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Energy Flow and Nutrient Cycles
This describes a specific level in a food chain. The term trophic refers to nutrition.
There are four important levels in most food chains:
- Producers: Organisms which convert some of the energy from the sun into stored chemical energy (usually plants).
- Primary consumers: Organisms that obtain energy by consuming producers. They are herbivores.
- Secondary consumers: Organisms which obtain energy by consuming primary consumers. They are carnivores.
- Decomposers: These organisms form the end point of every food chain. They are bacteria or fungi that obtain their energy by breaking down dead organisms from the other trophic levels.
Each description of a trophic level will describe an organisms role in the ecosystem. Organisms may occupy more than one trophic level, (e.g. when acting as omnivores).
Transfer of energy between trophic levels is relatively inefficient. Energy is transferred from one trophic level to another as organisms are consumed.
In primary producers the main energy input is from the solar energy. In a plant, not all of the solar energy available actually makes it into the leaf.
There is loss of energy by reflection from the leaf, transmission through the leaf, and because some of the energy is the incorrect wavelength.
The energy that is taken up by the producer is then fixed by photosynthesis, although again a proportion of this energy is lost as it is used up during photosynthetic reactions.
Of the energy that is fixed in photosynthesis some will be used during respiration whilst the remaining energy is the portion that is incorporated into the biomass. It is the energy that is incorporated into the biomass that is available for the next trophic level.
In the consumer a further series of energy losses occur. The consumer will take in a certain amount of energy from the trophic level beneath it.
This energy intake does not equal the amount of energy available in the biomass of this organism since feeding is an inefficient process. There will be a loss of energy through the production of urine and faeces, as well as losses through respiration and heat loss. This leaves a proportion of the energy consumed to be incorporated into the biomass.
It is generally accepted that only around 10% of the energy gained from the previous trophic level is passed on to the next level. All other energy is lost as described above. This limits the number of trophic levels in any food chain.
Ecological pyramids are used as a tool to illustrate the feeding relationships of the organisms, which together make up a community.
Pyramid of numbers
This is the simplest way of illustrating the feeding relationships within a community. The commonest form shows that the numbers of organisms occupying each trophic level decreases from producers to secondary consumers and beyond.
Two problems with this form of pyramid are that the numbers involved may be huge (in the hundreds of thousands) and some pyramids may be inverted.
Pyramid of biomass
This indicates the feeding relationship between organisms occupying different trophic levels with reference to their biomass.
Biomass can be measured as either wet mass or dry mass. Measuring the dry mass is more accurate as it does not include the variable water content of organisms.
The commonest form of the pyramid of biomass shows that the total biomass of organisms occupying each trophic level decreases from producers to secondary consumers and beyond.
There is still the problem that a pyramid of biomass can be inverted and also it does not take account of changes over time. The sampling must all be carried out at one moment in time and therefore indicates the standing crop and not the productivity.
Pyramid of energy
This is the most accurate representation of the feeding relationship between the organisms at different trophic levels. It takes into account the energy gains and losses over a period of time.
These consider how inorganic nutrients cycle through the various trophic levels and remain constantly available.
The carbon cycle
Carbon dioxide in the atmosphere and dissolved carbon dioxide in the oceans provide the major source of abiotic carbon for organisms.
The carbon is fixed from the carbon dioxide by photosynthesis to form organic compounds such as carbohydrates, proteins and lipids in producers.
The fixed carbon dioxide is then taken up by primary consumers and passed on to secondary consumers and beyond.
Carbon can be returned to its abiotic source via respiration, combustion of fossil fuels, and death and decay by decomposers.
The nitrogen cycle
The abiotic source of nitrogen is atmospheric nitrogen gas.
Nitrogen fixing bacteria convert atmospheric nitrogen to nitrates in the soil via ammonia and nitrites.
Nitrates can be absorbed from the soil by plants, which convert the nitrates and incorporate the nitrogen into organic nitrogen compounds.
The organic nitrogen compounds are passed on to other trophic levels through feeding.
Death and decay of plants and animals returns the nitrogen to the soil as organic nitrogen compounds.
Nitrifying bacteria will produce nitrates from these organic nitrogen compounds.
Denitrifying bacteria are able to return nitrogen to its abiotic source by converting nitrates to nitrogen gas.