- How do the number of individuals change as you go from one trophic level to the next?
- What trophic level do herbivorous insects occupy?
- What trophic level do carnivorous insects and spiders occupy?
- Does the number of insect species have an effect on the number of spider species in an ecosystem?
- Does the total number of insects have an effect on the total number of spiders in an ecosystem?
In every ecosystem, there are food chains that transfer energy from the sun to a producer, then to consumers. These food chains may be woven together to form the food web that more accurately shows the feeding relationships in the ecosystem. Every time that energy is transferred from one organism to another in an ecosystem, some energy is lost to the environment as heat. Because of this, food chains may also be portrayed as trophic energy pyramids, with producers on the bottom with the most energy and number of individuals. As you go up the food chain from primary to secondary and tertiary consumers, the number of organisms and the amount of energy decreases. In this experiment, the student will determine if the invertebrate ecosystem shows this trend by comparing the number of herbivorous insects to the carnivorous insects and spiders.
- Insect and spider field guide(s)
- Meter tape or stick
- Sweep net (purchase, borrow, or make your own)
- Container with small holes for keeping invertebrates until they are identified
- Create your hypothesis. Do you think there will be a relationship between the number of insect species and the number of spider species you find? Do you think there will be a relationship between the total numbers of insects and species?
- Choose a location for your experiment. An open field that has not been mown or disturbed for a year or so is often a good choice. Time of day and year will also be important for a successful experiment. A warm afternoon in late spring, summer, or early fall would be best. Use a 10 meter tape to mark your first plot. Take your sweep net and point it down. With a back and forth motion (as if you were sweeping with a broom), agitate the grass and vegetation. Keep the net in constant motion to keep the arthropods from escaping. Sweep as you walk slowly along the right hand side of the 10m tape. When you reach the end, turn slowly and walk along the other side. Once you have thoroughly swept both sides of the 10m tape for arthropods, pick up the net quickly and shake the insects and spiders into the container you have prepared. Put the lid on the container. Look at the arthropods you have collected. Identify them and count the number of each type that you find. Write your results in your data table. If you do not collect many arthropods the first time, try again, this time with a little more force and vigor to sweep up the insects and spiders. When you are finished recording the number and type of invertebrates that you find, return them to their habitat.
- Repeat the experiment a total of 5 times in 5 different locations within the ecosystem. Once you have finished the data collection, use the field guide or other resources to identify what type of consumer each invertebrate is. Write this information into your data table. Create a bar graph to illustrate the number of each type of consumer you found. Compare your results to your hypothesis to determine if you found the expected relationship between the numbers and types of consumers.
Terms/Concepts: Ecosystem; Food chain; Food web; Trophic pyramid; Energy pyramid; Pyramid of numbers; Invertebrates; Arthropods; Insects; Spiders; Carnivore, Herbivore, Producer; Habitat; Primary, Secondary, Tertiary, and Quaternary Consumers