Back in the prehistoric days of computer games, I recall playing one that simulated the competition for survival among unicellular organisms. Something like that. I thought of it this morning when I was walking the dog, as I noticed that you can see similar patterns on lawns that are beginning to return to, ahem, a natural state. All the best weeds spread via rhizomes or stolons or what have you, so they tend to try and expand from a few well-established bases. Where two weed patches come in contact with one another, there is naturally a struggle to see which one will prevail, nature being "red in tooth and claw" and all.
And what of the poor grass? Ironically, it seems that it gets along better if the lawn hasn't been mowed for some time: if left alone, domestic grass reaches about one foot in height, allowing it to easily overtop competition from things like violets and most types of clover. When mowing is the only care a lawn sees, grass is cruelly overshadowed by faster-growing plants and those that need less water to thrive. That said, when there is sufficient water available even the weediest lawn can look remarkably decent with a good even mowing—or, even better, with the dedicated chomping of a flock of sheep or goats. Maybe we should get some!