Design a simple controlled experiment to test how a limiting factor (for example, water) affects plant growth in an ecosystem context.

Design a simple controlled experiment to test how a limiting factor (for example, water) affects plant growth in an ecosystem context.

• A. Vary water while keeping light, soil, and temperature constant; measure growth over time; include a control with optimal water; compare results.
• B. Vary light while keeping water constant.
• C. Test all factors at once without a control.
• D. Vary temperature only.

Answer: (D)

To understand how a limiting factor like water affects plant growth, the key is to manipulate that factor while keeping everything else constant. By varying the amount of water across several treatments and holding light, soil, nutrients, temperature, and other conditions steady, you can observe how growth changes in response to water alone. Monitoring growth over time—things like height, biomass, or leaf area—lets you see a dose–response pattern: as water availability changes, growth changes accordingly. Including a control group that receives an optimal watering amount provides a baseline to compare against and helps show whether water is truly limiting growth. This setup isolates the effect of water, which is essential for identifying how a limiting factor operates in an ecosystem.

Other approaches don’t isolate the factor of interest. Varying light while keeping water constant tests light effects, not water’s. Testing all factors at once without a control mixes up which factor caused any change. Varying temperature only focuses on heat effects, not on water as the limiting resource.