of energy
No of organisms, biomass or amount of energy contained at each trophic level can be represented in pyramids.
These provide a quantitative account of the feeding relationships in the community.
Pyramids of energy
o Most accurate way to represent feeding relationships
o Show the quantity of energy transferred from one trophic level to the next, per unit area or volume, per
unit time. This represents the total energy requirement of each successive trophic level in a food chain.
o As material passes up through the food chain, energy is lost (respiration as heat/excretion) so the size
of bars decreases sharply. Since only some of the energy is transferred from one level to the next, the
pyramid is never inverted as in biomass pyramids.
o Use of a set time overcomes the problems which arise when ecosystems are compared simply by
counting or measuring the standing crop of organisms.
o Easier comparison of efficiency of energy transfer from one trophic level to the next between different
o Obtaining the data can be complex and difficult.


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