Biology Fieldwork

A Level

Discussion

Uaing a pooter to collect invertebrates

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 terrestrial habitats

Inverted pyramids of biomass are fairly frequent in terrestrial environments.

Species that spend a great deal of time moving also expand larger amounts of energy and so convert a lower percentage of the energy they consume into biomass.

Seasonal changes occur in both the canopy and the floor. However, in deciduous woodland, they are likely to be most marked in the canopy community. As leaves become old and tough and eventually fall off the trees, the food source for many of the primary consumers in the canopy disappears. Individuals may die, the species surviving the winter as eggs or pupae; others hibernate or migrate to warmer climates. These changes in the number and types of primary consumer will result in changes in the whole canopy food web.

Although many of the plants in the ground flora die back in winter the detritivores in the soil and leaf litter community have a plentiful supply of food. In cold weather they move deeper into the litter and soil layer making it more difficult for larger predator such as birds and small mammals to find their prey. Eggs, pupae and even adults may be found hibernating in the litter and soil. These may include species more commonly associated with the canopy in summer months.

On the woodland floor many of the invertebrates are susceptible to losing water from their bodies and drying out. They tend to hide in cool damp places during daylight hours and become more active at night. In the tree canopy where it is more often warm and sunny a higher proportion of animals are likely to be fast-moving winged species with more resistance to drying out. They are often active during the daytime and rest at night.

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")`