Introduction

Ecosystems are comprised of dynamic interactions between the physical (abiotic) environment and the organisms (biota) that live there. The organisms within the ecosystem interact directly with one another through herbivory and predation, competition, and symbiosis, topics you will have covered in your introduction to ecology. Yet organisms in an ecosystem are connected to each other in many ways, sometimes interacting indirectly with one another. For example, an organism high up on the food chain changes the abundance of another organism’s predator or competitor. In addition, organisms and the physical environment affect one another, as when sunlight and nutrients help plants to thrive. Herbivores then eat the plants, and then carnivores eat the herbivores. Such top-down or bottom-up effects in an ecosystem lead to interactions known as trophic cascades. Humans interact with, are a part of, and derive benefits from natural ecosystems. Coastal salt marshes (Figure 7.1) serve as the basis for our explorations of trophic interactions and the ecosystem services they provide.

Part 1: Revealing Trophic Cascades in a Salt Marsh

This activity is based on the ecological investigations by Silliman and Zieman (2001) and educational activities by Beardsley and Cooper (2024). Silliman’s work in salt marshes and other coastal ecosystems continues through the Silliman lab, spanning over 20 years of research throughout the world, and includes roles of predators like sea otters and alligators.

1. First you will reinforce your understanding of the process of science. Go to Biointeractive’s How Science Works. Choose Basic and Intro. Read through the pages and answer questions 1-2 in Part 1 of the Lab Response Form.
2. Watch this video clip on salt marshes. Answer question 3 in Part 1 of the Lab Response Form.
3. Watch this video clip on top–down and bottom–up controls. Answer question 4 in Part 1 of the Lab Response Form.
4. Watch this video clip on the role of abiotic factors in the salt marsh. Answer questions 5-6 in Part 1 of the Lab Response Form.
5. Watch this video clip on a key discovery of an animal in the salt marsh, followed by this second video clip on plant–herbivore experiments in the salt marsh. Answer questions 7-8 in Part 1 of the Lab Response Form.
6. Watch this video clip on the results of the previous experiment. Answer questions 9-10 in Part 1 of the Lab Response Form.
7. Watch these video clips on the potential herbivore, its predator, and the role of the predator in this system. Answer questions 11-13 in Part 1 of the Lab Response Form.

Part 2: Restoring a Salt Marsh Ecosystem and Its Services

Coastal land loss is a significant problem in many parts of the world. The iconic Mississippi River Delta of the Louisiana coast is a heavily altered and utilized region. As a result, the coastline is vulnerable to storms and rising sea levels and has been disappearing at an alarming rate. Explore the effects of human activity and efforts to restore these coastal ecosystems through a partnership program between Louisiana and various federal agencies.

1. Go to Our Changing Delta. Scroll down the page to learn about the location and project associated with this marsh ecosystem.
2. Go back to the top of the page and select “Click Here to Begin” to take the virtual tour of the coastal area and its salt marsh habitats.
3. Click on the information buttons and arrows in this interactive. Read the information and watch the videos.
4. Answer questions 1-4 in Part 2 of the Lab Response Form.

References

Beardsley, Paul and Robert Cooper. 2024. Critiquing and Revising a Salt Marsh Food Web Model. (Nala Rogers and Esther Shyu, eds.) HHMI Biointeractive.

Coastal Wetlands Planning, Protection and Restoration Act. 2025. Our Changing Delta: Delta Management at Fort. St. Philip Watermarks 360 Tour. https://ourchangingdelta.centralcreativity.com/

Silliman, Brian R. and Jay C. Zieman. “Top-Down Control of Spartina alterniflora Production by Periwinkle Grazing in a Virginia Salt Marsh.” Ecology 82, 10 (2001): 2830–2845. https://doi.org/10.2307/2679964.

Lab 7 Response: Trophic Cascades and Ecosystem Services

Part 1: Revealing Trophic Cascades in a Salt Marsh

1. Describe how curiosity is important to a scientist. Include parts of the scientific process that you saw highlighted in this interactive.
2. Are observations and data that contradict your expectations or understanding of science wrong or useless? Explain, listing points made in this interactive.
3. Where are salt marshes found? What are some of the benefits of salt marshes to people and to the areas in which they occur?
4. Define top-down and bottom-up controls in an ecosystem. Explain which one was thought to control salt marshes and how.
5. What plant dominates the salt marsh? How do tides affect this plant? Consider short-term (daily) and long-term (yearly) effects.
6. What nutrient limits plant growth in the salt marsh?
7. Stop the second video clip linked in Step 5 of the instructions at 0:22 seconds. Draw and label the 4 different types of experimental plots.
8. What organism(s) were observed and measured for their response to these treatments?
9. What were the responses to fertilizer? How did snails influence the response?
10. What did these experiments reveal in terms of bottom-up vs. top-down control of the salt marsh?
11. What surprising information was discovered about what snails were eating? How were the plants affected?
12. Name the predator and describe its effect on snails. Briefly describe the experiment showing that it caused these effects on snails.
13. Describe the direct and indirect effects of the predator on this ecosystem.

Part 2: Restoring a Salt Marsh Ecosystem and Its Services

1. What is a delta? Where is this delta and what river helps form it?
2. Describe the ways in which humans and natural events altered this delta area.
3. Why was there a need to restore the delta area? Describe two different structural changes used to restore the delta marshes.
4. Some different organisms are featured in the videos. Which ones have been introduced? Look up and describe the effects of these introduced organisms on the salt marsh ecosystem. Would you consider them to generally have a positive, neutral, or negative impact on the ecosystem? Cite your sources.