How can animals act as natural indicators for the acidity of water in lakes and rivers?
Different aquatic animals have different tolerance to pH changes in the water they live in. For example, a wood frog will die if the pH of the water falls below 4 while a clam will die if the pH of the water falls below 6. So can anyone please explain to me how these animals can act as natural indicators for the acidity of water in lakes and rivers?
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Your explanation nearly answers the question for you. If the wood frog dies below a 4 ph (acid conditions), and all the wood frogs are dead, then the pH is probably below 4. If the clam dies below a pH of 6 (neutral is 7 so this isn't very acid at all) then you'd expect to find no clams either. However, if there were clams but no wood frogs (and you've seen wood frogs there before) you would predict that the pH was between 4 and 6. Each organism has a certain "range of tolerance" where they live quite contently, but at either end of the bell shaped curve they are stressed and eventually die. The pH of a lake is one of the abiotic limiting factors for both the wood frog and the clam. On the picture in the link you could substitute pH for precipitation and get the same curve for each of your organisms. There is probably an upper level of pH that is tolerable by each organism as well which might be used as a predictor of the upper limits of the pH ranges for each one.
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