Every world has an ecology; if you want your fantasy world to be believable, you’ll need to pay attention to how it works: how does each living group fit with all the others and with the world? What might your world design goals be in general?
What’s an Ecology?
Broadly defined, an ecology is the relations of organisms to one another and to their physical surroundings. If the ecology doesn't make sense in your world, you risk breaking immersion for the player (or reader). The more interested you are in making up a world rather than a setting or just an area to play in, the more likely you are to care about consistency and immersion. Perhaps ecology and self-consistency are more important for novels than games, but they are important in games for many people.Fantasy author Glen Cook (The Black Company series) once said he didn’t like maps, because they constrained his authorial freedom of action. And to some extent, creating an ecology means creating consistency in which the game master (or designer or author) cannot easily break their own rules without it being noticeable. This necessarily limits some creative freedom, although I would argue the tradeoffs are worth it for immersion and consistency.
Monster Basics
In world building you can look at this as a matter of survival. If there is a very powerful monster, or numerous and prolific species, or long-lived or aggressive species, how do other species exist, how do they survive contact with those species?An obvious example would be a world with thousands of dragons. Food chains are important. What do dragons eat and how do other creatures survive? Won’t the dragons eat all the other creatures? Relative power, ability to cast spells, ability to rise in levels, all come into play as well. For an example of how this can get rapidly out of control, see Raph Koster's commentary on Ultima Online's resource system-- an ecology that was massively disrupted when players murdered everything in sight.
Don't just throw together a bunch of statistics, try to fit the monster into the world. Monsters can be merely monsters or they can be monstrosities. What is a monstrosity? (see my article, "Make Monsters, Not Monstrosities,” in Dragon Magazine #59) A monstrosity doesn't seem to fit together or make sense, or strikes one as extraordinarily gross. It's a strange combination. I prefer to design monsters that seem more believable (see my Monster Workshop Part 1 and Part 2).
Species Basics
Typically, a fantasy world has more than one intelligent species and often many more. It also has some very powerful monsters, so an immediate question should be “how have humans survived in this environment?” Why haven't they been wiped out? There is evidence for this in our modern world – genetic data suggests that our human ancestors nearly went extinct 900,00 years ago, down to a group of only 1,280 individuals.You can ask the same question about other intelligent humanoids and other species, of course. In my world humans survive because they are more versatile than other creatures, and reproduce fairly fast. They depend on magic use and also on their ability to rise in level, which other species can rarely do. Further, few species can make magic items or use magic items. For an example of how this might work between species, see the research around how Neanderthals went extinct.
Differentiation
In your world design try to avoid duplication. Make sure to practice differentiation deliberately as much as possible; these are also game design goals of course, not just world design.In game design, whatever you think about differences between species, it's important to avoid treating them as a monoculture. This is bad for fiction and is a common problem with sci-fi settings, in which entire worlds are reduced to one population with one culture; our own planet is host to a dizzying variety. This is also bad for game design, where you want differentiation to provide variety, and if differentiation is not there you probably should eliminate the duplication to simplify the game. In some rulesets or campaigns humans are the only intelligent humanoid species. If all the humanoid species are practically identical, why have anything but humans in the game?
In the end, it’s really about what your design goals are for your world. If you use your world as a playground for ideas (and in some cases for new game masters, their games lack internal consistency to start but form more solid ecologies as they play in their world), then have at it. Many people have enjoyed RPGs that make little common sense, in favor of just having a good time. But if you want your world to have continuity, an ecology that makes sense will pay dividends in believability and consistency later.
Your Turn: How do you plan out ecologies for your world?
