Not sure I agree in nature it is the rule to have many species. Wherever I have observed a multitude of species it has been through man's intervention whether intentional or not. Canals connecting waterways that were not connected before, creations of reservoirs out of streams and rivers, indescriminant planting of fish, introduciton of species by ballast water, by bait buckets etc.
Many virgin fisheries in the far north (not many left) have very few species. I was fishing Devil's lake in North Dakota a few years back and was agast that the lake had no largemouth or smallmouth bass. Bait restrictions was strictly enforced. The only species in this vast lake were yellow perch, walleye, white bass, northern pike and crappie. The only minnows allowed for bait were fatheads.
