Water, Flowers and Nest Holes: Limits in a School Garden

Use a greenhouse watering trial and four years of bee records to explain how resources limit living things.

Answer Key

The greenhouse watering trial

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

Water, Flowers and Nest Holes

1Oksana Petrenko and the garden club at Northgate Middle School look after a small greenhouse and an outdoor garden. Their adviser, Mr. Kwabena Asante, gave the club a question that ecologists ask about every ecosystem: what happens to living things when a resource they need is scarce, and what happens when there is plenty of it?

2First, the club tested water. Inside the greenhouse, where no rain falls, they built four beds, each 1 m², side by side along the same sunny wall and filled with the same soil. They sowed 40 bush bean seeds of one variety in each bed. A drip line gave each bed a different amount of water every week: 5 L, 10 L, 20 L or 30 L. After 8 weeks, they counted the living plants, measured each plant's height and counted its pods (Table 1). Bean plants use water, carbon dioxide and light to make their food. A plant that is short of water often closes the tiny pores in its leaves, so it also takes in less carbon dioxide.

3Outside, the club studies blue orchard mason bees, a native bee that does not live in a hive. A female nests in a hollow tube, such as a hole in dead wood or a paper tube in a nest block. She gathers pollen and nectar from flowers, packs a ball of it at the back of the tube, lays one egg on the ball and seals it in with a mud wall. That makes one cell. Then she builds the next cell in front of it. Females do not share a tube, and each larva eats only the food ball in its own cell. The new adults come out the next spring.

4The club put up a nest block with 60 paper tubes in spring 2022. That fall they planted a strip of native wildflowers beside the garden, and in fall 2023 they made the strip about twice as big. In 2025 they added a second block, for 120 tubes in all. Each spring, the club counted the plants in bloom within 30 m of the blocks, the tubes that were filled and the cells inside them (the paper tubes can be unrolled and closed again). The next spring, they counted the new bees that came out and weighed a sample of the new females (Table 2). In late spring 2024, club members watched many females land on the full block and fly away again.

Table 1. Bean Beds After 8 Weeks (40 seeds sown in each 1 m² bed)
Water per week (L)	Plants alive	Average height (cm)	Pods per plant
5	22	18	3
10	31	27	6
20	37	38	11
30	38	40	12

Total pods in a bed = plants alive × pods per plant.

How many pods did the bed that got 20 L a week produce in all? 407 pods

Which pattern does Table 1 show?

A. Each step up in water doubled the pods per plant, from the 5 L bed to the 30 L bed.
B. From 5 L to 20 L, survival, height and pods all rose; from 20 L to 30 L they rose a little.
C. More water made the plants taller, while the number of plants alive stayed near 30 in each bed.
D. The 10 L bed did best, because its plants had fewer neighbors sharing the water.

The 30 L bed got half again as much water as the 20 L bed, but its results were only a little better. Which explanation fits the passage and Table 1 best?

A. Above about 20 L, water was no longer scarce, so another need, such as light or space, limited the plants.
B. The 30 L bed was given too much water, so its roots rotted and many of its bean plants died early.
C. The 30 L bed sat in a shadier spot than the 20 L bed, so its plants could not use the extra water.
D. The 30 L bed had fewer plants alive than the 20 L bed, so each of its plants had more room to grow.

Suppose the club adds a fifth bed that gets 15 L of water a week and treats it like the others. Which result is most likely after 8 weeks?

A. About 39 plants alive, with about 13 pods per plant
B. About 45 plants alive, with about 9 pods per plant
C. About 34 plants alive, with about 8 pods per plant
D. About 20 plants alive, with about 2 pods per plant

Use paragraph 2. Circle the three things the club kept the same in every bed.

Water given each weekVariety of bean seedPlants alive after 8 weeksNumber of seeds sownPods on each plantBed size and soil

Four years of mason bees

Use paragraphs 3 and 4 and Table 2 to answer the questions.

Table 2. Mason Bee Records (new bees come out the following spring)
Year	Plants in bloom	Tubes offered	Tubes filled	Cells per filled tube	New bees next spring
2022	40	60	18	3	48
2023	150	60	42	6	230
2024	300	60	60	7	380
2025	300	120	85	7	540

Average mass of a new female: 80 mg from the 2022 cells, 105 mg from the 2023 cells, 110 mg from the 2024 cells and 108 mg from the 2025 cells.

How many tubes were left empty in 2025? 35 tubes

The club offered 60 tubes in both 2022 and 2023, yet the new bees rose from 48 to 230. Which explanation fits the data best?

A. More flowers meant more food, so more females nested and built cells.
B. The 2022 females were heavier, so each one of them laid fewer eggs.
C. The club added tubes in 2023, so more females found a place to nest.
D. Fewer plants bloomed in 2023, so the bees spent less time flying.

What most likely limited the number of new bees from the 2024 cells?

A. Food, because fewer plants were in bloom than in 2023
B. Food, because each filled tube held fewer cells than it did in 2023
C. Nest space, because the club took away half of the tubes that spring
D. Nest space, because every tube was filled and females left the block

Cells built = tubes filled × cells per filled tube.

How many cells did the bees build in 2024 altogether? 420 cells

10.

In 2026 the club keeps 120 tubes, but a dry spring leaves only about 150 plants in bloom, as in 2023. Which prediction do the data support best?

A. Fewer cells per tube and lighter new females than in 2025, with empty tubes
B. More cells per filled tube than in 2025, because the bees have twice as many tubes
C. About the same results as in 2025, since the number of tubes stays at 120 again
D. More than 100 tubes filled and heavier new females, because the bees have more room

11.

A classmate says, “The bees did better in 2025 than in 2024 because they had more flowers.” Do you agree? Write a claim, use numbers from Table 2 and paragraph 4 as evidence, and explain how resources affected the bee population from 2022 to 2025.

I disagree. The flowers did not change from 2024 to 2025: there were 300 plants in bloom both years, and the cells per tube stayed at 7. What changed was nest space. In 2024 all 60 tubes were filled and females were seen leaving the full block, so nest tubes limited the population. In 2025 the club offered 120 tubes, 85 were filled, and the new bees rose from 380 to 540. Earlier, from 2022 to 2023, food was the limit: blooming plants rose from 40 to 150, and cells per tube rose from 3 to 6 and new females got heavier (80 mg to 105 mg). So a population is limited by whichever resource is in shortest supply, and that resource changed from flowers to nest tubes.