Accurate knowledge of the number and different types of species in a given community or ecosystem is the basis for understanding how a system functions, and thus how the removal or addition of species may alter that functioning. Knowing which species are present is requisite for understanding the ecological roles of the critical community members---what they eat, who eats them, and how they alter the community in which they live. Not knowing how many species are in a community sorely limits the ability to predict the fate of that community under different kinds of anthropogenic stresses. Not knowing the names of the species in a community severely limits the ability to compare different systems and to understand the biology and ecology of such organisms by comparing them to their better-known relatives.
Species diversity is sorely undersampled, underdescribed and, in some cases, is undergoing dramatic revision in the marine realm, as indicated in the examples given below.