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Grubs, Clover and Root Bacteria: Interactions Under a Practice Field

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Grubs, Clover and Root Bacteria: Interactions Under a Practice Field

A runner's twisted ankle leads to a study of predators, partners and rivals.

Grubs and their predators

Read the passage and study Table 1. Then answer the questions.

Loose Turf on the Practice Field

1Farida Nasser runs the 800 meters for the Lakeshore Middle School track team. The team warms up on a grassy practice field next to the track. Last September, Farida twisted her ankle when a patch of turf slid out from under her shoe. The athletic trainer, Ms. Renata Szabo, wrapped the ankle and told her to rest for a week. Farida spent that week with the groundskeeper, Mr. Ignatius Moreau, trying to find out why the turf had come loose. She wondered whether the problem had something to do with the plants and animals living in the field.

2Mr. Moreau pulled back the loose turf. Underneath were dozens of white grubs, the C-shaped young of beetles. Beetles lay their eggs in the turf in midsummer, and the grubs that hatch feed until the soil turns cold in late fall. Grubs live in the soil and eat grass roots, and turf with its roots eaten away can peel up like a rug. He also pointed to small holes and torn patches. Starlings and crows probe the turf for grubs during the day, and skunks dig for them at night.

3In August, Mr. Moreau had set up a test on the part of the field with the most grubs. He placed wire cages over 6 plots, each 1 square meter, to keep birds and skunks out but let rain and light in. He chose plots that all had about the same number of grubs at the start. He left 6 more plots open nearby. In August and again in October, he cut a small square of soil from each plot and counted the grubs in it. In October, he also measured how much of each plot's turf was loose from root damage and how much was torn up by digging. Table 1 shows the averages.

Table 1. Caged and Open Plots (averages of 6 plots)
Plots	Grubs per sample, Aug.	Grubs per sample, Oct.	Loose turf, Oct. (%)	Dug-up turf, Oct. (%)
Caged	12	13	45	0
Open	12	3	10	12

1.

From August to October, the grubs per sample in the open plots went down by . In October, the caged plots had more grubs per sample than the open plots.

2.

Which kind of interaction between the birds and skunks and the grubs do paragraph 2 and Table 1 show?

A. Competition, because the birds and grubs both need the same soil to liveB. Mutualism, because the birds and skunks loosen the soil for the grubsC. Predation, because the birds and skunks find the grubs and eat themD. Competition, because the birds, skunks and grubs each feed on grass roots

3.

The caged plots had 45% loose turf, but the open plots had only 10%. Which explanation fits Table 1 best?

A. The cages shaded the grass, so its roots grew weaker and more of the turf came loose.B. The grubs in the open plots ate less, since they spent much of their time hiding from birds.C. The predators ate many grubs in the open plots, so fewer grass roots were eaten there.D. The predators dropped new grass seeds in the open plots as they dug for their food.

4.

Mr. Moreau thinks about fencing skunks out of the whole field and hanging nets to keep birds off it. Based on Table 1, what is the most likely result?

A. More grubs would likely survive, so more turf would come loose even with less digging.B. The grub numbers would stay the same, because grubs do not move between the plots.C. Fewer grubs would survive, because the grubs depend on birds and skunks to loosen the soil.D. The turf would get safer to run on, because the animals would stop digging holes in it.

Clover and its root bacteria

Read the paragraph and study Table 2. Then answer the questions.

White clover grows among the grass on the field. When Farida dug up a clover plant, she found small pink bumps called nodules on its roots. Each nodule is packed with bacteria. The bacteria take nitrogen gas from the air in the soil and change it into a form the plant can use to build proteins. The plant sends sugars from its leaves to the nodules. To test what each partner gets, Farida's science class grew clover seedlings for 6 weeks in sterilized sand with no nitrogen fertilizer. Pots in Group 1 got a small amount of nodule bacteria from the field's soil, and pots in Group 2 got none. Group 3 pots got the same small amount of bacteria, about 0.5 million per gram of sand, but no clover. All pots got the same water and light.

Table 2. The Pot Test After 6 Weeks (averages)
Group	Clover?	Bacteria added?	Nodules per plant	Clover dry mass (g)	Bacteria (millions/g)
1	Yes	Yes	22	0.48	30
2	Yes	No	0	0.12	0
3	No	Yes	—	—	0.5

5.

The clover plants in Group 1 had times the dry mass of the clover plants in Group 2.

6.

What do Groups 1 and 3 show when you compare them?

A. The sand in Group 3 held more nitrogen, so the bacteria there had more to eat.B. The bacteria multiplied far more in sand with clover roots than in sand with none.C. The bacteria slowed down the clover's growth, so the clover in Group 1 stayed small.D. The bacteria in Group 3 made their own sugar, so they did as well as those in Group 1.

7.

Which kind of interaction do the clover and its nodule bacteria have?

A. Competition, because the bacteria fill the roots and leave less room for the cloverB. Mutualism, because the clover gets nitrogen it can use and the bacteria get sugarC. Competition, because the clover and the bacteria both need nitrogen from the soilD. Predation, because the bacteria feed on sugar they take from the clover's roots

Grass and clover side by side

Read the paragraph and study Table 3. Then use everything on this sheet to answer.

For the last three years, Mr. Moreau has spread nitrogen fertilizer on the east half of the practice field but not on the west half. Both halves get the same mowing and watering. On the east half, the grass now grows thick and tall, and the few clover plants there grow in its shade. In the school greenhouse, extra clover pots with no grass that got the same fertilizer grew well and stayed green.

Table 3. Plant Cover on the Two Halves of the Field
Half of field	Nitrogen fertilizer	Grass cover (%)	Clover cover (%)
East	Yes, 3 years	92	4
West	None	64	30

8.

Subtract the clover cover on the east half from the clover cover on the west half.

Clover cover on the west half was percentage points higher than on the east half.

9.

Why is there so much less clover on the fertilized east half? Choose the explanation that fits all of the evidence.

A. With extra nitrogen, the grass grew thick and tall and took the light and space the clover needed.B. The clover's nodule bacteria moved to the grass on the east half, so the grass got their nitrogen.C. The fertilizer harmed the clover directly, so it died back on the east half wherever it landed.D. The east half was mowed shorter, so its clover was cut down before it could spread across the turf.

10.

Far from the school, a mountain glacier has melted back and left bare gravel with very little nitrogen that plants can use. Alder shrubs have root nodules with bacteria like the clover's, and many other plants there do not. Based on the patterns on this sheet, which prediction is most likely?

A. Plants without nodules would likely spread first, because alders would use up the gravel's nitrogen.B. Alders would likely grow poorly, because nodule bacteria need the rich soil of a grassy field.C. Alders and plants without nodules would likely grow equally well, because both get the same light.D. Alders would likely do better early, because their partners supply nitrogen other plants lack.

11.

Farida's coach says, “The crows, starlings and skunks are pests. They only damage our practice field.” Do you agree? Write a claim, use evidence from Table 1, and explain your reasoning about the interactions among the predators, the grubs and the grass.