spec points

• the efficiency of biomass transfers between trophic levels

  • efficiency = biomass transferred/biomass intake x 100

• how human activities can manipulate the transfer of biomass through ecosystems.

efficiency of energy transfers

• very large proportion of Sun's energy isn’t made available to photosynthetic plants, because

  • Light falls away from plants

  • Light passes through leaves or is reflected away

  • Light is a mixture of wavelengths, and only certain wavelengths stimulate photosynthesis

• During photosynthesis, primary producers such as plants and algae convert light energy to chemical energy in biological molecules

• The storing of this chemical energy as plant biomass makes a certain amount of energy available to the next trophic level, the primary consumers

• Only a small percentage of plant biomass becomes biomass in the primary consumer because:

  • Not all the plant's biomass is eaten by the primary consumer

  •  Not all the consumer's biomass intake is digested

    • Think about the energy content of cow dung, which can be dried and used as a heating/cooking fuel, as it contains a lot of undigested biomass e.g. cellulose

  • The primary consumer converts a lot of chemical energy to movement and heat, and only a small amount to new biomass in its own body

• The efficiency of biomass transfer from one trophic level to the next is low, typically around 10%

• Scientists use 3 calculations for measuring plant productivity in ecosystems:

  1. Primary productivity

  2. GPP

  3. NPP

primary, gross, and net primary productivity

1. primary productivity measures energy produced by photosynthetic organisms in ecosystem

2. gross primary productivity: measures total amount of organic matter produced by photosynthetic organisms in ecosystem. Plants also use some of matter they produce for respiration. Remainder is available for heterotrophs

3. net primary productivity (NPP): measures amount of matter produced that is available for heterotrophs. NPP for ecosystem is generally measured in grams per square meter (g/m2)

net primary productivity (NPP): rate which energy is transferred into organic molecules that make up new plant biomass, that is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account

• NPP = GPP - R

  • GPP = gross primary productivity

  • R = respiratory losses

gross primary productivity (GPP): rate at which energy is incorporated into organic molecules in the plants in photosynthesis, that is the chemical energy store in plant biomass, in given area/volume, in given time

• thus NPP = GPP - R

net primary production is available for plant growth + reproduction as well as to other trophic levels in ecosystem ex. decomposers + herbivores

net production of consumers (N) such as animals can be calculated by N = I - (F+R) where:

• I = chemical energy store in ingested food

• F = chemical energy lost to environment in faeces + urine

• R = respiratory losses to environment

how can human activities can manipulate the transfer of biomass through ecosystems

• Human activity can adjust the efficiency of transfer of biomass between trophic levels, usually to maximise it in the context of maximising agricultural productivity

For producers, arable farmers can adopt these methods

• Providing artificial light in greenhouses on overcast days

• Optimising planting distances between crops

• Irrigation to maximise growth in dry weather

• Use of fertilisers

• Selective breeding for fast growth

• Use of fungicides/pesticides

• Fencing to exclude grazers

• Ploughing and herbicides to kill weeds

• Plant crops that store energy in edible form e.g. seeds, fruit, tubers

Livestock farmers can adopt these methods for primary consumers (grazers)

• Use of good quality feeds / food supplements

• Use antibiotics and vaccines to reduce disease

• Control predation with fencing or with indoor animal husbandry

• Reduce competition for grazing e.g. rabbits, deer

• Indoor husbandry to reduce energy loss from movement or from getting cold outside