The species found in a particular place (“species assemblages”) differ from those found in other places, and figuring out why this is so has occupied the minds of ecologists since the mid-20th century. Currently two theories dominate: the niche theory, and the neutral theory. The niche theory holds that species assemblages result from species migrating into a particular place, and then either thriving or leaving based on how good of a match they are to the habitat and other living organisms (the “niche”) in that place. In contrast, neutral theory predictions of species assemblages assume that each species has similar fitness and purely random processes determine which species end up in a particular place. In a new paper, assemblages of small, cryptic fish species in the Indo-Pacific were examined to test whether niche theory or neutral theory best explains patterns in species association. The results strongly support niche theory: species in each broad region contribute to the assemblages, but the distinct microhabitat preferred by individual species has a strong influence on species assemblages. The research is published in the journal Coral Reefs by Gabby Ahmadia, SAFS professor Luke Tornabene, David Smith, and Frank Pezold.