(I'm off to London for the weekend so will be scarce around here until Tuesday.)
Parasites. By some estimates, up to 75% of identified species are actually parasites of some kind, feeding directly on other organisms. Predators also feed on other organisms. The difference is that predators kill then eat their targets; parasites may damage their host but don't usually kill them directly. There's a sliding scale of lethality. Outright lethal parasitism such as Ophiocordyceps unilateralis or parasitoid wasps do indeed kill their hosts. Others may not kill their hosts but prevent them from reproducing in order to divert the host organism's metabolic surplus towards the castrating parasite. But successful parasites don't kill their hosts, they may even help them: look at the role human gut bacteria play in fermenting undigested carbohydrates into more easily absorbed forms. The most successful parasites become symbiotes—in return for nutrients and shelter they contribute to the host organism's survival. By far the most successful are the endocellular symbionts such as mitochondria, without which complex eukaryotic organisms (from amoebae to human beings) wouldn't exist. It's parasites all the way down, too: some parasites are themselves parasitized by so-called hyperparasites.
Anyway, my starter question is this: what is your favourite parasite? (And why?)
ADMINISTRATIVE NOTE: Ha ha, very funny, no: congressmen and estate agents are not parasitic species, whatever one may think of their niche. I'm deleting all suggestions of human beings. Stick to wildlife. And hey, why no plants yet?