http://www.physorg.com/news2552.html
It was stated that the variety of vegetation varies directly as to the affluence of the area, but no reason was identified. I would like to suggest a reason.
There are people who are fascinated by succulent plants, as my wife and I are. We can afford to and do visit specialist nurseries to increase our collection, which comprises 100 or more species in our yard and another 1000 or so in pots.
In addition, landscapers will visit affluent neighborhoods offering large specimen plants. Our neighborhood has many yards with such large plants.
Thus do the variety of plants increase in our civic ecology.
Our yard is also visited by the pink-faced lovebirds which are native to Namibia.