Lesson 1: Crayfish and Freshwater Ecosystems, Investigating Crayfish and Freshwater Ecosystems

Introduce the expert visitor if one is present and tell students they will be starting an exciting new unit about crayfish and their habitats (where they live).

Optional: Explain to students they will first find out what they already know. Pass out the “Crayfish Trivia” handout to each student and allow 10–15 minutes for them to complete it. Tell the students they are not expected to know the answers, so they should just do the best they can. This activity serves many purposes, including evaluating current student knowledge, helping students focus on topics to be discussed, and evaluating change in understanding over time. Collect the handouts. This activity could also be done later as a review game or assessment.

Ask students to turn to a neighbor and brainstorm everything they can think of about crayfish, where they live, what they eat and what eats them, etc. They should record all their ideas on a piece of paper, without worrying about if they are right or wrong. Circulate around the room, answering questions, if necessary. After about five minutes, ask for a few to share their best ideas. Then explain to students that this lesson will be all about the fascinating places where crayfish most often live, called freshwater ecosystems.

Open the “Crayfish and their Ecosystems” PowerPoint presentation and you and/or the visitor can lead a brief interactive discussion about crayfish and their ecosystems, drawing on the student ideas shared earlier and the information in the slide notes to talk about the important roles crayfish play, and how they get what they need from their environment, including food, water, shelter, space, and oxygen. If available, you can show the students live crayfish and/or any other organisms from freshwater ecosystems that interact with them, such as variety of plants, fish, turtles, or frogs/tadpoles.

Ask the class to arrange their desks in groups of four, if necessary, and pass out blank paper (one sheet per student) and coloring supplies.

Ask the students to each share the name of one of their favorite species from freshwater ecosystems with each other and demonstrate how they can write its name in large letters in the top third of a blank piece of paper using a pencil. Then they can make the names dark enough to read from across the room with a marker or other coloring supplies. This can be an animal or plant that they have learned about in the presentation or seen in nature. Each student should choose a different organism, and one or more students in each group should choose a plant, because plants are so important for almost every ecosystem.

Note: To help students understand what to do, you can show them the “Rainbow Trout” example provided, create your own example, and/or show student samples. For more advanced students, it may be valuable to have them write the common and scientific names of the organism they choose. This will teach them the importance of understanding scientific names, as they provide a universal code for identifying species.

Ask the students to create a basic illustration of their organism below the organism’s name on the paper. They can use available reference sources such as books and the Internet for reference and/or live specimens if you are lucky enough to have some. Tell students they will only have about five minutes (or however much time you want to allow) to create their illustrations, but that they will be able to add more details and color later if they wish. Depending on where you live you might suggest:

• Animals and plants presented in the PowerPoint presentation, including those shown in the food web diagrams
• Options of freshwater plants such as those presented in the PowerPoint presentation

While the students finish their illustrations and/or short research projects, ask the groups to choose 2–3 species and choose volunteers to represent the group to act out a simple food chain for the rest of the class. Write the term on the board and ask one of the student groups to send a representative to the front of the room (or the center of the circle if you’d like to ask the groups to arrange themselves in one) to play the role of an animal at the top of the food chain, a large predatory one that eats other animals. Ask the student to try to make themselves look and/or act like the animal they are playing.

Ask another group to send a representative to play a different animal that eats other animals, but that might be eaten by the first animal. Ask the second student to act out their animal, while the first gets ready to try to eat it. Ask the class if they know a word used for animals that eat other animals and a word for the animals that get eaten. Write or type the words predator and prey on the board. Then ask the groups to identify another animal that might get preyed upon and what predator might eat it; have a student representative come to the front of the room (or center of the circle) and ask one of its predators to move near its prey, as well. Ask if students know the name for a meat eater—carnivore—and a plant eater—herbivore—and write those words on the board below “predator” and “prey.” Then ask if they know the name for an animal that eats many types of food. Discuss the term omnivore and write that on the board, too.

Ask the class what important parts of the freshwater ecosystem food chain are missing. Where do the prey species get their energy from? Instead of calling on a student raising her/his hand, tell the class that at the count of three, all of them should shout out the organisms (living things) they think are most important for the ecosystem. Count 1-2-3, and hopefully many of them will shout PLANTS!—or something else important, like algae (a type of plant) or insects.

Ask for volunteers from the groups to play the role of freshwater plants—the producers—and invite those students to join the food chain simulation while you write the word producers on the board, as well. Ask the class to again shout out—at the count of three—where the plants get their energy from, and hopefully many of them will shout THE SUN! or PHOTOSYNTHESIS! Write the words Sun (perhaps within a quick doodle depicting it as a large circle with rays coming out of it) and photosynthesis (perhaps within a quick doodle of a leaf) on the board. Ask the students playing plants to act like they are soaking up the sun’s energy so they can convert it into food—sugar, starch, and other nutrients—that supports the whole ecosystem.

Explain that all other organisms that don’t produce their own food are called consumers, and write that word below carnivore and herbivore in the middle.

Ask students what prey species in freshwater ecosystems might eat, and they may suggest smaller organisms such as tadpoles or insect larvae. Hopefully, one will also say dead things, the way scavengers like crayfish eat, or you can suggest dead organisms and ask students which living organisms eat them. Explain that this function of eating dead matter is very important for keeping the water clean and with enough dissolved oxygen for animals to breathe. Also explain that freshwater ecosystems can be very complex, with many hundreds of animal and plant species, all interconnected through a complex food web and supported by nonliving things such as sunlight, water, and air. Write food web under food chain and explain that it is the interaction of many food chains and cycles.

Ask for a round of applause for the ecosystem actors, and they can take their seats. Explain to students that they will now create a more complex model of the food web—or web of life–that will better represent the rich biodiversity of a healthy freshwater ecosystem. Write the word on the board, and if time allows, talk about the prefix bio- (life) and the root diversity (variety).

Ask the second student to do the same thing, passing the ball to another organism it interacts with while holding the end of the yarn; continue until all the students are connected in the web of life, completing the model of the freshwater ecosystem.

Ask the students to step back and/or gently pull on the yarn until the web is taut. Then ask the students to remain still. Explain that in a moment, the student who started the web will tug on it, and only those students who feel a tug will tug back. Ask the student playing the plant to begin the process, and continue until all the students can feel a vibration moving through the web. Then ask students to choose an organism that might be less critically important for the ecosystem and ask that student to drop the yarn.

Continue this “organism removal” process several more times, then ask students a few questions to promote critical thinking and generate discussion:

• How did removing organisms from the freshwater ecosystem impact the web? Possible answer: Organisms that depend on the food web are impacted and the web changes shape.
• When were the changes to the web most dramatic? Possible answers: When there were fewer species; losing one of them had a greater impact on the ecosystem. When certain species that had multiple interactions were lost.
• When was the web the most stable and why? Possible answers: The web was most stable when there was the largest number of species. In general, the more biodiversity, the more stable the environment and the less it is impacted by changes in the environment.
• How might humans impact the web if they were added to it? Possible answers: They might cause more species to leave the web. This would be especially true if humans don’t try to minimize their impact and to protect the biodiversity of the ecosystem.

Pass out cards with the names of freshwater species and other important components of freshwater ecosystems. Students can use these to get them started on their short research projects to prepare to create the “web of life,” or the cards themselves could be used for the activity if you are limited for time. Sets of cards can be found online, using the web resource “of Pond Life Images”Pond Connections” from the New Mexico Department of Game and Fish.

Have students work with a partner to create a visual diagram of freshwater ecosystems. Pairs will need a large sheet of paper or poster board to share; consider having a few stacks of used sheets around the room from which students can choose (to use the backs of them). Completed diagrams—or the best of them—can be displayed on the classroom walls or on a hallway bulletin board or other display.

Consider directing students to use different colored arrows for the different types of interactions on their diagrams, and write this on the board with color-coded markers or chalk, if available, or type it to display on the screen or interactive whiteboard: Orange to connect the sun with producers (plants) — Green to connect herbivores to plants — Red to connect predators to their prey — Brown to connect decomposers to the plants and animals they break down after they die.

Optional: Show students the Sagebrush Ecosystems poster/graphic available from the U.S. Fish and Wildlife Service on the Greater Sage-Grouse Education page as an example of one type of visual diagram they could create. This resource is located in materials.

Optional: Students can include humans in their diagrams, if desired.

Students can work together to create a large mural of a diagram depicting freshwater ecosystems with their illustrations and/or nonfiction writing about them. Diagrams can be created on classroom walls and/or other walls in the school or larger community. Yarn and/or arrows can be used to show ecosystem interactions, and students can help to illustrate additional important aspects of the ecosystem, such as the sun, algae, bacteria, and detritus.

Show a short video clip about crayfish and/or freshwater ecosystems, such as: “Queen Nerdling Presents Freshwater Ecosystems” and “I Speak for the Fish: Facing the Wrath of a Crayfish”. These can be found in materials.

Students can write fictional stories or poems about one or more organisms from freshwater ecosystems.

Have each student choose a freshwater ecosystem organism to research in depth. They can research elements, such as what the organism needs to survive and how human activities have impacted it over time. Provide a rubric so students know how they will be evaluated on the project, and findings could be shared with the rest of the class through written reports and/or oral presentations.

For younger grades, read a story or nonfiction book with your class about crayfish. Examples include:

• “Crayfish” by Meg Gaertner: | LINK
• “Crayfish” by Phillis W. Grimm: | LINK
• “The Life Cycle of a Crayfish” by Bobbie Kalman: | LINK

Ask students to reflect on the lesson in writing and/or orally, including about what they learned and what you, as the teacher, might do to improve the lesson next time. Use completed student diagrams to evaluate student understanding of the concept of freshwater ecosystems. Review the short research projects about an organism from freshwater ecosystems and its interactions with other organisms. Use student participation in class discussion and activities, including the simulation of a freshwater web of life, to determine student understanding.