Learning target: I can graph survey data from streams with and without beaver ponds, calculate averages and differences, and use the patterns to make a claim about how a change to a physical component affects populations.
Both tables on this sheet are teaching datasets: the numbers were invented to look like a real stream survey, but no one measured them. Part A compares streams with and without beaver ponds. Part B follows one stream before and after beavers moved in. You may check calculations with a partner, but write your own answers. Show every calculation, keep units, and round to one decimal place. When a question asks what the data show, answer with numbers.
You will need: This worksheet, Pencil, Ruler, Calculator
In July 2024, ecologists surveyed eight short sections of mountain stream, called reaches, in the same valley during the same week. Four reaches had an active beaver dam with a pond behind it. The other four were similar reaches nearby with no beavers. All eight were at about the same elevation, ran through the same kind of forest, and were sampled with the same nets in the same way. In each reach the team measured water depth and water speed, then counted three kinds of animals: tadpoles of the Pacific chorus frog, which develop in still or very slow water; stonefly larvae, insects that cling to rocks and need fast, cold water with plenty of oxygen; and dragonfly larvae of kinds that hunt small animals in still water.
| Reach | Beaver pond? | Water depth (cm) | Water speed (cm/s) | Chorus frog tadpoles | Stonefly larvae | Dragonfly larvae |
|---|---|---|---|---|---|---|
| A | yes | 120 | 5 | 64 | 2 | 18 |
| B | yes | 95 | 8 | 48 | 4 | 14 |
| C | yes | 140 | 3 | 71 | 1 | 22 |
| D | yes | 105 | 6 | 57 | 5 | 16 |
| E | no | 30 | 45 | 6 | 38 | 3 |
| F | no | 25 | 52 | 2 | 44 | 1 |
| G | no | 35 | 40 | 9 | 31 | 4 |
| H | no | 30 | 47 | 3 | 39 | 2 |
| Average with beaver ponds (A to D) | Average without beavers (E to H) |
|---|---|
| Water depth (cm) | |
| Water speed (cm/s) | |
| Chorus frog tadpoles | |
| Stonefly larvae | |
| Dragonfly larvae |
Reach J is in the same valley. Until the autumn of 2020 it was a fast, shallow stretch like reaches E to H. That autumn a beaver family built a dam there. The team had surveyed reach J every July since 2017, so they have four summers of data from before the dam. In 2021, the first summer after the dam, the pond was still filling and the stream bed was still changing.
| July survey | Chorus frog tadpoles (per 10 sweeps) | Stonefly larvae (per kick sample) |
|---|---|---|
| 2017 | 4 | 41 |
| 2018 | 7 | 36 |
| 2019 | 3 | 44 |
| 2020 | 5 | 39 |
| 2021 | 31 | 18 |
| 2022 | 52 | 6 |
| 2023 | 58 | 4 |
| 2024 | 61 | 3 |
Average: add the four values, then divide by 4 (example with made-up numbers: 10 + 12 + 14 + 8 = 44; 44 ÷ 4 = 11.0). 'How many times as many': divide the bigger average by the smaller one (30 ÷ 6 = 5 times). Percent lower: subtract the smaller from the bigger, divide by the bigger, then multiply by 100 ((20 − 5) ÷ 20 = 0.75; 0.75 × 100 = 75 percent). Write each step on its own line so you can check it.
Ecologists often combine both kinds of evidence in one design called before-after-control-impact: they survey places that will get a change and similar places that will not, both before and after the change. A restoration group plans to install beaver dam analogs (rows of wooden posts with willow branches woven between them, which slow water the way a beaver dam does) in two of reaches E to H next autumn. Design the study: which reaches get structures and which are left alone, what you would measure, in which years, and what pattern of results would convince you the structures caused any change. Sketch the line graph you would expect if they work.