To explore defense mechanisms involved in predatory/prey relationships.
The periodical cicadas that emerge in various parts of the United States every 13 or 17 years provide an excellent opportunity for you to engage your students in a discussion about predatory/prey interactions. Students can reflect on the life cycle and behavior of the cicada and develop understandings about how their unique life cycle and behavior actually helps them to reproduce and survive in spite of the fact that they have very few of the typical defense mechanisms with which students might be familiar, such as the use of mimicry or camouflage.
Found only in North America, periodical cicadas are closely related to the annually appearing cicadas found in most of the world. The difference between them is in their life cycles: annual cicadas appear every year but the seven species of periodical cicadas, all genus Magicicada, live underground as nymphs for 13 or 17 years, depending on the kind. They exist by sucking sap from tree roots and rarely move more than a few feet during their underground existence. During this period, they molt every few years, and grow larger. Then, just before their aboveground cycle begins, all the members of a local population, called a brood, tunnel toward daylight.
There are 15 broods in North America, each emerging in varying years. They also vary tremendously in size. Some engulf huge swaths of the eastern continent; one emerges only in a few woodlots in a couple of northern New York counties. Generally, each brood contains three different Magicicada species, each singing its own distinct song.
While teaching this lesson, it is important for you to be aware of some student misconceptions. Research suggests that upper elementary-school students may not believe food is a scarce resource in ecosystems, thinking that organisms can change their food at will according to the availability of particular sources. Students of all ages think that some populations of organisms are numerous in order to fulfill a demand for food by another population. (Benchmarks for Science Literacy, p. 342.)
A more advanced discussion of the evolution and survival strategies of the periodical cicada can be found in Cicadas' Bizarre Survival Strategy.
You may introduce the lesson by talking about the emergence of Brood X this spring and summer. It is likely that students have heard or read about this event and a brief discussion in which students can relay what they might already know about periodical cicadas can help set the stage for the rest of the lesson. If you live in an area in which Brood X will make its presence felt, encourage students to discuss any experiences they might have already had. Remember that there are 15 different broods found in North America. It is likely that students may have experienced the emergence of other broods in previous years, depending on where they lived.
Using the Periodical Cicada Survival student esheet, students should view a slide presentation that describes the typical life cycle of a 17-year cicada.
After students have viewed the slide show, discuss the final slide, which states the following:
"While the cicadas remain underground, they are safe from most predators. However, as soon as they emerge, they are vulnerable to predators ranging from birds to snakes to small mammals. They don't fly well, they are not combative, and apparently, they taste pretty good. But in spite of this, enough of the nymphs manage to survive and burrow into the soil, waiting to emerge again when the time is right!"
Ask students to think about how it is that cicadas manage to beat the odds and survive in sufficient numbers to ensure the survival of the brood. Make a list of their speculations or comments on chart paper and save it to compare to their later, more informed reflections.
The rest of the lesson will explore in more depth prey defense strategies, particularly as employed by cicada species. The Periodical Cicada Survival student esheet will provide information on general defense strategies with which students may already be familiar. You may wish to review this with the class and answer any questions students might have. Students will also be directed to listen to a Science Update interview with a researcher who talks about how the emergence of the cicadas in vast numbers is an example of a survival defense strategy called predator satiation.
After students have completed the activities found on the esheet, ask students to think again about how it is that cicadas manage to beat the odds and survive in sufficient numbers to ensure the survival of the brood. Compare their answers to earlier reflections on this topic. Ask them to also reflect upon how their answers might have changed and why.
Use the "Knowledge Check" section of the Periodical Cicada esheet to assess student understandings of the concepts taught in this lesson. Answers to the questions posed in this section are provided below:
- What does the cicada eat for most of its life? (Juice from tree roots.)
- About how long does the periodical cicada spend as an adult? (About four weeks, if it doesn't get eaten.)
- How long do periodical cicadas live underground? (13 or 17 years, depending on what kind they are.)
- Why does the periodical cicada emerge from the ground? (It emerges to mate and reproduce.)
- Which is more vulnerable to prey, an adult cicada or an underground nymph? Why? (An adult cicada is more vulnerable because it is above ground where there are many animals that prey upon it.)
- The emergence of the periodical cicada in such large numbers is an example of which defense strategy? How does it help the periodical cicada? (Predator satiation helps the cicada because even though predators feast on them, there are still enough adults left to mate and reproduce.)
- What is another defense strategy described in this lesson? How does it work? (Students may mention any of the following: chemical defenses, physical defenses, protective or warning coloration, mimicry, or camouflage.)
- Where does the female cicada lay its eggs? (She lays them in the new growth of trees.)
- What happens to the eggs? (They hatch into nymphs that fall from the tree branches and burrow their way underground.)
The following resources contain a wealth of information about periodical cicadas: