Please excuse the 2 cents from me with no scientific background, but a question popped up to me, so I'm asking it.
If it's a principle of some sort that life tends to fill all the niches available to it, then isn't there a certain tendency toward complexity. Yes, microbes outnumber us significantly, and if mitochondria is a "species," as Lyn Margulis suggests, then very simple life really outnumbers us, but there are niches for complex life that bacteria and other single-celled forms of life will never be able to fill. So there would be a "tendency" towards complex life, just because there are niches they can fill, right?
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