That biology class you slept through because you were going to be a writer anyway? Yeah…that was a bad decision if your goal was to be a fantasy or science fiction writer. One of the easiest mistakes fantasy and science fiction writers make is having plant and animal life mismatched to their ecological regions.
More often than not, it happens because the writer thinks their new creation would just be cool, and they don’t give a whole lot of thought to how that plant or animal would survive in its environment. Remember my story about setting my flourishing metropolis? People need water to survive. So do a fairly decent number of plants and animals. And those that don’t have made some sort of adaptation to make surviving with little accessible water possible. A prime real-world example of this is the cactus, which soaks up and stores water for drier periods. Can you imagine putting celery, a plant that thrives in moist conditions, in a desert? It would be dead in a matter of days.
This isn’t to say you can’t have a plant or animal in an ecological setting it normally wouldn’t survive in. But you do need to think about how this organism got into that setting, why it decided to adapt to that setting, and how it adapted to that setting. Also, you have to remember that these adaptations don’t often happen in a single generation. They happen gradually over the course of at least a few generations, depending on how major a change it is. It has to make sense within the context of your world’s ecology to make sense to the reader. (This is one of those sticky points that will ruin a reader’s suspension of disbelief.)
As you’re developing the world your story takes place in, remember to think through placing plants and animals. The more logical your decisions are in your own mind (because you really don’t have to show it to readers unless you’re writing hard science fiction or giving behind-the-scenes interviews), the more believable it will be to your reader, and the less often you will shock the reader out of the story.