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Grass, Bison and Dung Beetles: Matter and Energy on a Prairie Refuge

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Grass, Bison and Dung Beetles: Matter and Energy on a Prairie Refuge

Follow the grass a bison eats and test what dung beetles do with the dung.

Where a bison's food goes

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

Survey Summer at Red Hollow

1Red Hollow Wildlife Refuge protects 4,000 hectares of tallgrass prairie and a herd of about 200 bison. Each summer, refuge biologist Dr. Aurelio Castañeda runs animal surveys with student volunteers. This year, seventh grader Signe Halvorsen helped count bison, birds and insects, and she used the refuge's data to build a model of how matter and energy move through the prairie.

2In Signe's model, grasses and wildflowers are the producers. They use energy from sunlight to make sugars from carbon dioxide and water, and their roots take up nitrogen and other minerals from the soil to build new leaves and roots. Bison are consumers that eat the grass. Dung beetles, along with many fungi and bacteria, are decomposers. Dung beetles roll or bury balls of dung, eat part of it and lay their eggs in it, and the fungi and bacteria break down what is left. Every living thing in the model uses some of the energy in its food to move, grow and stay alive, and that energy ends up as heat.

3The refuge staff estimate that a young bison eats about 10 kg of dry grass a day. Table 1 shows their model of where the matter in 10 kg of dry grass goes after a young bison eats it. The part that is broken down for energy leaves the body in its breath and urine as carbon dioxide and water.

4In June, Signe and the staff set out 12 fresh bison dung pats in one pasture. Each pat had a dry mass of about 200 g. Six pats were covered with a cage of fine screen that kept dung beetles out but let in rain, air, fungi and bacteria. The other six were left open. Six more spots with no dung were marked for comparison. After 6 weeks, the team weighed the dung left, tested the soil under each spot for nitrate, a form of nitrogen that plant roots take up, and later harvested the grass growing in a ring 30 cm wide around each spot (Table 2).

Table 1. Where 10 kg of Dry Grass Goes in a Young Bison (refuge model)
What happens to it	Dry mass (kg)
Leaves the body in dung	4.5
Built into the bison's body as it grows	0.2
Broken down; leaves as CO₂ and water	5.3

1.

Use Table 1. Of every 10 kg of dry grass a young bison eats, how many kilograms leave its body, either in dung or as CO₂ and water? kg

2.

A visitor says that most of the grass a bison eats turns into more bison. Which response fits Table 1 best?

A. Just 0.2 kg of each 10 kg becomes bison; most leaves as dung or as CO₂ and water.B. The visitor is right, since the 4.5 kg of dung is stored in the bison's body.C. About half becomes bison, and the other half is turned into energy and disappears.D. The visitor is right, since 5.3 kg of each 10 kg is built into the bison's body.

3.

Use paragraph 2 and Table 1. What can happen next to the carbon atoms in the 5.3 kg row?

A. They sink into the soil as CO₂, where dung beetles eat them.B. As CO₂ in the air, grass can build them into new sugars.C. They are turned into heat energy, so they stop being matter.D. They stay inside the bison and build up as stored heat.

4.

Suppose each of the refuge's 200 bison eats 10 kg of dry grass a day and leaves dung as Table 1 shows.

About how many kilograms of dung (dry mass) does the whole herd leave on the prairie each day? kg

The dung beetle test

Use paragraphs 2 and 4 and Table 2 to answer the questions.

Table 2. Dung Beetle Test (averages of 6 spots; pats started near 200 g dry)
Spot	Dung left at 6 weeks (g)	Soil nitrate (mg/kg)	Grass in ring (g)
Open pat (beetles reach it)	40	18	48
Screened pat (no beetles)	150	9	33
No dung	none added	7	30

5.

What percent of the starting dung was gone from the open pats after 6 weeks? %

6.

What does comparing the open pats with the screened pats show?

A. The screen kept the rain off, so the screened dung dried out and lasted longer.B. Fungi and bacteria break down dung faster when the beetles are kept away.C. Dung beetles take nitrogen out of the soil, so less nitrate was left under the open pats.D. Dung beetles speed up the breakdown of dung and move more of its nitrogen into the soil.

7.

Why did the team also test six spots with no dung?

A. To keep the bison from stepping on the screened patsB. To show the soil and grass with no dung addedC. To measure how much dung a bison leaves in 6 weeksD. To give the dung beetles a safe place to lay eggs

8.

Circle the two results that, together, best show that matter from the dung ended up in the grass around the open pats.

9.

Which statement about energy fits Signe's model and the test best?

A. Sunlight energy passes in food from grass to bison to decomposers and ends up as heat.B. The energy in the dung was made by the dung beetles as they buried it under the soil.C. Bison get their energy from the nitrate in grass, which is why the grass grew more near the pats.D. Dung beetles return the energy in dung to the soil, where grass roots take it up again.

10.

Suppose most of the dung beetles in one pasture died out, while the bison kept grazing there. Which prediction for the next few years fits the model and Table 2 best?

A. The grass would grow faster, since the beetles would no longer take nitrate from the soil.B. The bison would get less energy from sunlight, since beetles pass energy to the grass.C. Dung would break down more slowly, so less nitrate would reach the soil near the pats.D. The dung would break down just as fast, since the fungi and bacteria would still be there.

11.

A visitor says, “Dung beetles just clean up waste. The grass and the bison would get along the same without them.” Do you agree? Write a claim, use evidence from Table 2, and use Signe's model to explain how matter cycles and how energy flows on the prairie.