From the beginning, life seems to have been based on the consumption of nutrients. The wide range of different processes for obtaining those nutrients is reflected by the vast biodiversity on Earth. Many organisms have specialized in obtaining nutrients by consuming other organisms through predation. Many parasites prey upon other organisms by an intimate form of predation.
Needless to say, but predation harms the prey and parasitism harms the host. The differences between predation and parasitism are dependent on the situation. Predation can be fatal to animal prey whereas it is usually not fatal to plant prey.
I am not merely playing with words here. The distinction between a predator and a parasite became very real to me when I was working with an undeniable parasite, Paragonimus kellicotti, a lung fluke in mammals. This trematode ingested host blood from its position in its host's lung.
I wondered how its tetrapyrrole metabolism compared to that of the blood-sucking leech, Hirudo medicinalis. I wanted to use the leech as an easier to raise stand-in for the fluke. But is this leech a predator or a parasite? Is the female mosquito a predator or a parasite? Mosquitoes consume a blood meal in just a minute or so. In my lab, leeches fed on guinea pigs for a few hours to over a day. The lung fluke can persist in place for years. Is the difference between a predator and a parasite a mere matter of time? Or is it internal versus external? How about a blow fly maggot eating dead flesh on a living host? Or an ichneumon wasp larva developing in a live caterpillar? What about fleas and ticks?
If it is difficult to make a distinction between predation and parasitism, then perhaps there is no distinction, just a spectrum. If indeed that is the case, then because predation is certainly a property of life, parasitism is also a property of life.