ACT science practice test 36

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


An invasive species is a species that is not native to an ecosystem and whose introduction has harmful environmental, economic, and/or human health effects.

Eichhornia crassipes (water hyacinth) is an invasive species of floating aquatic weed found in freshwater waterways in tropical and temperate regions worldwide. It is highly tolerant of fluctuations in water level, nutrient availability, pH, and temperature. This allows it to grow rapidly and outcompete native aquatic plant species for resources. Dense floating mats of E. crassipes further alter aquatic communities by reducing dissolved oxygen levels and access to light. Decomposing matter from E. crassipes mats increases sediment deposition in waterways.

Ecological studies have shown that the growth of a plant can be influenced by competition with different species of neighboring plants. A group of scientists carried out the following studies to determine the effects on the growth of E. crassipes when paired with three other, more benign, aquatic weed species.

Study 1

Scientists collected growth data on E. crassipes mats in the Kagera River in Tanzania. Scientists marked off 1 square meter (m2) sample areas containing E. crassipes alone and in combination with three other aquatic weeds common to the Kagera River.

To determine the effects of the other three weed species on E. crassipes growth, scientists analyzed five growth parameters. Fresh weight was determined by removing and immediately weighing 10 E. crassipes plants from each area. Plant height was measured from the base of the plant to the tip of the tallest leaf. The total number of E. crassipes plants within a sample area was recorded as plant density, which was then multiplied by fresh weight to determine total biomass. The number of leaves per plant was also recorded. Table 5.1 lists the averages for each growth parameter for E. crassipes growing alone and in combination with the three other aquatic weed species.

TABLE 5.1 Kagera River Data

Source: http://www.academicjournals.org/ijbc/fulltext/2011/August/Katagira%20et%20al.htm.

Study 2

Scientists transplanted young E. crassipes, Commelina sp., Justicia sp., and V. cupsidata plants from the Kagera River to a greenhouse. In the greenhouse, E. crassipes potted alone and in combination with the other three weed species were allowed to grow in water from the Kagera River for four months. At the end of the four-month growth period, the parameters of fresh weight, plant height, and leaves per plant were all determined by the same methods used in Study 1.

Table 5.2 lists the averages for each growth parameter for E. crassipes growing in the greenhouse alone and in combination with the other aquatic weed species.

TABLE 5.2 Greenhouse Experiment Data

Source: http://www.academicjournals.org/ijbc/fulltext/2011/August/Katagira%20et%20al.htm.

1. According to the passage, species identified as invasive are always:

A. aggressively growing plants.
B. disruptive to an ecosystem.
C. introduced by humans.
D. economically profitable.

2. According to the passage, water hyacinths upset freshwater ecosystems by doing all of the following except:

A. increasing sediment deposition in waterways.
B. outcompeting native plants for resources.
C. altering the pH of aquatic environments.
D. limiting aquatic organisms' access to sunlight.

3. Which weed combination was tested in Study 1 but not Study 2?

A. Water hyacinth alone
B. All four aquatic weeds together
C. Water hyacinth with V. cupsidata
D. Water hyacinth with Justicia sp.

4. In Study 1, plant density was measured as:

A. the total number of E. crassipes plants in 1 m2.
B. the total number of weed plants in 1 m2.
C. fresh weight divided by water volume in 1 m2.
D. fresh weight divided by plant volume in 1 m2.

5. Which weed combination serves as the control group in Study 1?

A. E. crassipes with Justicia sp.
B. E. crassipes with all three other weeds
C. E. crassipes with V. cupsidata
D. E. crassipes alone

6. In Study 1, V. cupsidata caused the greatest reduction in:

A. all E. crassipes growth parameters.
B. E. crassipes fresh weight only.
C. all growth parameters except fresh weight.
D. E. crassipes height and density only.

7. Based on the data in Table 5.1, which weed exerts the least competitive pressure on E. crassipes?

A. Justicia sp.
B. V. cupsidata
C. Commelina sp.
D. The combination of all three weeds.

8. In Table 5.2, Commelina sp. and Justicia sp. are both shown to have:

A. a stronger effect on fresh weight than V. cupsidata.
B. no effect on E. crassipes plant height.
C. the same effect on fresh weight as V. cupsidata.
D. a positive effect on E. crassipes plant height.

9. In Study 2, the water hyacinths grown alone exhibited a greater average:

A. number of leaves than in Study 1.
B. plant height than in Study 1.
C. fresh weight than in Study 1.
D. total biomass than in Study 1.

10. Total biomass was not included as a growth parameter in Table 5.2 because:

A. plant density was not measured in Study 2.
B. the fresh weight values recorded in Table 5.2 were too low.
C. the plants used in Study 2 had no biomass.
D. total biomass is not a good indicator of plant growth.

11. Which of the following statements is supported by the data collected in both studies?

A. V. cupsidata has the most negative effect on water hyacinth growth.
B. Commelina sp. has a positive effect on water hyacinth growth.
C. Water hyacinth growth is not affected by the presence of other weed species.
D. Justicia sp. has no effect on water hyacinth growth.

12. Based on the data in Table 5.2, the most significant impact of growing E. crassipes in combination with other weeds in a greenhouse environment appears to be the production of:

A. shorter plants.
B. lighter plants.
C. fewer leaves per plant.
D. fewer plants.

13. The greatest advantage of the experimental design in Study 2 is that scientists were able to:

A. choose on which weed species to focus their observations.
B. record data more frequently than could be done at the Kagera River.
C. control for other environmental factors that may affect plant growth.
D. obtain more precise measurements for each of the growth parameters.

14. According to Table 5.1, the presence of all three competitor weeds within the same square meter appears to have:

A. a greater effect on E. crassipes fresh weight than the presence of any single competitor weed.
B. an effect approximately equal to the sum of the effects of each single competitor weed on fresh weight.
C. a lesser effect on E. crassipes fresh weight than the presence of any single competitor weed.
D. an effect approximately equal to the mean of the effects of each single competitor weed on fresh weight.

15. Ecologists have found that introducing a competitor to an ecosystem is sometimes more effective in reducing an unwanted population than introducing a predator. Based on the results of this pair of studies, increasing the presence of which of the following species can be predicted to best reduce the water hyacinth population?

A. Justicia sp.
B. V. cupsidata and Commelina sp.
C. Commelina sp. and Justicia sp.
D. V. cupsidata