Environmentally Transient Organisms, or ETOs are animals that travel in between biomes regularly. They can fill a niche in more than one biome , but only when they are in that biome. There are two types of ETO classification: nomadic and migratory.
Nomadic ETOs wander wherever they like in search of resources. These ETOs can fill a niche in all of the environments that they wander through, since there will probably always be a few in any given biome within their range. Many organisms that are large or higher sized are nomadic, because one biome cannot fully support them. It is assumed most players will become nomadic ETOs at some point because of their curiosity. Nomadic ETOs usually cycle in their wanderings, but these cycles are not to be confused with yearly migrations. The wanderings of nomadic AI will most likely be expressed by Levy flights.
Migratory ETOs go from one biome to another on a yearly (or otherwise climatically influenced) basis. They are part of only the biomes at their destinatons, since they do not stop for long in between them. Migrations usually contain many organisms moving together.