Biology Fieldwork

A Level

Discussion

Fieldwork at the seashore

3. Discussion

Ecological pyramids

Not all energy is transferred from one trophic level to the next. The number of organisms, biomass and energy at each trophic level can be represented as a pyramid.

In many ecosystems organisms can readily be sampled, identified and counted.

Different organisms can be assigned to different trophic levels (such as primary and secondary consumers), and the data collected in the field used to construct ecological pyramids.

  • Pyramids of number - these show the number of individuals at each trophic level
  • Pyramids of biomass - these show the biomass at each trophic level (total dry mass of organisms). Dry mass is found by placing specimens in an oven for 4 hours at 80° C to drive off all moisture. Historic data on dry biomass is often available.
  • Pyramids of energy - these show the total energy content (in kilojoules) of organisms at each trophic level. Energy content is found by placing specimens in a bomb calorimeter, and measuring the amount of energy released through their combustion. Historic data on energy content is often available.

Ecological pyramids in marine habitats

It is difficult to give a comprehensive flow of the energy on a shore as there are so many factors affecting it, such as the time of year.

Productivity

Productivity is the rate at which energy passes through each trophic level in a food chain. it is measured in units of energy per area per period of time, typically kJ per m2 per year.

  • Gross Primary Productivity (GPP) is the rate at whih plants convert light energy into chemical energy
  • Net Primary Productivity (NPP) is GPP minus respiration loss. NPP represents the energy available to the primary consumer.

Efficiency of energy transfer

The efficiency of an energy transfer at any stage in the food chain can be calculated as

`"energy efficiency" = 100 xx ("energy or biomass available before the transfer" / "energy or biomass available after the transfer")`