Okay, if you are anything like me, at least part of the reason you love speculative fiction so much, is that it gives you mental access to a new world. This can be to read about, or write about. I know I was trying to make up worlds and the things that inhabit said worlds since I was very young.
Maybe your story is close to modern day, in a recognized city, with no made up creatures. It is possible to write speculative fiction that way. It’s even possible to write good speculative fiction that way. Maybe you have no desire whatsoever to write imaginary lands filled with mythical creatures and plants. In which case, you may find this post boring. Sorry. But hey, you’re here, might as well read it anyway, right? Maybe for a different story, you will want to use this.
If you aren’t setting your story in a recognized area then you have some leeway into the geography you are using. And sometimes even if you are, you still have some room. I have two books set on a fictional island in a real lake outside a real town. That I’ve never been to, which is a bit of a problem, and on my list. At some point. If your story is supposed to be set in New York City, you have a little less wiggle room, but even then, most people take a few liberties with geography.
Now the main problem is that most people don’t care to learn much about the rules of geography. That can lead to stupid mistakes like putting a forest next to a desert with no transition zone. TV Tropes has a trope called Patchwork Map, of people doing things like that. It’s an amateur mistake, that makes the writer look foolish, unless there’s a good excuse for it. I’m sure you aren’t here for a geography lesson, and I’m no expert either. But try to keep in mind a few things.
One, when moving from one type of land to another, there should be a transition or a buffer zone. Trees in a forest start out sparser and further apart, before becoming denser. Deserts become more sand and rock, and less plants. It’s a slow process. Also, unless there is a really good explanation, even a buffer zone isn’t going to explain how a rain forest is next to a desert.
Two, natural boundaries, like rivers, oceans, and mountains, do not form in nice, straight lines. Coasts are crooked, rivers meander, and mountains curve. Why? Because nature very rarely forms in straight lines.
Three, natural boundaries are good ways to move from one type of land to another. Particularly mountain chains. Climate on one side of a mountain chain can be radically different from the other side. I’m told it has to do with rain getting stuck on one side of the mountain.
Okay, that’s geography. Now comes the fun part. Plants and animals to inhabit your world. The first thing you need to ask yourself is what purpose does that plant or animal serve, and why does it manage to thrive in your world. If you believe in evolution, what caused your plant or animal to evolve here, and what niche does it fill. If you believe God, or some other deity, was involved in creation, then for what purpose was this created, and how is it made for its purpose.
What preys on this plant or animal, and what does it prey on? The answer to one of those questions may be nothing. A non carnivorous plant really can’t be said to prey on anything. An apex predator, an animal at the top of the food chain, may not have any other predators. Both answers can’t be nothing, and it is perfectly possible and even likely for your creation to be in the middle of the food chain.
Plants are easy to make up, and unless you go too far out of the normal realm of plant life, most people aren’t even going to blink. You do not need to tell us the plant growth cycle, show us how it photosynthesizes or anything like that. You tell me it’s a shrub with blue-green leaves, and the bark can be boiled to make a healing tea for fevers, fine. As long as you are consistent, I don’t care. You tell me that plant gets up and walks, we may have an issue. I want to know how.
Animals are a little more complicated and for one major reason. The big, dangerous predators are the most fun to make, and the most interesting to throw at your characters. So why is that a problem? Okay, you have the big, awesome Razortooth, that’s a combination of a wolf, a bear, and a lion, how can you go wrong? It’s faster than a horse, and they travel in packs! They climb trees and swim! They are aggressive and have poisonous spikes! Great conflict for your characters.
Well, if it’s fast than a horse, can climb trees and swim, how are your characters supposed to deal when they do run into one? Or more. Can they fight them? Chase them off? What do they eat? A robin eats one or two times it’s body weight each day. Sure, birds eat a lot, but the fact remains that most animals eat a lot. The bigger the animal, the more it will have to eat. Unless you have a lot of smaller prey animals, your Razortooth is going to be in very small numbers. Why does it have poisonous spikes? Those are a defensive tool, an apex predator wouldn’t need them.
Research is your friend here. For one of my stories, I decided to forgo the usual horses as pack animals and transport, and go with what I called Pack Lizards instead. I basically looked up as much as I could about lizards and took the interesting parts. They are about ten feet long, can change colors, can climb cliff faces, give birth to live young, and are capable of giving birth asexually in rare instances. The only thing I made up was the length. Have fun, dream big, but make sure there’s a foundation for your plans.