[This is the next portion of this series on managing the complexity of different world elements.]
This indicates the number, variety, and quality of the “species” in your story. One presumes a baseline of humans and Earth-found animals/plants. If on another world entirely, that may be different. The primary things to look for include—number of “unusual” species, number of “sentient” species, number of “humanoid” species, and details of each species. “Unusual” is anything that is not found in our reality and thus must be explained (or not, if part of the background of an unusual world, but it adds to background complexity). “Sentient” is anything that is intelligent and aware enough to participate in the society depicted, even as a marginalized figure (compared to species which may be different but are treated as animals, pets, familiars, food, etc.). “Humanoid” is anything which resembles a human more than not, and which is portrayed by a human actor, even with makeup.
Low complexity species systems are usually a single species in an otherwise realistic world. It may even be a single being from an otherwise extinct or non-existent species (such as a created being). But a single species, even if unusual, sentient, and humanoid, is easy enough to convey to the reader. Occasionally, there maybe more than one species if the other species are small in number, similarly unique, or otherwise marginalized by the story in such a way that they have little impact and thus do not require a lot of exposition.
Medium complexity species systems come in a few varieties. The first is a society dominated by a single sentient/humanoid species but surrounded by numerous other species, which are marginal enough in society as to not “clutter” the main characters of the story. The second is a society concerning a small number of sentient/humanoid species, but without a lot of ‘secondary’ species, and usually these species are “unusual” in only specific, mostly simple ways.
High complexity species systems are dominated by a larger number of unusual species, with several varieties of sentient and/or humanoid species. This creates the scenario in which a humanoid character presented to the audience could be any number of species, each with their own high complexity of details and traits. It also usually serves to complicate the society surrounding this variety of sentient species (even if not all humanoid, thus allowing for simpler audience identification of species), which we will get to in the “Society” section. High complexity systems can be modulated if a single species dominates the world in such a way that nearly all the major characters are from the species and any new character will be presumed to be this species unless shown otherwise, and those cases are rare.
The line between “medium” and “high” complexity can be difficult to sort out. Usually, the “society” element will push a species system towards one or the other, depending on how complex the society is and how it treats the different species. For example, in a simpler society, they all live the same way and thus their differences are not emphasized and remarked upon enough to require constant detail.
This is the only category which may be completely non-existent in a fantasy/science story, because the species may just be humans and Earth species, even in space.