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Biomass transfer

Don’t plants also use their own photosynthesised goodies (glucose) to provide energy for their own business (growth, reproduction, etc.) via respiration, and waste stored energy in their tissues upon their death? Of course they do. So less must be available for whatever eats the plant. And whatever eats the plant will also lose energy through excretion for example, so whatever eats this herbivore will have even less energy available to themselves.

Therefore, at each trophic level in the energy transfer (feeding) hierarchy there is a net loss of energy. This results in a pyramid:

The plants at the bottom are the photosynthesising primary producers. They hold the most energy (Joules) and are fed on by herbivores – primary consumers.

Notice only about 10% of that energy is available one trophic level higher. This is taken by carnivores feeding on herbivores – secondary consumers.

At the very top of the pyramid a mere 0.001% of the original 10,000 J remains (10 J). This is taken by tertiary consumers feeding on carnivores.

Because such tiny amounts of energy are left at the highest level, it’s rare to find quaternary consumers or above.

Notice how the above pyramid is based on energy alone. There are 2 other types of pyramid: biomass and numbers. A numbers pyramid is based on simple numbers e.g. 1,000 plants at the bottom eaten by 100 herbivores eaten by 10 carnivores eaten by 1 omnivore.

A biomass pyramid on the other hand takes into account the dry mass of the organisms. This can look something like this:

As you can see, there is a clear correlation between these types of pyramid. However, you can get irregular “pyramids”. For example, a single tree can feed several hundred insects, etc.

Energy transfer and its efficiency

As seen previously, plants produce a great deal of energy which is used up increasingly at every trophic level. This is the basis on which decisions are made in agriculture and rearing of domestic livestock.

In the wild, both plants and animals are subject to a lot of energy loss due to pests, physical activity or insufficient nutrients. This results in a relatively inefficient flow of energy between trophic levels. We think of this in terms of net productivity.

Net productivity is equal to gross productivity minus respiratory loss.

In both terms, productivity refers to the amount of leftover useful tissue such as cereals or animal flesh.

Gross primary production (GPP) is the chemical energy stored in plant biomass in a specific area or volume, while net primary production (NPP) is the chemical energy store in plant biomass after accounting for respiratory losses (R) to the environment.

This gives NPP = GPP – R

NPP can end up in plants’ own growth and reproduction or it can be taken up by herbivores and decomposers.

Consumers such as animals have their own net production termed N which becomes equal to:
N = I – F + R, where
I = chemical energy stored in Ingested food
F = chemical energy lost to the environment in Faeces and urine (stay classy)
R = chemical energy lost in respiration

The rates of productivity for primary and secondary production respectively are primary and secondary productivity. These can be expressed as energy per area per time, e.g. kJ ha-1 year-1, which is kilojoules per hectare per year.

As opposed to the wild, in human-made growth environments this respiratory loss is kept as low as possible. Extreme measures are taken to achieve this, which include:

1. The use of chemical pesticides and biological agents to kill all or certain organisms which may infect or feed on plants e.g. insects, fungi, small animals
2. Intensive rearing of animals which includes keeping them indoors and in confined spaces to prevent their energy being lost on movement; and administering antibiotics to prevent mass spread of infection

To enhance plant growth fertilisers are used, whether natural (manure) or artificial.





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