Worksheet

When the Beavers Moved In: Arguing About Cutbank Creek

Grade 7 life science: use creek surveys before, during and after a beaver pond, plus an upstream comparison, to argue how ecosystem changes affect populations.

Grade 7ScienceInteractive · Printable

Aligned toMS-LS2-4

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1 NGSS standard
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10 questions
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Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Short answer

About this worksheet

What students practice

Students use survey data to link changes in a creek's physical conditions to changes in its populations.
Students use a comparison stretch and a reversal to rule out other causes.
Students construct an argument from evidence about how one change affected several populations.

How to use

Read the passage together and list the physical and living parts of the creek on the board. Students then work through the tables alone and share their arguments in small groups.

Curriculum

NGSS standard covered

MS-LS2-4

Argue from evidence that changes to physical or living parts of an ecosystem affect its populations.

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Name Date

When the Beavers Moved In: Arguing About Cutbank Creek

Use six years of creek surveys to argue how a beaver pond changed the creek's populations.

Before and after the dam

Read the passage and Table 1, then answer the questions.

A Pond Where the Riffles Were

1Since 2019, the ecology club at Cedar Valley Middle School has studied two 200 m stretches of Cutbank Creek: a lower stretch behind the school and an upstream stretch 1 km away. Each July, students measure the water's depth, speed and temperature, scoop insect nymphs from the creek bed with nets, and sort them under dissecting microscopes in the classroom. They also count tadpoles with net sweeps, take one-liter water samples to count tiny drifting animals called zooplankton under a microscope, and help a state fish biologist, Ms. Hana Kowalczyk, count brook trout. Seventh grader Lucas Ferreira keeps the club's records.

2In September 2021, a pair of beavers moved into the lower stretch. Beavers eat bark, twigs and water plants, and they build dams of branches and mud that back up the water into a pond deep enough to protect their lodge. By the next July, a dam about 1 m high had turned the fast, shallow riffles of the lower stretch into a still pond. The upstream stretch had no beavers and no dam. In April 2025, a spring flood broke the dam, and the beavers moved away.

3Ms. Kowalczyk explained what some of the creek's animals usually need. Most of the mayfly nymphs in Cutbank Creek cling to rocks in fast, cool riffles, where moving water brings them plenty of oxygen. Brook trout do best in cold water, and they are often stressed when the water stays above about 20 °C. Dragonfly nymphs and green frog tadpoles are most common in still, shallow water with plants to hide in. Zooplankton drift with the water, so fast currents tend to carry them downstream.

4Table 1 shows the lower stretch each July. Table 2 adds more counts from the lower stretch and compares them with the upstream stretch, where the club used the same nets, sample sizes and survey dates every year.

Table 1. Lower Stretch Each July (the dam was built after the 2021 survey)
Year	Depth (cm)	Current (cm/s)	Water temp. (°C)	Mayfly nymphs/sample	Dragonfly nymphs/sample
2020	30	60	16	48	2
2021	28	62	17	45	3
2022	120	5	21	12	14
2023	150	3	22	6	22
2024	150	3	22	5	25
2025	40	50	18	20	8

From 2021 to 2023, the mayfly nymphs in the lower stretch fell by per sample, and the dragonfly nymphs rose by per sample.

Which pattern does Table 1 show from 2021 to 2024?

A. The water got deeper, slower and warmer, while mayfly nymphs fell and dragonfly nymphs rose.
B. The water got deeper and colder, while mayfly nymphs and dragonfly nymphs both fell.
C. The water got deeper and slower, while both kinds of nymphs stayed near their 2021 counts.
D. The water got shallower and faster, while mayfly nymphs rose and dragonfly nymphs fell.

Use paragraphs 2 and 3. Why did the mayfly nymphs in the lower stretch fall after the dam was built?

A. The beavers ate the mayfly nymphs while they fed along the creek bed.
B. A cold winter in 2022 killed most of the mayfly nymphs along the creek.
C. The mayfly nymphs grew into dragonfly nymphs in the warmer pond water.
D. The pond's slow, warm water replaced the fast, cool riffles the mayflies need.

Table 2. More July Counts (trout in the whole lower stretch)
Year	Brook trout, lower	Tadpoles/sweep, lower	Zooplankton/L, lower	Upstream mayflies/sample	Upstream temp. (°C)
2021	25	1	3	50	16
2022	17	9	60	46	17
2023	11	15	90	47	16
2024	10	16	85	48	17
2025	15	3	12	49	17

From 2021 to 2024, the number of brook trout in the lower stretch fell by %.

Ruling out other causes

Use both tables and the passage.

A club member wonders whether a change in the weather, not the beaver pond, caused the changes in the lower stretch. Which evidence argues best against this idea?

A. In the lower stretch, dragonfly nymphs rose from 3 to 25 per sample between 2021 and 2024.
B. Upstream, with no dam, mayflies stayed at 46 to 50 per sample and the water at 16 to 17 °C.
C. In the lower stretch, the pond was 150 cm deep in both the 2023 and 2024 surveys.
D. In the lower stretch, brook trout fell from 25 to 10 between the 2021 and 2024 surveys.

Zooplankton in the lower stretch rose from 3 per liter in 2021 to 90 per liter in 2023. Which explanation fits paragraph 3 best?

A. The still pond no longer swept them downstream, so they could build up in the water.
B. The faster water in the pond carried more zooplankton down from the upstream stretch.
C. The warmer water made the brook trout eat more zooplankton, so more of them hatched.
D. The slower water turned the mayfly nymphs into zooplankton as they drifted in the pond.

The dam broke in April 2025. Which statement best describes the lower stretch in July 2025?

A. Trout fell again, because the water got much colder after the flood broke the dam in April.
B. Mayflies were back to about 45 per sample, the same as their 2021 count, within three months.
C. Mayflies and trout rose, and dragonflies and tadpoles fell, but none were at 2021 counts.
D. Dragonfly nymphs kept rising, as they had while the pond was there.

Suppose beavers build a dam like this one in the upstream stretch in 2026. Which TWO results would you predict there by July 2027?

A. Fewer brook trout
B. A faster current
C. More dragonfly nymphs per sample
D. More mayfly nymphs per sample
E. Fewer zooplankton per liter

Complete the chain of causes for the lower stretch. Use words from the word bank. Two words are left over.

Word bank: slowed · sped up · warmer · colder · mayfly · dragonfly

Beavers built a dam → the current and the water got → nymphs and brook trout declined → still-water animals such as nymphs and tadpoles increased.

10.

A member of a local fishing club says, “The beaver dam harmed the whole creek. Every population in the lower stretch went down while the pond was there.” Write an argument for or against this claim. Make a claim, use evidence from both tables, and explain how changes to the physical parts of the creek affected its populations.