Change through time. There are more bacteria on earth than any other organism. There are more bacteria in one person s gut than there are humans on earth.
Biotic environments change Organisms change environments, too.
1.8 BYA photosynthetic bacteria made the oxygen that poisoned anaerobic bacteria, and cleared the way for eukaryotes. 450 MYA photosynthetic eukaryotes made oxygen that formed ozone layer and cleared the way for terrestrial organisms. Reasons poorly understood. Why are species so well adapted for survival and reproduction in their own particular environments. Some variations within and between species are genetically based.