Model Ecosystems
How does energy flow through an ecosystem?

An ecosystem consists of a community of living organisms interacting with each other and the environment. The source of energy that fuels most ecosystems is the Sun. Plants use the Sun's energy to produce food in a process called photosynthesis. Organisms that use energy from the Sun or energy stored in chemical compounds to produce their own nutrients are called autotrophs. They are also called producers because most other organisms depend on autotrophs for nutrients and energy. Heterotrophic organisms that cannot make their own food may obtain nutrients by eating other organisms. A heterotroph that feeds only on plants is called an herbivore. Herbivores are also called first order heterotrophs. Carnivores that feed on herbivores are called second order heterotrophs. Carnivores that feed on other carnivores are called third order heterotrophs. A food chain is a simple model of how energy and matter move through an ecosystem.

Each level of production and consumption in a food chain is a trophic level. The autotrophs form the first trophic level, first order heterotrophs (herbivores) constitute the second trophic level, second order heterotrophs the third trophic level, and third order heterotrophs are layered on top.

In the pyramid of energy, the energy moves in only one direction and decreases at each succeeding trophic level. The total energy transfer from one trophic level to the next is, in general, only about ten percent or less. This is called the energy conversion efficiency. Organisms fail to capture and eat all the food available at the trophic level below them. The food consumers ingest is used to metabolize and build body tissues. Some food is given off as waste. The energy lost at each trophic level enters the environment as heat.

A pyramid of biomass expresses the weight of living material at each trophic level. Biomass is calculated by finding the average weight of each species at that trophic level and multiplying the weight by the estimated number of organisms in each population. In terrestrial ecosystems, biomass decreases as the trophic level increases. In contrast to terrestrial ecosystems, freshwater and marine ecosystems have less primary producer biomass than biomass present at higher trophic levels, leading to an inverted biomass pyramid. This is because algae and phytoplankton have a shorter lifespan, are more edible than terrestrial plants, and are more rapidly grazed. Their biomass does not accumulate.

In this exploration, you will study and analyze five simplified model ecosystems: a deciduous forest, a hot desert, a freshwater lake, grassland, and an Antarctic ocean shore. Many more plant and animal species would be involved in a real-world ecosystem. The field notes for each model ecosystem present a profile of the plant and animal inventory for each ecosystem.

1. Select the icon next to the animals' and plants' names from the Field Notes tablet and drag and drop them to the appropriate trophic level in the ecosystem pyramid.

2. Click the Check button when all the names have been placed in the ecosystem pyramid to verify the accuracy of animal and plant assignments to the appropriate trophic level. For each accurate placement, the names of the animals are replaced with pictures and the number of each kind of animal is displayed beneath it.

3. Click the Pyramid of Energy button to display numbers indicating amounts of energy.

4. Click the Pyramid of Numbers button to display numbers indicating numbers of plants and animals.

5. Analyze this data by calculating the conversion efficiency for each trophic level for each of the five ecosystems and record the results in the Data Table. The energy conversion efficiency is calculated by dividing the energy at the higher trophic level by the energy at the lower level to obtain a ratio. Enter the value as a decimal number.

6. When all the data are recorded in the Data Table and analyzed, answer the Journal Questions.