You are conducting a long term study of ecosystem functioning in a freshwater lake. You and your field assistants gather data for many biotic and abiotic factors, including species diversity, water and benthic quality, and turbidity. Over the ten years of your study, you find that the abundance of water lilies is correlated with several other factors. When water toxicity and turbidity are high and water pH is abnormally low, water lilies quickly become scarce, followed by periods where fish, amphibians, and water birds are also scarce or absent. In years water nutrient levels are very high, water lilies quickly become abundant, followed by years where wading birds are scarce. From these data, you conclude that water lilies are
A. an indicator species.
B. an endemic species.
C. an umbrella species.
D. a keystone species.
E. a flagship species.
Answer: A
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