Productivity and farming
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Gross primary production
the chemical energy stored in plant biomass, in a given area or volume energy per unit area, e.g. J m–2 mass per unit area, e.g. g m–2
Net primary production
The chemical energy stored in plant biomass after respiratory losses to the environment have been taken into account
Net primary production explained in detail
Plants convert light energy to chemical energy in the form of glucose during photosynthesis; the total energy stored in this way is gross primary production (GPP) Some of the energy stored during GPP is lost to the environment, e.g. when waste heat is generated during respiration The energy that remains in the plant tissues after these energy losses is the NPP NPP is important because it represents the energy that is AVAILABLE to consumers in the ecosystem
Net primary production calculation
NPP = GPP - R Where: GPP = gross primary production R = respiratory loss to the environment
Why is there a low % of energy transfer between trophic levels?
Whole organism not consumed Some organisms parts are indigestible and lost in faeces Energy lost through other excretion e.g., urine Heat loss to the environment through respiration Highest in mammals and birds which maintain high core temperatures
Net production of consumers
The energy that is transferred to the tissues of consumers is the net production of consumers This is also known as secondary production
Calculation for net production of consumers
N = I - (F + R) Where: I = the chemical energy in ingested food F = the chemical energy lost to the environment in faeces and urine R = the respiratory losses to the environment
Energy is lost in the form of faeces and urine because:
consumers are not able to digest all of the food they eat, so some is egested as faeces
What are the effects of inefficient energy transfer?
Biomass decreases up the food chain Net production is very low at the top of food chains Food chains only support 4/5 trophic levels
% efficiency of energy transfer
= (Net production of trophic level / net production of previous trophic level) x 100
primary production
transfer of energy to the tissues of plants
Productivity can be defined as
the rate of primary or secondary production
secondary production
transfer of energy to the tissues of consumers; also known as net production of consumers
How can farming practices be improved to increase efficiency of energy transfers?
Simplifying food webs Reducing respiratory loss
Simplifying food webs
Pests feed on crops, reducing crop biomass and meaning that crop plants need to expend energy on herbivory defences rather than on growth This reduces the NPP of crops and therefore the energy available to humans Insecticides kill pests that consume crops Simplifies food webs
Reducing respiratory loss
Herbicides kill weeds that compete with crops for light energy Livestock in Small, heated enclosures to reduce respiratory losses by restricting movement More energy for growth Diets supplemented with antibiotics to reduce infection Less respiratory losses from disease