Difference Between Parasitism And Predation

Despite about ane year's worth of posts about parasite environmental, this blog has never defined the term 'parasite.' D'oh! You might think, "Pft, the definition is obvious!" But actually, it isn't, and it isn't without controversy, either. I'm going to talk most a bunch of types of natural enemy, and and then I'll nowadays a really good dichotomous key at the end.

Predator:

Let's offset with predators. Like parasites, predators are organisms that acquire energy by taking that free energy from other organisms. Therefore, we have a human relationship that positively affects one organism (the predator) and negatively affects the other organism (the casualty). Predators have these of import characteristics:

ane) One predator eats multiple casualty during the predator's lifetime.

2) Predators tend to be bigger than their prey.

three) Predators tend to kill their prey.

Micropredator:

You'll observe that I said that predators "tend to" exist bigger than their prey and "tend to" impale their prey. They don't e'er! A very good example of this is a vampire bat that takes blood meals from cows. A single vampire bat will take blood meals from multiple cows during its lifetime. It is eating multiple prey, therefore, it is a predator. But it doesn't kill the cows, and it isn't bigger than them. It'due south a micropredator.

Parasite:

Parasites are different from predators because parasites merely take resource from i host, whereas predators eat many prey. A good example of this is the trematode parasite Schistosoma mansoni. An adult schistosome parasite lives within of only one human being host. It is never going to clamber out and go infect a unlike human.

You might be thinking, "Waaaaait… Schistosoma mansoni has a complex life cycle! Information technology infects humans AND snails! That's two hosts!" Yep. But the rule is that parasites only infect 1 host during each phase of the life cycle. One homo. Ane snail.

Here are another common characteristics of parasites:

ane) They are smaller than their hosts.

2) They don't usually impale their hosts.*

Ok, and so, the killing bit is disruptive and wishywashy. I'll come back to it below.

Parasitoid:

Similar a parasite, a parasitoid infects simply one host per life phase. But parasitoids ever kill their hosts.

Parasite vs. Parasitoid:

So, what's the difference betwixt a parasite and a parasitoid? If you're nigh to have an exam or something and you desire a quick answer, say that parasitoids e'er impale their hosts and parasites don't normally kill their hosts. Y'all'll find that in many introductory environmental textbooks.

In practice, we don't really employ that definition. The term parasitoid is unremarkably applied to certain insects that accept gratis-living developed stages that lay eggs inside a host, and the eggs proceed to parasitize and somewhen impale the host.

In that location are many "parasites" that always kill their hosts, and we nevertheless call them parasites and not parasitoids. Why, Scientists? Why do you do this thing? Well, it just doesn't make sense to have a rule that says that parasites don't kill their hosts. For example, if a parasite (say an acanthocephalan) in an intermediate host (a pillbug) makes the host more than likely to become eaten by the next host (a bird) in the life cycle, and so the parasite is often the crusade of the host'due south decease. Those kinds of parasites are called trophically-transmitted parasites.

A Very Nice Dichotomous Guide:

Lafferty and Kuris (2002) came upward with a really prissy dichotomous key for classifying natural enemies. They used 4 dichotomies, but I'm only going to use the offset three:

1. "Does the enemy assault more than one victim?"

2. "Does the enemy eliminate victim fitness?" ('Eliminating fitness' could be killing the victim or sterilizing the victim so that information technology cannot reproduce.)

3. "Does the enemy require the decease of the victim?"

Definition of Parasite Diagram

Figure adapted from Lafferty and Kuris (2002) – link below.

You might remember from one of my previous posts that nosotros tend to split parasites into microparasites and macroparasites. As I described in that post, for microparasites, we care most presence/absence of infection, and for macroparasites, we intendance about intensity of infection. The fourth dichotomy used by Lafferty and Kuris (2002) is "does the enemy cause intensity-dependent pathology?" They include the fourth dichotomy in their effigy. It's actually useful, just I didn't include it hither to avoid confusion. Click the PDF link below to see their version.

Reference:

Lafferty, K.D., and A.M. Kuris. 2002. Trophic strategies, animal diverseness and body size. TREE 17(eleven): 507-513. (Straight link to PDF download)