# One Model to Rule Them All?

A common topic on this blog has been how to classify the different types of natural enemies. Where do we draw the line between predators, parasites, micropredators, parasitoids, etc.? What characteristics do we look for to determine which type of enemy we’re looking at? In a previous post, I showed you a classification scheme that used four criteria to divide up the predators, parasites, and parasitoids: (1) “Does the enemy attack more than one victim?” (2) “Does the enemy eliminate victim fitness?” (3) “Does the enemy require the death of the victim?” and (4) “Does the enemy cause intensity-dependent pathology?” In a later post, I talked about a classification scheme that extended this concept from considering only natural enemies to including other types of symbionts, such as mutualists. In that classification scheme, there were only two criteria: (1) the relative duration of the association and (2) the effects of the symbiont (predator, parasite, mutualist, etc.) on the fitness of other partner.

So, how many criteria do we need? To answer this question, we really need a precise way to categorize each type of natural enemy. And nothing gives a precise definition like an elegant mathematical expression! But if you’ve done any mathematical modeling of enemy-victim interactions, you know that there are tons of models out there: multiple predator-prey models, multiple parasite-host models, multiple parasitoid-host models, etc. And it can be tricky to figure out how these models relate to each other.  At least, it used to be tricky! Last week, Lafferty et al. (2015) published a general consumer-resource model that can be simplified to produce any specific enemy-victim model that you want. And they have a neat little program that can do the algebra for you! (At least, Kevin showed that program at ESA 2015. I didn’t see a link to it in the article.)

Now that we can define all of the natural enemy types using a common mathematical model, what criteria do we use to differentiate between the types? The first criterion is whether the enemy can try again if it has a failed attack (“predators”: autotrophs, detrivores, scavengers, predators, social predators, and micropredators) or whether one failed attack results in natural enemy death (“parasites”: parasitoids, parasitic castrators, macroparasites, microparasites, and decomposers. The decomposer thing is blowing my mind.) The second criterion is how many victims each enemy attacks at a given life stage (predators attack many victims, but parasites attack one victim), which is similar to the relative duration of association, and the third criterion is how the enemy affects the victim’s fitness (predators kill their victims, micropredators do not kill their victims). Those second and third criteria are similar to the previous classification scheme used by Lafferty and Kuris (2002), but the first criterion is a new one. And I’m not sure if Lafferty et al. (2015) would argue that we don’t need the intensity-dependent fitness cost criteria, or not. So it looks like we need three or four criteria. Cool stuff!

References:

Lafferty, K.D., G. DeLeo, C.J. Briggs, A.P. Dobson, T. Gross, and A.M. Kuris. 2015. A general consumer-resource population model. Science 349 (6250): 854-857.