Indicator Species are organisms that signal changes in ecosystem health by...

• A. A predator that indicates food chain length
• B. An organism used to monitor water quality exclusively
• C. A plant indicator of soil type only
• D. Organism signaling ecosystem health changes by presence, absence, or abundance

Answer: (D)

Indicator species reveal changes in ecosystem health by their presence, absence, or changing numbers. They’re useful because these organisms respond to shifts in environmental conditions—like pollution, habitat loss, climate, or nutrient levels—and thus provide an early warning about how well an ecosystem is doing. For example, lichens are sensitive to air pollutants, so a decline or disappearance can signal poorer air quality; amphibians and aquatic insects often reflect water quality because they’re sensitive to pollutants and habitat changes. The idea is that the status and abundance of these organisms mirror the broader health of the environment. The other options are narrower or misleading: a predator indicating food chain length doesn’t directly measure overall health; focusing only on water quality or only on soil type misses the broader signaling role indicator species have across ecosystems.