Which species is commonly used as an indicator of environmental health, particularly in relation to disease presence?

• A. Tigers
• B. Frogs
• C. Eels
• D. Bacteria

Answer: (B)

Indicator species reveal how healthy an environment is by responding to changes in pollution, habitat quality, and disease. Frogs are especially good at this because they spend life stages in both water and on land, and their skin is highly permeable. This makes them very sensitive to contaminants, pesticides, pH shifts, and other stressors that reflect the condition of water and surrounding habitats. When ecosystems are stressed, frog populations often decline, and disease outbreaks such as chytridiomycosis can spread more readily, linking disease presence to environmental health. So their health and numbers act as an early warning of broader ecological problems that could affect many species, including humans.

Other options don’t fit as cleanly. Tigers indicate habitat fragmentation and prey availability but not disease presence as a primary signal. Eels can reflect river health in some contexts but aren’t the classic disease-related indicator. Bacteria are microbes and, while they can signal pollution, they aren’t a wildlife indicator species in the same ecological sense used for assessing environmental health.