low plant diversity
What? A normal yard.
Where? This is my neighbor's yard, across the street from me in the seward neighborhood of Minneapolis.
So? There is only one type of tree growing, and some grass. The grass doesn't look that great and there are some dirt patches. There are probably a few types of weeds growing. My guess is that the plant diversity here is low because human activities have made the soil inhospitable to a lot of plants. Also, any plants that would randomly grow in a yard are considered weeds and are either sprayed with pesticides or pulled out of the ground. If you're going to promote plant diversity in a city yard, you have to put a lot of effort into it.