Nature recycles crap to make substances divine again through a process called evolution. Zombies don't get to stay around very long, not without constant human intervention to go against Nature's evolutionary intent. Zombies would not survive in nature, but they could with the help of mankind, such as gene therapy "vaccines" and booster shots every 6 months to force their genetics against the will of Nature. The only Zombie that Nature would allow is one which is symbiotic with its environment.