Mites, Fungi and Mulch: Interactions in a School Greenhouse

Use three garden club studies to explain predation, mutualism and competition.

Study 1: Mites on the bean benches

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

The Garden Club Investigates

1The garden club at Ridgeview Middle School looks after a small greenhouse, a vegetable garden and a row of young apple trees. Last year, seventh grader Hamza Qureshi and the club's adviser, Ms. Lorraine Achterberg, ran three studies to find out how the living things in and around the greenhouse affect one another.

2Spider mites are tiny relatives of spiders, about half a millimeter long. They pierce the cells of a leaf and suck out what is inside, leaving pale specks, and a female can lay a few eggs every day. Predatory mites are slightly larger mites that hunt. They catch spider mites and their eggs and suck them dry. The club grew 40 bean plants on each of two greenhouse benches with the same light, water and temperature. In week 0, both benches had about 4 spider mites per leaf. That week, the club released predatory mites bought from a garden supplier onto Bench A only. Every week, Hamza used a hand lens to count the mites on 20 leaves per bench and recorded the averages (Table 1).

3Many plants have partners in the soil. Some fungi grow fine threads into the roots of a plant and far out into the soil around it. The threads take up phosphorus, a nutrient plants need to grow, and pass some of it to the roots. The plant passes some of the sugar it makes in its leaves to the fungus. To test how much the tomato seedlings gain, the club grew them for 8 weeks in four groups of 5 pots of sterilized potting mix. Two groups had a mix low in phosphorus, and two had phosphorus fertilizer added. In each pair, one group got spores of the fungus and the other did not. All pots got the same water and light (Table 2).

4Outside, the club planted 15 young apple trees of the same size and kind. Five trees had lawn grass growing right up to the trunk. Five had a 1 m circle of bare soil that was hoed every week to keep grass out. Five had a 1 m circle of bare soil covered with wood chips. In July, after three weeks with no rain, the club measured the water in the soil 20 cm down. In the fall, they measured each tree's new shoot growth (Table 3).

Table 1. Mites on the Bean Benches (average per leaf)
Week	Spider mites, Bench A	Predatory mites, Bench A	Spider mites, Bench B
0	4	1	4
1	7	2	9
2	10	4	18
3	6	6	34
4	3	5	55
5	2	3	70
6	2	2	72

Bench B got no predatory mites. In week 6, 10% of the leaves on Bench A and 85% of the leaves on Bench B were pale and speckled.

In week 5, Bench B had how many times as many spider mites per leaf as Bench A? times as many

From week 2 to week 5, the spider mites on Bench A dropped from 10 to 2 per leaf. Which explanation fits the passage and Table 1 best?

A. The leaves on Bench A turned pale and gave the spider mites too little to eat.
B. Predatory mites multiplied and ate spider mites faster than new ones hatched.
C. The spider mites on Bench A crawled across to Bench B, where no predators lived.
D. The greenhouse got cooler after week 2, which slowed the spider mites on both benches.

Circle the two pieces of data that, together, are the best evidence that the predatory mites kept the spider mites low on Bench A.

Both benches started with about 4 spider mites per leaf.On Bench B, with the same light, water and temperature, spider mites rose to 72 per leaf.Hamza counted the mites on 20 leaves per bench each week.On Bench A, predatory mites rose to 6 per leaf in the same weeks that spider mites fell.Each bench held 40 bean plants.

By week 6, there were only 2 predatory mites per leaf on Bench A, down from 6. Suppose the club now moves 20 bean plants with many spider mites from Bench B onto Bench A. Which prediction fits the pattern in Table 1 best?

A. Spider mites on Bench A rise at first, and then predatory mites increase and bring them back down.
B. Predatory mites on Bench A keep falling, because predators eat less when prey become plentiful.
C. Both kinds of mites stay at 2 per leaf, because the bench has room for that many mites.
D. Spider mites on Bench A disappear within a day, because each predatory mite eats the prey nearby.

Study 2: Tomatoes and root fungus

Use paragraph 3 and Table 2 to answer the questions.

Table 2. Tomato Seedlings After 8 Weeks (averages of 5 pots)
Potting mix	Fungus spores added?	Dry mass of plant (g)	Roots with fungus (%)
Low phosphorus	No	1.2	0
Low phosphorus	Yes	3	65
Phosphorus added	No	3.4	0
Phosphorus added	Yes	3.4	15

In the low-phosphorus mix, the plants with the fungus had how many times the dry mass of the plants without it? Do not round. times

Which kind of interaction do the tomato plants and the root fungus have, based on paragraph 3 and Table 2?

A. Predation, because the fungus threads grow inside the roots and feed on them
B. Predation, because the plants take phosphorus out of the fungus threads
C. Competition, because the plant and the fungus both take phosphorus from the mix
D. Mutualism, because the plant gets phosphorus and the fungus gets sugar

In the mix with phosphorus added, the fungus made no difference to the plants’ dry mass. Which explanation fits Table 2 best?

A. The fertilizer kept the fungus spores from growing into these plants’ roots.
B. The plants already had plenty of phosphorus, so the fungus added little they needed.
C. The fungus took so much sugar that it slowed the plants down to the same mass.
D. The plants in that mix got more water, so they needed less phosphorus.

A rainforest grows on very old soil that holds little phosphorus. Based on Table 2, which prediction about its trees is best supported?

A. Trees with root fungus partners likely grow better than trees without them.
B. Trees without root fungus likely grow best, since the fungus takes their phosphorus.
C. Trees likely grow the same with or without the fungus, since the soil is the same.
D. Root fungus partners likely matter less there than in soil with fertilizer added.

Study 3: Grass and young apple trees, and patterns elsewhere

Use paragraph 4 and Table 3. Then use the whole sheet.

Table 3. Young Apple Trees (averages of 5 trees)
Ground around the trunk	Soil water in July (%)	New shoot growth (cm)
Grass up to the trunk	8	15
Bare soil, hoed	17	38
Bare soil with wood chips	20	44

How many more centimeters of new shoots did the trees in hoed bare soil grow than the trees with grass up to the trunk? cm

10.

Why did the club include the trees in hoed bare soil, with no wood chips?

A. To give the club extra apples to pick in case the other trees failed
B. To find out whether wood chips make apple trees grow more than grass does
C. To compare with the grass group, so that removing grass is the one difference
D. To show how much water the wood chips soak up and keep from the roots

11.

Which statement about the grass and the young apple trees do paragraph 4 and Table 3 support best?

A. The grass is a predator, because its roots feed on the apple tree roots.
B. They compete, because grass roots took much of the scarce soil water.
C. The trees harmed the grass, because their leaves took the grass’s light.
D. They are partners, because the grass shaded the soil and kept it damp.

12.

The same kinds of interactions show up in very different ecosystems. Sort each example.

Barn swallows and tree swallows catching the same flying insects over a pondA lynx catching and eating a snowshoe hare in a northern forestA cleaner shrimp on a coral reef eating parasites off a fish’s skinDesert shrubs whose roots draw on the same small supply of soil waterA lichen, in which a fungus shelters algae and shares the sugar they makeA great horned owl swooping down on a rabbit at the edge of a field

Competition	Predation	Mutualism

13.

A club member says, “The root fungus is a pest. It steals sugar from the tomatoes, so we should keep it out of our pots.” Do you agree? Write a claim, use evidence from paragraph 3 and Table 2, and explain your reasoning about the kind of interaction.