Ecology, Part 1: Trophic Structure of Ecosystems
Objectives
- Define the trophic levels in an ecosystem, and identify the trophic level of specific organisms in various ecosystems.
- Compare the number of individuals, biomass, amount of energy, and the efficiency of energy transfer (i.e., the percentage of energy transferred and converted to new biomass between levels) at each trophic level.
- Investigate the concept of trophic cascades, and explain how a keystone species can indirectly affect the biodiversity and nutrient cycling of an entire ecosystem.
This lab includes three activities through which you will explore the organization of ecosystems.
I. Trophic Levels and Trophic Pyramids
Ecological pyramids are diagrams that are used to show the pathways of energy and biomass in an ecosystem. These energy relationships form the trophic structure of an ecosystem. Organisms are classified as autotrophs or heterotrophs based on their nutritional strategy, and they are also assigned a specific trophic level according to their position in the food chain. You will learn the definition of each trophic level in this activity, and then practice the concept by assigning various organisms to the correct trophic level in several different ecosystems.
Key Terms
| Term | Definition |
| Trophic structure | The pattern of energy relationships in an ecosystem (based on how each species gains energy) |
| Ecological pyramid | A diagram showing the pathways of energy and biomass in an ecosystem. This is one method of depicting trophic structure—other methods include food chain diagrams and food web diagrams |
| Trophic level | Position in the trophic structure that indicates the function of organisms in the ecosystem (e.g., producer, consumer, decomposer) as well as their relationship to the primary energy source (e.g., primary, secondary, tertiary, etc) |
| Autotroph | Organisms that can produce their own food from abiotic sources |
| Heterotroph | Organisms that must consume their food from biotic sources (i.e., other organisms) |
Activity 1
This activity defines each trophic level and then allows you to practice placing various organisms in the correct trophic level for an ecosystem in Panama. Afterward, you will analyze the trophic structure using the questions provided as a guide.
You may download the PDFs needed for the activity if provided by your instructor, or, if necessary, access the materials as follows:
- First, access and download the activity guide, “Modeling Food Webs in Darién, Panama.” The activity guide PDF is fillable using your device, or it may be printed and completed by hand.
- Next, access and download the images you will need.
These materials are from from the Howard Hughes Medical Institute’s BioInteractive activities.
Activity 2 (Optional)
If time permits, your instructor may also conduct the following activity, or it may be used as a back-up next week if the weather prevents us from conducting our outdoor ecology lab. This activity is a more advanced analysis of trophic structure in a different ecosystem, the Silver Springs ecosystem in Florida. Data on the energy contained in each trophic level of the ecosystem will be provided, and you will manipulate the variables to see the effects of changes at particular levels.
You may download the Word and Excel documents needed for the activity if provided by your instructor, or, if necessary, access the online instructions, “Ecosystem and Eutrophication Lab.” These materials are from Lumen Learning.
II. Keystone Species and Trophic Cascades
In many ecosystems, one species may have a disproportionate effect on ecosystem dynamics. For example, the removal of a particular species may cause significant changes in biodiversity and nutrient cycling throughout the ecosystem, and may even result in a collapse of the entire system. Depending on the role of this species, it may be called a foundation species, a keystone species, or an ecosystem engineer. This activity focuses on keystone species, whose presence is the “key” to some aspect of ecosystem dynamics. For example, the biodiversity of the entire ecosystem generally decreases dramatically when the keystone species decreases in number or disappears. Such amplified effects are called trophic cascades, and may begin at either the top or the bottom levels of the trophic structure.
Key Terms
| Term | Definition |
| Keystone species | A species that is not necessarily abundant, but has a disproportionate effect on biodiversity, nutrient cycling, or other key ecosystem processes |
| Foundation species | A species that provides food, shelter, and other resources to many other species in the community. Often a producer, such as the pine trees in an alpine ecosystem |
| Ecosystem engineers | A species that physically alters the environment in such a way as to significantly change the composition of the ecosystem. For example, reef-building corals |
| Trophic cascade | A series of changes in the population sizes of organisms at many different trophic levels, often precipitated by a change in just one species that has many indirect, cascading effects |
| Top-down control | Predators (or other high-level consumers) exert a strong influence on the dynamics of lower trophic levels. |
| Bottom-up control | Abiotic resources (or producers) exert a strong influence on the dynamics of higher trophic levels. |
Activity 3
In this “Click and Learn” online interactive activity from the Howard Hughes Medical Institute, you will predict whether specific organisms have positive or negative effects on other organisms in adjacent trophic levels. You will then trace the indirect effects of this interaction through the rest of the ecosystem, and predict how the ecosystem will change with the removal or addition of a particular species.
Use the “Click and Learn” activity to explore the concept of trophic cascades. When you are finished, answer the six questions below. You will notice that similar questions are asked (and answered) in the activity.
1. Throughout most of the 1800s, sea otters were hunted to near-extinction all along the Pacific coast of North America. How did this impact those ecosystems?
2. How did the diet of each of the following change after the near-extinction of the otters?
| Species | Scientific name | Dietary changes |
| Glaucous-winged gull | ||
| Bald eagle |
3. Why did the ecosystem of the Aleutian Islands of Alaska change from grassland to tundra after the introduction of Arctic foxes? Explain the trophic cascade that caused this to occur.
4. Why did lakes in the midwestern U.S. change color after the removal of bass by overfishing? Explain the trophic cascade that caused this to occur.
5. Why did the forest disappear after the construction of a dam in Venezuela? (Hint: it is not because the forest was drowned by water.) Explain the trophic cascade that caused this to occur.
6. Did the African savanna ecosystem experience more fires or less fires after humans developed a vaccine for rinderpest? Explain the trophic cascade that caused this to occur.
