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AP Biology 8.7 Disruptions in Ecosystems- Exam Style questions - FRQs - New Syllabus

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Question

Buffelgrass, an invasive grass species in southwestern desert ecosystems, is threatening the saguaro cactus, a keystone species in these ecosystems. Buffelgrass is drought-tolerant and can survive wildfires. However, the dry buffelgrass also acts as fuel for wildfires, causing the fires to be more severe. Older saguaro cacti can survive wildfires; however, many of the young cacti cannot.
Scientists conducted an experiment to determine whether they could control the abundance of the buffelgrass population. The scientists identified several native grass species that, when grown with buffelgrass, might reduce the abundance of buffelgrass. They grew buffelgrass in the presence of several different native grass species in greenhouses, in either nondrought \( \left(\text{watered every } 3 \text{ days}\right) \) or drought \( \left(\text{watered every } 9 \text{ days}\right) \) conditions. After \( 12 \) weeks, they measured the height and dry weight of the buffelgrass in each treatment group.
(A) Describe the effect that removing a keystone species will have on an ecosystem.
(B) Identify a control group the scientists should include in their experiment.
(C) State the null hypothesis of the experiment in which buffelgrass is grown in the presence of native grass species.
(D) Scientists have found that the population growth rates of native grasses are much slower than the population growth rate of buffelgrass following a wildfire. The scientists claim that wildfires will therefore increase the abundance of buffelgrass plants in the ecosystem. Based on the information given, justify the scientists’ claim.

Most-appropriate topic codes (AP Biology \( 2025 \)):

• Topic \( 8.5 \) — Community Ecology: Effects of removing a keystone species on ecosystem structure and stability — part (A)
• Science Practice \( 3 \) — Questions and Methods: Identifying an appropriate control group and stating a null hypothesis — parts (B), (C)
• Topic \( 8.3 \) — Population Ecology: Population growth rates and changes in abundance under different conditions — part (D)
• Topic \( 8.7 \) — Disruptions in Ecosystems: Invasive species and wildfire-driven ecosystem change affecting species abundance — context and part (D)
• Science Practice \( 6 \) — Argumentation: Justifying a scientific claim with evidence from ecological interactions and population dynamics — part (D)
▶️ Answer/Explanation

(A)
Removal of a keystone species reduces biodiversity, diversity, or resilience of the ecosystem.
Removal of a keystone species can cause the ecosystem to collapse.
Summary: because many other organisms depend strongly on a keystone species, removing it can greatly disrupt the whole ecosystem.

(B)
An appropriate control group would be a treatment group with only buffelgrass planted.
A treatment group with no native grass species present is also acceptable.
This control would allow the scientists to compare buffelgrass growth with and without competition from native grasses.

(C)
There will be no difference in the abundance, dry weight, height, or size of buffelgrass when grown alone as compared with when grown with native plants.
There will be no difference in the abundance, dry weight, height, or size of buffelgrass grown in drought and nondrought conditions.
Summary: the null hypothesis states that the native grasses will have no effect on buffelgrass growth.

(D)
With fewer native plant species after a wildfire, buffelgrass will have less competition or no competition for resources.
With fewer native plant species after a wildfire, buffelgrass will have more resources available for growth, development, and reproduction.
Because buffelgrass survives wildfire and grows faster afterward than native grasses, its abundance in the ecosystem is likely to increase.

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