Have you ever seen Star Wars?
My favorite scene in the entire "trilogy" takes place in a Mos Eisley cantina in a New Hope. It took me a few years to understand why and that particular reason is what today's lesson be about.
Every universe needs what I can "inactives." These consist of living things that are present in the narrative but don't have any speaking lines. The cantina scene in Mos Eisley demonstrates the sheer overwhelming diversity of the locale, filled with creatures from other worlds. This conveys to the reader specifically, "look at thus world, but also see how it is just one of many worlds spanning the cosmos." The characters of your story in this context are no longer players in a drama without consequence. People, things, civilizations are on the line.
Plants are the first element of supplying the world with indigenous life. Because plant life in any ecosystem is the foundation of the entire chain of being, it is important to get plants dialed in and completed in concept. Little details like a character picking up a flower, or seeing a tree sway in the wind helps ground the character in a world. Also show how that particular character effects the world. If a ship crash-lands on an alien planet then describe the wood splintered trees in the wake of the crash. Another thing too is to have descriptions of how the plants interact with local wildlife. Obviously all stories are different. But this concept is as true as a story set on another planet as it is a story set in New York City. The trees in Central Park or the hanging gardens off the fire escapes are good details to consider to help bring the narrative alive.
Creatures and Animals are not the same thing. So if you were wondering in the title was a typo it wasn't! The distinction lies between one being sentient and the other not. Stories need marketplaces, civic buildings, roads, and theaters to convince the reader that the characters or protagonist aren't the only inhabitants of the city. Non-intractable characters contextualize the character into a society of individuals. Animals, likewise elevate the characters of your book into special positions. Animals don't have responsibilities or duties or stakes in the plot, but because they are there they enhance the importance of the main character and his/her actions that move the story along.
I hope to expand on the lesson into other categories soon. For now though take into consideration these hints and tips, and I promise that you will see an improvement in your narrative.