3.5.3 Energy and ecosystems

The ultimate source of all energy within an ecosystem is the sun. We have seen in 3.5.1 how plants absorb light energy and CO2 and turn it into biomass (well, they turn it into glucose which they then turn into biomass). The rest of the sugars synthesised are used by the plant as respiratory substrates (glucose is a substrate of respiration).

Biomass
Biomass is the total mass of living material in a specific area at a given time. The presence of varying amounts of water male fresh biomass unreliable - when comparing biomass, dry biomass should be used. However, because obtaining dry biomass requires killing the organism only  small sample is used and this is hardly representative. Biomass is measured using dry mass per given area in a given time. The unit is g m^-2 where an area is being measured and gm^-3 where a volume is being measured. You can measure the chemical energy store of dry mass using calorimetry:

1. A dry sample is weighed and burnt in pure oxygen in a sealed chamber known as a bomb
2. The bomb is surrounded by a water bath
3. The heat of combustion causes a small temperature rise in this water
4. We know how much energy is required to raise the temperature of 1g of water by 1°, we can calculate the energy released from the mass of burnt biomass (in kJ kg^-1)

Productivity
Plants actually only convert between 1%-3% of the light energy available to them from the sun. The other 97%-99% is not absorbed because...

• Over 90% of the Sun's energy is reflected back into space by clouds/dust or it is absorbed by the atmosphere
• not all wavelengths of light can be absorbed/used for photosynthesis (the best are 640-700nm wavelengths of light)
• light simply may not fall on a chlorophyll molecule
• there may be a limiting factor on the rate of photosynthesis.

The gross primary production (GPP) is the total quantity of chemical energy store in plant biomass, in a given area/volume, in a given time. Some plants use as much as 20-50% of this energy for respiration - the remaining chemical energy store is the net primary productivity (NPP)...

net primary productivity (NPP) = gross primary productivity (GPP) - respiratory losses (R)

This net primary production is then available for plant growth, but also to other trophic levels if they eat the plant. Usually, around 10% of the available energy is used in growth by primary consumers, secondary and tertiary consumers use around 20% of the available energy. This is because:

• Some parts of the organism is not consumed (e.g roots)
• Some parts cannot be digested so are lost in faeces (e.g we can't digest sweetcorn)
• Some energy is lost in excretory materials (e.g urine)
• Some energy lost as heat from respiration --> lost to the environment

The net production of consumers can be calculated using the equation...

N = I - (F + R)

N = net production

I = chemical energy store of ingested food

F = energy lost in waste materials (faeces/urine)

R = energy lost in respiration

This explains why the total amount of energy available is less at each level as you move up the food chain, and why most food chains only have four or five trophic levels.

Farming practices
As we can see from above, energy transfer along the food chain is pretty dismal. Farming practices are designed to increase yields (the amount of harvest) by increasing the efficiency of energy transfer along the food chains which produce our food. 

Any practice that reduces the respiratory loses (energy lost as heat when respiring) will increase the yield in a human food chain. One practice that achieves that is the intensive rearing of domestic livestock. Energy conversion can be made efficient by ensuring that as much energy as possible from respiration goes into growth rather than other activities. This can be achieved by:

• restricting movement --> less energy is used in muscle contraction
• keeping environment warm (most intensively reared species are homeothermic) --> reducing heat loss from the body
• Controlling feeding --> animals receive the optimum amount and type of food for maximum growth
• no predators --> no loss to other organisms in the food web

Another practice that increases the efficiency of energy transfer is to reduce losses to non=human food chains - this can be done by simplifying food webs by reducing/eliminating organisms which compete with the plant/animal that is being farmed. E.g. pest control removed weeds/pests. The cost of pest control must be balanced with the benefits it brings.