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I think there are a few more theories on this - but I can't quite remember them right now.. Anyway one major reason is "redundancy". Consider two different ecosystems, one with a few species, each species has a different ecosystem function. What do you think would happen if one of those species died (perhaps a disease wipes out an entire population)? Then think of another population that has many species so it has some have a functional overlap, what do you think would happen if you lost one species in this ecosystem?
I'm thinking that a disease might have a more devastating effect on an inbred population than one which is more diverse. An example of this might be zoo-bred cheetahs, which are very susceptible to FeLV (feline leukaemia virus). This might be because the cheetahs came from a restrcited gene pool (ie only a few individuals).
That's true - the more genetic diversity within populations the more stable it will be too because it has more variations and so it is more likely that not all individuals will be effected by a change - like disease.