Net primary prductivity and efficiency of biomass transfer

what is net primary productivity

• The amount of chemical energy that is available to heterotrophs in an ecosystem

• The rate at which the light is used to make organic material by producers is determined by the rate of photosynthesis.

• Small percentage of light from the sun results in production of plant material. 1-3%

why energy lost from sun to plant

• Light passes through leaves or is reflected away

• Light hits non-photosynthetic parts of the plant e.g. bark or flowers

• Light is a mixture of wavelengths and only certain wavelengths are absorbed in photosynthesis

• Plants release energy during respiration, some of which is lost to the environment in the form of heat

Why no trophic level 4 or 5

At trophic level 4 or 5 the organisms would need to move over

large distances and more energy is required to find enough food

and mates. Survival is impossible as little energy is left for growth

and reproduction

what is gross primary productivity

Gross primary productivity is the rate of chemical energy fixed during photosynthesis by all producers in an ecosystem, measured in KJ m-2 year -1

We can measure it in units gC m-2year-1 grams of carbon assimilated per metre squared per year.

Could also look at energy kJ m-2year-1photosynthesis.

why net primary productivity difficult to measure?

Productivity is difficult to measure. The mass of samples should be taken and then these are scaled to represent theecosystem.

why biomass used in plants

• Plants use 25% of the biomass they produce in respiration.

They do this to release energy for their own metabolic needs.

• They respire and they breakdown glucose to make ATP.

Respiration is exothermic. It increases the internal energy store of surroundings. The remainder of the material is stored in new plant tissue. The energy stored in biomass is known as net primary productivity.

GPP-R = NPP

why different ecosystems will lead to differences in NPP?

As the light levels vary in different latitudes. Nearer the poles light levels will be lower than closer the equator.

What is NPP dependent on?

• Light availability

• Temperature

• Water

Not all parts of the earth contribute equally to the to the overall NPP of the earth. If you add up NPP of earth and then look at the NPP of specific ecosystems you can see how much they contribute. Tropical rainforest 5% of earth contributes 30%

How humans effect net primary productivity?

Human population growing rapid rate and humans consuming 40% of world NPP

How to calculate energy flow in the ecosystem

• It is difficult to measure energy transfers for whole ecosystems because assumptions made in calculating the energy in all the organisms.

• Identify most abundant species in whole ecosystem. Find out how many there are and average body size and how much energy is stored in the body.

• You also need to work out how much of the biomass is transferred to decomposers

why dry biomass 

• because the amount of water stored in tissues can vary

how to measure dry biomass

• Dry biomass can be measured by drying a sample of the organism in an oven at a low heat and weighing the sample at regular intervals until the mass becomes constant; this will be the dry biomass

  • The organism needs to be dead for this process to be carried out, so an estimate is often used

• Depending on the transfer being studied, it may then be necessary to multiply up the sample to take into account the size of the area or number of organisms being studied

how to calculate efficiency of biomass transfer?

Efficiency of biomass transfer = (biomass transferred biomass intake) 100

• Biomass transferred refers to biomass that has passed to the higher trophic level while biomass intake refers to biomass of the lower trophic level that has been consumed

• Efficiency of biomass transfer is given as a percentage 

How energy flow may differ in different weathers/seasons

• Temperate regions more biomass in plant and animal bodies in summer. More biomass in decomposers in winter.

• Tropical regions will have varying biomass in different trophic levels in wet vs dry season. These factors need to be taken into account when estimating energy stores in the different trophic levels

Values used to measure biomass for energy transfers

The values used are mean values and have large standard errors. A small standard error indicates that the mean is close to the true value. Large standard error means values are not very reliable