Energy Flows in Ecosystems
The food chains and food webs discussed on another page are fairly trivial and are only qualitative.
At A Level it is necessary to be able to discuss feeding relationships in ecosystems in quantitative terms.
We have to consider the number of individuals of each species, the biomass of each species and the quantity of energy present in each species and why these values change as we move up a food chain
Pyramids
Various types of data can be collected from an ecosystem.
The simplest example involves counting all of the individuals of each species
The number of individuals at each trophic level in an ecosystem can then be show as a:
1. Pyramid of Number
In this example (from http://www.sturgeon.ab.ca/rw/Pyramids/pyrakind.html) the number of individuals at each trophic level are shown. The length (or area to be more accurate) of each bar is proportional to the number of individuals.
This often produces an upright pyramid - but not always
In the diagram below, two pyramids of number are shown.
In example A the pyramid is upright while in example B (which shows a food chain from an ecosystem present on a single tree) the number of individuals at the producer level is relatively small (it is in fact one - the tree). This latter pyramid is said to be inverted.
Image from http://www.envf.port.ac.uk/geog/teaching/environ/sect2-3c.htm
The inverted pyramid may give a misleading impression of the ecosystem so the pyramid of biomass also shown in the diagram is preferred.
With certain constraints the pyramid of biomass will always be an upright pyramid so the pyramid of biomass shown corresponds to both of the pyramids of number in the diagram
2. Pyramid of Biomass
To produce a pyramid of biomass the dry weight of each species present is calculated.
The dry weight of an individual is the mass of organic material contained within that individual - the determine this the organism is dried in an oven to remove the water then weighed. This is done because the water content of an organism (which has no impact on the amount of energy or nutrients available to the ecosystem) varies considerably
The length (or area to be more accurate) of each bar is proportional to the dry weight (biomass) of all the individuals at that trophic level
The pyramid of biomass will always be upright since all organisms have to consume more biomass than they contain.
The exceptions to this rule occur when measurements are taken at particular times e.g. the biomass of producers may be relatively low in winter (when there is little sunlight) when compared to the biomass of consumers which persists all year round.
Showing pyramids of biomass for the whole year (and thus taking into account seasonal variations) will produce upright pyramids.
The units for pyramids of biomass are: dry weight of organic matter (per square metre)
3. Pyramid of Energy
The most informative pyramid is the pyramid of energy
This shows the amount of energy (in kiloJoules [kJ]) present at each trophic level.
This pyramid is also always upright (given the same caveats as for the pyramid of biomass)
The full units for a pyramid of energy are: kJ m-2 year-1 (sometimes kcal m-2 year-1 where kcal is a now obsolete measure of energy)
Why are there losses of biomass and energy as we move up trophic levels?
Pyramids of biomass and energy show a reduction with movement up the trophic levels.
Biomass is approximately proportional to energy because the biomass is predominantly organic material (with some inorganic material such as ions) so this discussion considers the two interchangeably
The reason that not all energy (or biomass) at a lower trophic level is passed to a higher trophic level is because the energy is lost to:
- heat generated by respiration
- not all of the biomass consumed can be used by the consumer - some material is indigestible and passes out of faeces
Of these two the energy lost to heat in respiration is truly lost to the ecosystem.
Some of the energy contained in indigestible material will be consumed by decomposers and returned to the ecosystem (Don't forget that the decomposers also respire and some of the energy they consume is lost as heat)
In general only 10% of the energy consumed at a particular trophic level is available to the next trophic level - the rest is used to make indigestible material or is lost as heat
When energy from the sun is taken into account we witness a massive loss of energy - the vast majority of energy from the sun is not utilized by the producers
A real world example
The following image shows the flow of energy through an actual ecosystem; a river in Silver Springs, Florida. The image and the pyramids derived from it are from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FoodChains.html
Note that these data use kcal for energy
The data shown above has been used to produce this pyramid of biomass
and this pyramid of energy
Productivity and production
The Silver Springs data above also includes information about the productivity of the ecosystem
Some definitions:
Productivity:
- is the rate (per unit area per unit time) at which biomass is produced. So:
- primary productivity is:
- the rate at which producer (autotroph) biomass is produced per unit area per unit time
- secondary productivity is:
- the rate at which consumer (heterotroph) biomass is produced per unit area per unit time
Gross Production
- is the amount of energy entering a trophic level. It is measured per unit area per unit area so is also a rate.
- Gross Primary Production (GPP) is:
- the total amount of energy fixed by photosynthesis
- Gross Secondary Production is:
- the total amount of energy consumed by heterotrophs
Net Production
- is the rate of production of new biomass per unit area per unit time
- i.e. it is equivalent to productivity
- Net Primary Production (NPP) is:
- the rate of new biomass available for consumption by heterotrophs
The difference between gross production and net production is the respiration rate (R)
So we have the equation:
GPP = NPP + R