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)

Controlling variables to see how a single factor affects growth is the key idea. To test water as a limiting factor, you set up groups of plants that are identical in every other regard—same species, soil, light, and temperature—and only change how much water they receive. This lets you observe how growth responds to different water levels without other factors muddying the result. Include a control group that gets an optimal amount of water so you have a baseline to compare against. Over a defined period, measure growth (such as height, biomass, or leaf area) at regular intervals. If plants grow more with more water up to a point or stall when water is scarce, you’re seeing the direct impact of water availability on growth in an ecosystem context.

Varying another factor, like light, while keeping water constant would tell you about light’s effect, not water’s. Testing all factors at once without a control makes it impossible to say which factor caused any observed changes. Varying temperature only focuses on temperature, not water.