GED Science Test: Ecosystems Basics

By Murray Shukyn, Achim K. Krull

You will need to have some basic background knowledge in ecosystems to succeed on the GED Science test. An ecosystem is a community of plants, animals, and other living things that interact with one another and their physical environment.

Tracking energy flow in ecosystems

Every living thing requires energy to live, grow, and reproduce, so energy is a key component in ecosystems. The sun generates nearly all the energy that flows through an ecosystem. Plants (producers) capture the sun’s energy and use it to grow and reproduce; consumers eat the plants, berries, nuts, and fruit (and/or other consumers); and decomposers break down the waste products to return nutrients to the ecosystem that plants require to continue to produce.

Energy flows through the system by passing from one trophic level to the next. A trophic level represents an organism’s position in a food chain or food pyramid, with plants at the bottom and omnivores at the top:

  • Primary producers (plants, algae, and some bacteria) use solar energy to produce organic plant material through photosynthesis.

  • Primary consumers are herbivores (animals that feed exclusively on plants).

  • Secondary consumers eat primary consumers and may eat primary producers, as well. Animals that eat primary consumers are carnivores. Those that eat consumers and producers are omnivores.

  • Tertiary consumers eat secondary consumers and may eat at lower trophic levels, as well.

Most land-based ecosystems have as many as five trophic levels. Marine-based systems tend to have more — as many as seven.

Consumers obtain a small fraction (about 10 percent) of the energy captured by the organisms they consume at lower trophic levels. The other 90 percent of the energy is used by the organism that’s consumed to fuel its growth, survival, and reproduction or is converted to heat.

So, for example, if you eat a salad, you’re getting about 10 percent of the energy that the plants in your salad captured from the sun. If you eat a hamburger, the cow obtained 10 percent of the energy captured by the plants it ate, and you’re getting 10 percent of the energy from the cow, so you’re only getting 1 percent of the energy captured by the plants the cow ate.

Conservation of energy

According to the law of conservation of energy, energy can’t be created or destroyed; it can only be transformed. Plants, for example, use solar energy to fuel photosynthesis, which converts energy in the form of light into chemical energy in the form of the molecules created from water, carbon dioxide, and other nutrients. These molecules form the substance of a plant.

When an animal eats and digests the plant, those molecules are broken down, releasing energy that fuels growth, movement, and reproduction. Some of the energy is said to be “lost” in the form of heat, but it’s never really lost as in “gone for good.” The energy isn’t recycled and pumped back into the ecosystem, but it rises into the atmosphere. The important point to remember is that energy is never destroyed.

Forms of energy

Although energy is never destroyed in an ecosystem, it can be converted into other forms of energy:

  • Chemical: Through photosynthesis, plants use water and nutrients from the soil to convert solar energy into chemical energy that’s stored in the molecules that comprise the plant and fuel its growth.

  • Electrical: Some of the energy animals obtain from the food they eat is converted to electrical energy that’s used in communication systems within the body. For example, your body essentially has its own electronic pacemaker that keeps your heart beating, which explains why you need to replenish electrolytes after exercise.

  • Mechanical: Much of the energy animals obtain from the food they eat fuels internal mechanical processes and movement.

  • Nuclear: Nuclear energy is stored in the nucleus of an atom. The sun converts nuclear energy into massive amounts of radiant energy.

  • Radiant: Radiant energy is electromagnetic energy that flows into the ecosystem from the sun.

  • Thermal: Thermal energy (heat) is released as animals digest plant material, breaking down the molecules in the plant material.

Exploring relationships among organisms

Organisms in an ecosystem develop various types of relationships, either living together in harmony or not:

  • Predator-prey: In predator-prey relationships, one organism (the predator) eats the other (the prey); for example, a bear eating a salmon. The relationship isn’t all that bad for the prey because it thins out the population, preventing them from starving, and it strengthens the genetic pool, making the prey evolve into stronger, faster, and smarter creatures over time.

  • Symbiosis: In symbiotic relationships, organisms live together in one of the following three ways:

    • Mutualism: Both organisms benefit from the relationship; for example, a flower and a bee. The bee gathers pollen it can use to make honey and the flower is pollinated by the bee carrying pollen from one flower to the next as it gathers pollen, helping the flower reproduce.

    • Commensalism: One organism benefits from the relationship, and the other is neither harmed nor helped. For example, some barnacles attach themselves to whales to obtain transportation and greater access to food without harming or helping the whale in any way.

    • Parasitism: One organism (the parasite) benefits, and the other (the host) is harmed in the process. For example, if a tapeworm becomes attached to a host (human or animal), the tapeworm benefits, and the human is harmed. (Interestingly, however, scientists are beginning to wonder whether certain critters considered parasites truly are; for example, people infected with pinworms or hookworms tend to have a lower risk of developing autoimmune disorders, such as allergies and asthma, which is a definite benefit.)