Here's an example of how much a species matters. On the left is an aerial photo of a productive lake taken when the abundance of Daphnia , a 2mm-long, planktonic crustacean was very low. On the right is the same lake when the abundance of Daphnia was very high.

As you can imagine, ecosystem processes were very different in the lake when these two photos were taken.

As far an anybody knows, Daphnia is the only planktonic crustacean that can cause such a dramatic change in lake attributes. The data appear in:

Sarnelle, O. 1993. Herbivore effects on phytoplankton succession in a eutrophic lake. Ecological Monographs 63:129-149. 

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