I'm wondering if predation is an evolutionarily risky strategy for a species? Predators get a bonus because their prey does a lot of work collecting calories from the environment, the predator just takes it from them. This would make predation an advantageous strategy as long as there's prey around (obviously), but leaves the predator unable to harvest energy from the environment in the absence of prey (to the extent that they are specialized for hunting).
What I'm getting at is that being a predator complicates the value-chain/food cycle, it makes the success of the predator species dependent on the success of the prey species, and thus more vulnerable to collapse.
No surprises here. It just occurred to me that it was interesting that genes would take the risker evolutionary path on occasion... I suppose that's another way of explaining the relatively greater number of herbivore species.
Actually, come to think of it, are there a greater number of herbivore species? I'm quite sure that the actual number of herbivore animals on the planet must always be greater than the number of predetors, but could it be that there are more predetory species? That should be a relatively easy question to answer.