Thanks for the comprehensive reply, red51 .
My view was set quite high, and I'm culling the critter population as well as the plant population. The islands were a bit overloaded...
I thought Snake Island would be an interesting theme, and provide nice places to hide my chests of gold. So I added snakes to the islands. To feed the snakes I added lots of rats. Wild boars had to be added to eat the snakes. Thick foliage was added to hide all that, along with bandits to eat the boars. By then the islands were rather crowded.