Is it plants in your yard? Is it barnacles on a rocky sea shore? Is it a forest floor?
It doesn't matter what it is, as long as you're picturing it! (Mine's a prairie.)
Are you picturing something?
Okay, now the question is: Why are each of those particular species there?
Some of the earliest ecologists (e.g. Frederick Clemens) nearly a century ago thought it likely that there was one proper community for any given place. The climax community. Species live where they are a part of the singular perfect combination of species for the conditions in that place.
This idea wasn't popular long before it was challenged by a new idea: that random chance also pays a major role in determining which species actually end up where. Ecologists have since found over and over again that the arrival order of species to a place can be critical in determining which species will eventually live there. Your climax community might be "forest" or "prairie," but that won't tell you which species are there.
One of the earliest experiments that demonstrated the importance of arrival order was conducted by [*edited* J.P. Sutherland in 1974] with barnacles. The experiment was terribly simple—he submerged tiles into the near-shore ocean and watched to see what organisms showed up. He found that whichever barnacles happened to show up first were the ones that persisted on the tiles. This had nothing to do with their competitive ability, or the tile, or anything like that—it was just the random chance of which polyps floated by at the particular moment which gave them first dibs. All these barnacles have to do is take up space, and the space remains theirs.
This first come, first serve scenario is referred to as a “priority effect.” But it’s not the only way early arrivers can determine which species can join a community (or not) later. The earliest species can also change the environment to make it more, or less, favorable for other species.
On a bare rock, lichens are the only species that can colonize initially, since other species (true plants) need some sort of substrate (dirt) to root into. The lichens break down the rock to make that substrate which the plants can later live on.
Likewise, certain plant species might be able to survive in soil that has very little nutrients, because they are able to make their own (e.g. N-fixing legumes). Once those species have added nutrients to the soil, other species that can’t make their own nutrients can now move in. This is called facilitation—when the earlier species make conditions more favorable for the later species.
Of course, organisms don’t always play nice with other organisms. Like barnacles, plants can also take up physical space that doesn’t give other, late-to-the-game plants a chance. Even when space isn’t limited, plants can use up resources like water, sunlight, or nutrients so much that it prevents other plants from living nearby. Finally, some plants take an extreme route, exuding chemicals that actually inhibit the growth or survival of other species (called allelopathy).
These are all important considerations for ecological restoration, because we’re trying to dictate which species establish first. We get rid of the bad guys (often with herbicide) and try to get the good guys established before more bad guys show up. This doesn’t always work out as we plan… but we’re working on it!
Next time you're out and about in nature, ask yourself— why are these organisms here, in this spot? And think about it—maybe they just got there first.