Thirsty Months: Interactions at a Desert Refuge's Water Holes

0 / 11

Thirsty Months: Interactions at a Desert Refuge's Water Holes

Use camera counts and a flower test to explain competition, predation and mutualism.

Rock Tank through the year

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

Water Holes in the Dry Season

1Saguaro Flats National Wildlife Refuge protects a wide valley of Sonoran Desert in southern Arizona. Rain falls there mainly in two seasons: gentle winter rains and short, heavy summer storms from July to September. In between, from April to June, the desert is hot and almost rainless. Many animals depend on tinajas, natural rock basins that hold rainwater. Seventh grader Mirela Kostova spent a year helping refuge biologist Mr. Ruben Atencio check the refuge's 12 tinajas and sort photos from motion-triggered cameras.

2One camera watched Rock Tank, the largest tinaja on the refuge, which usually holds water all year. Mule deer and javelina, pig-like mammals that live in herds, came there to drink. The camera also caught chases, in which a mule deer or a javelina drove an animal of the other species away from the water's edge, and attacks by coyotes and bobcats on rabbits, quail and young javelina. Each month, Mirela and Mr. Atencio counted how many tinajas still held water and averaged the camera counts (Table 1).

3In spring, the refuge's chuparosa shrubs bloom with tube-shaped red flowers. Costa's hummingbirds hover at the flowers and drink their sugary nectar, and pollen dusts their heads as they feed. To test what flower visitors do for the shrubs, Mirela marked 90 flowers that had not opened yet. She covered 30 with fine mesh bags that kept out every visitor, covered 30 with coarse mesh cages that let bees in but kept hummingbirds out, and left 30 open. Both kinds of mesh let through almost all of the sunlight. A few weeks later, she counted the flowers that had formed fruit (Graph 1).

4Mr. Atencio told Mirela that biologists find the same kinds of interactions in very different ecosystems, from coral reefs to grasslands. Wherever living things share a habitat, they may compete for the same resources, eat one another or help one another, and the same patterns tend to repeat.

Table 1. Rock Tank Camera (weekly averages; attacks are totals for the month)
Month	Tinajas with water	Mule deer visits/week	Javelina visits/week	Chases per week	Attacks seen (month)
February	11	14	20	1	0
April	7	30	38	4	1
June	2	70	84	19	5
August	12	12	18	1	0

1.

Add the mule deer and javelina visits for each month. In June, the camera recorded more visits per week than in February.

2.

Which pattern does Table 1 show?

A. The more tinajas that held water, the more visits and chases the camera recorded at Rock Tank.B. The fewer tinajas that held water, the more visits and chases the camera recorded at Rock Tank.C. Visits rose from February to June, but the number of chases stayed near 1 per week.D. Mule deer visits rose from February to June, while javelina visits fell over the same months.

3.

Which kind of interaction do the chases at Rock Tank show, and why?

A. Predation, because the animal that chases another away is hunting it for foodB. Competition, because the animals all need the water, and in June it is scarceC. Mutualism, because chasing spreads the animals out so that both get more waterD. Competition, because deer and javelina hunt the same rabbits near the water

4.

Attacks by coyotes and bobcats at Rock Tank were highest in June. Which explanation fits the data best?

A. Deer and javelina visits fell in June, which left the smaller prey alone at the water.B. Attacks were most common in the month when the most tinajas on the refuge held water.C. With only 2 tinajas holding water, prey gathered at Rock Tank, so predators found them more often.D. Prey were spread over more tinajas in June, so predators ran into them more often at each one.

5.

Next year, a dry winter leaves only 4 tinajas with water in April. Based on Table 1, how many chases per week would you most likely expect at Rock Tank that April?

A. More than 19, more than in JuneB. About 1, as in FebruaryC. About 4, as in this AprilD. Between 4 and 19

6.

Sort each change by how it would most likely affect competition for water at Rock Tank.

Hummingbirds and chuparosa

Use paragraph 3 and Graph 1.

7.

How many more flowers formed fruit when left open than when caged so that only bees could reach them? flowers

8.

What do the open flowers and the bees-only flowers show when you compare them?

A. The flowers formed fruit on their own, so the visitors made little difference.B. The cages shaded the bees-only flowers, so they formed fewer fruit than the open ones.C. Hummingbird visits helped many more flowers form fruit than bee visits alone did.D. Bees carried more pollen than hummingbirds, so the bees-only flowers did best.

9.

Which statement best describes the relationship between Costa's hummingbirds and chuparosa shrubs?

A. Mutualism: the hummingbird gets nectar to eat, and the shrub gets its pollen carried.B. Mutualism: the hummingbird gets seeds to eat, and the shrub gets its leaves cleaned.C. Competition: the hummingbird and the shrub both need the nectar to survive.D. Predation: the hummingbird feeds on the shrub, so the shrub forms fewer fruit.

The same patterns in other ecosystems

10.

On an African savanna, most water holes dry up late in the dry season, and zebras, antelope and lions all drink at the few that are left. Based on the Rock Tank pattern, what would you predict at those last water holes?

A. More animals at each hole, more fights over the water and more hunting by lionsB. More animals at each hole, but less hunting, because lions avoid large groups of preyC. Fewer animals at each hole, because animals drink less often when water is scarceD. Fewer fights over the water, because zebras and antelope eat different grasses

11.

A refuge visitor says, “Mule deer and javelina don't compete with each other. They mostly eat different plants, so they never need the same thing.” Do you agree? Write a claim, use evidence from Table 1, and explain the pattern of interactions at Rock Tank through the year.