Are Fish Communities Randomly Composed or Ecologically Structured?

Ecologists often analyze communities in order to determine the importance of biotic, abiotic and/or spatial factors in structuring species composition. An implicit assumption in these analyses is that the communities differ from random.  Several studies have explicitly tested and shown fish communities to be non-randomly structured, whereas other studies have shown that they do not meet this assumption. I examine this question of whether fish communities are non-random in their species composition using many datasets drawn from a range of different types of aquatic ecosystems (e.g. lakes vs streams, temperate vs tropical).  My results show a strong departure from randomness across this range of systems, although different types of systems differ in their degree of non-random structure.  Most systems also demonstrate a strong degree of nestedness in their species composition.  I show how underlying characteristics of fish communities have led to discrepancies in the literature about whether fish communities are randomly structured or not.