the planet is horribly overpopulated.
india and china alone hold over half the world's people. and that was caused by horrible poverty (yes china's economy is improving, but the bulk of them still live terribly)
humanity has long reached the scientific carrying capacity of earth. its only downhill from there.
starvation and disease usually kill off animals that overpopulate an area. and we can already see it happening. though likely not fast enough :(