Materials & Prep
Prepare one projected or board copy of the population data, graph paper or a data handout, and two colored pencils per student or pair. Set up the board with the labels prey, predators, claim, evidence, and reasoning.
Opening phenomenon
Display only the first four months of this population record.
Display
Month Prey Predators 0 80 8 1 100 8 2 130 10 3 120 14
AskIf predators keep increasing, will they eventually kill all the prey? Write “yes” or “no” and one reason. Students commit to a prediction before discussing it. Tell them to watch for evidence that supports or challenges their prediction.
Turn and talkPartners compare predictions and identify which population changed first. Accept a quick point to the prey or predator column as well as a spoken answer.
Model reading population data
Display
Month Prey Predators 0 80 8 1 100 8 2 130 10 3 120 14 4 85 16 5 55 12 6 45 7 7 70 5 8 105 6 9 135 9
DefineA predator eats other organisms, called prey. A population is the number of members of one species in an area at a particular time.
Think aloudFrom month 0 to month 2, prey increase from 80 to 130 while predators increase only from 8 to 10. At month 3, prey are already decreasing, from 130 to 120, while predators continue increasing to 14. This delay matters. Predators do not instantly respond to a prey change. The common mistake is to match the highest predator number with the highest prey number. Instead, I compare the direction of change and look for which population changes first.
ConfrontAt month 6, predators are down to 7, but the prey do not stay at zero. The prey begin increasing from 45 at month 6 to 70 at month 7, 105 at month 8, and 135 at month 9. The data challenge the idea that predators kill everything and the story ends. When prey become scarce, predators have less food, so the predator population later decreases. With fewer predators, more prey survive and the prey population can recover.
Check for understandingAsk students to point to the month when prey are highest and the month when predators are highest. Expected: prey are highest at month 2, and predators are highest at month 4. Ask why those peaks do not occur at the same time. Expected: predator numbers rise after prey numbers have already risen, so there is a time delay.
Guided graphing and interpretation
ModelPlot the first two points for each population on a graph with month on the horizontal axis and population size on the vertical axis. Use one color for prey and one for predators. Explain that each point represents the population measured at that month, and connecting points helps show the pattern over time.
Have studentsIn pairs, graph all ten months from the displayed table. Label both lines and circle the highest and lowest point on each line.
ScaffoldProvide this organizer for students who need a defined entry point: “Prey increase from month __ to __. Predators then increase from month __ to __. When prey reach __ at month __, predators are __ at month __. Later, prey __ because predators __.” Students may use the table instead of the graph for any value.
Look forStudents should see that prey peak at month 2, predators peak at month 4, prey reach their low point at month 6, and prey recover as predators remain low. Correct the misconception that a higher predator population immediately causes prey to disappear by asking students to locate the prey value at month 3, when predators are already increasing.
Collaborative claim-evidence-reasoning
PromptWrite a claim explaining how predator and prey populations affect each other over time. Support the claim with at least two values from the table and explain the cause-and-effect pattern in your reasoning.
ModelA strong claim is, “Predator and prey populations can cycle because prey increases provide more food for predators, while increasing predators later reduce the prey population.” Evidence could include, “Prey increase from 80 at month 0 to 130 at month 2, and predators then increase from 10 at month 2 to 16 at month 4. After prey fall to 45 at month 6, predators fall to 5 at month 7.” The reasoning connects the numbers: more prey can support more predators, and fewer prey provide less food, so predator numbers later decrease. With fewer predators, more prey survive and the prey population can rise again.
Have studentsPairs compare their claims and underline the data values in their evidence. Each pair must explain why the predator peak follows the prey peak rather than occurring at the same time.
CirculateListen for students who say only “predators kill prey.” Prompt them to complete, “When prey become scarce, predators later...” Expected: “have less food, so their population decreases.”
ExtensionStudents who finish early use the data to explain why the predator population is still 9 at month 9 even though prey have recovered to 135. They should infer that the predator population may be beginning another increase, but state that this is a prediction rather than a direct observation from the table.
Closing check
Exit ticketUse the new data below to explain the predator-prey relationship over time in three or four sentences. Include one claim, two specific pieces of evidence, and reasoning that connects the evidence.
Year Prey Predators 0 50 15 1 75 10 2 110 12 3 90 18 4 55 16 5 35 8
Ask students to answer: “What cycle is beginning or continuing in this data? Explain why the predator population changes after the prey population changes.”
Look forA complete response should identify that prey rise from 50 to 110 before predators rise from 10 to 18, then prey fall to 35 as predators remain high, and predators later fall to 8 when prey are scarce. The reasoning should explain that prey provide food for predators, while predators reduce prey numbers, creating a repeating pattern rather than a one-time ending.