Ecological Pyramids
Environmental Science
10/03
Name _________________________________ Period _____
Introduction
A food chain represents passage of energy through populations in a community. A trophic level represents a feeding level of
one or more populations in a food web; those organisms in an ecosystem that are the same number of food chain steps from the
energy input into the system. Producers occupy the first trophic level. The second trophic level consists of all the primary
consumers (herbivores). The third level is comprised of the secondary consumers (carnivores).
An ecological pyramid shows the trophic structure of an ecosystem as a graph representing biomass, organism number, or
energy content of each trophic level in a food web. The base of the pyramid represents the producer trophic level, and from there
the consumer trophic level is stacked, with the apex representing the highest consumer trophic level (tertiary consumer).
There are several types of ecological pyramids. A pyramid of numbers is based on the number of organisms in each trophic
level. A pyramid of biomass is based on the weight (biomass) of organisms at each trophic level at one time. There is usually a large
mass of plants supporting a medium mass of herbivores and a small mass of carnivores. A pyramid of energy is based on the total
amount of energy in each trophic level and is always pyramidal.
In general, about 10 percent of energy at a particular trophic level is incorporated into the next trophic level (10% rule). Thus,
1,000 kg (or kcal in an energy pyramid) of plant material converts to 100 kg of herbivore tissue, which converts to 10 kg of first
carnivores, which can support 1 kg of second level carnivores. This rapid loss of energy is the reason food chains typically have
from three to four links, rarely five. This rapid loss of energy is also the reason there are few large carnivores in any given
ecosystem.
Procedures
1. Construct ecological pyramids using the data and scale provided for each scenario.
2. Identify and define each type of ecological pyramid constructed.
3. Describe what each ecological pyramid represents about the given ecosystem.
Ecological Pyramids
1. In a grassland ecosystems there are 5,482,424 grasses, 708,624 insects, 354,904 birds and 3 bobcats
present. Allow 1mm to represent 100,000 individuals; each level of the pyramid should be 1 cm in
height.
2. In a given portion of an ocean ecosystem phytoplankton supply 8,000 kilocalories, zooplankton
supply 800 kilocalories of energy, fish species supply 80 kilocalories and sea lions supply 8
kilocalories. Allow 1 mm to represent 100 kilocalories ; each level of the pyramid should represent 1
cm in height.
3. In a wetland ecosystem there is approximately 800 g/m2 of wetland plants, 40 g/m2 of insects, 10 g/m2
of mice and 1.5 g/m2 of snakes. Allow 1mm to represent 10 g/m2; each level of the pyramid should be
1 cm in height.
Discussion
1. Consider the following ecological pyramid of numbers for a given forest ecosystem. Label each level
in terms of the trophic level it represents. Explain why it takes on the shape it does.
2. Consider the following ecological pyramid of biomass for a given ocean ecosystem. Label each level
in terms of the trophic level it represents. Explain why it takes on the shape it does.
3. Transferring energy from one trophic level to another consumes energy. Approximately how much
energy is available from one trophic level to the next?
4. Determine the energy available at each level if the primary producers provide 600,000 calories for the
first trophic level. Use the following formula to calculate the energy transfer among tropic levels. Show
all of your work.
EN = (10%)(EN-1)
5. Describe at least three factors that are responsible for the energy consumption between trophic levels.