You're right, food "chains" are actually food webs, with predator/prey interactions amongst and between levels. Apex predators do exist in natural systems though. In India and parts of Asia for example, tigers are (or were) at the top of the food web. In the oceans (depending on the geographical location) either orca or great white sharks occupy the apex predator position.
EDIT: As Skookum mentioned, introduction of an exotic predator into an ecosystem can "create" an apex predator. Here in New Zealand for example we have no native mammalian predators. The introduction of stoats, cats and dogs immediately placed those species at the top of local food webs.
EDIT again: looks like there's a bit of confusion around the definition of "predator". A predator is an animal which hunts and kills another animal for food. The operative words here are "kill" and "food". Viruses and bacteria are pathogens, not predators: these use hosts to replicate their genetic material, not for food (and in most cases don't kill the host). Parasites (leeches, mosquitos etc) do feed on their hosts but ideally don't kill them. The example of the dolphins torpedoing the shark is defensive behaviour to protect the pod: they had no intention of eating the shark and were most likely attempting to drive it off rather than specifically trying to kill it. That's not predation.
· 8 years ago