5.2 Ecology - interdependence of organism and environment

A feeding relationship in which a carnivore eats a herbivore, which itself has eaten plant matter, is called a food chain. Light is the initial energy source. This is generated by photosynthesis, the main route by which energy enters an ecosystem.  In a food chain the arrows point to the consumers thus indicating the direction of energy transfer:

oak-> oak beauty caterpillar -> caterpillar-hunting beetle -> common shrew -> red fox

Energy is transferred through the living organisms of an ecosystem when organisms eat other organisms, e.g. producers are eaten by organisms called primary consumers. The level at which an organism feeds is called the trophic level (producers, primary consumer, secondary consumer etc.) 

Food chains show simple lines of energy transfer whereas food webs show lots of overlapping food chains in an ecosystem.

Energy locked up in things that can't be eaten (bones, poo) gets recycled back into the ecosystem by decomposers that break down dead or undigested material

Only a limited amount of the energy transferred between trophic levels is available to be transferred to the next organism in the food chain. Only 10% of what is eaten by a consumer is built into the organism's body meaning around 90% of the total available energy is lost. 

Some of the available energy (60%) is never taken in by the organisms in the first place. E.g:

Plant's can't use all the light energy that reaches their leaves as some is the wrong wavelength, some is reflected and some passes straight through the leaves.

Some parts of food, e.g. roots or bones, aren't eaten by organisms so the energy isn't taken in. 

Some parts are indigestible so pass through organisms and come out as waste, e.g. faeces. 

The rest of the available energy (40%) is taken in (absorbed) - this called gross productivity but not all of this is available for the next trophic level either:

30% of the total energy available (75% of the gross productivity) is lost to the environment when organisms use energy produced from respiration for movement or body heat. This is called respiratory loss. 

10% of the total energy available (25% of the gross productivity) becomes biomass (e.g. it's stored or used for growth) - this is called the net productivity and is the part that can be consumed by the consumer in the next trophic level. 

Net Productivity = Gross Productivity - Respiratory Loss 

% efficiency of energy transfer between trophic levels = net productivity of a level / net productivity of previous level x 100 

When talking about producers (organisms that produce energy such as plants which get their energy from the sun) net productivity is called net primary productivity (NPP) and gross productivity is called gross primary productivity (GPP) so the equation becomes:

NPP = GPP - Plant Respiration