Step one on the race to my qualifying exams: Know your definitions!
The first paper on my reading list is Trophic strategies, animal diversity and body size by Kevin Lafferty and Armand Kuris from 2002. This paper creates a framework for categorizing “natural enemies” (organisms that obtain their energy by taking it from other organisms). The categories that first come to mind are predators and parasites, but many organisms do not neatly fit into these groupings.
For example, how do you categorize a lamprey? These primitive animals have mouths like suction cups, which contains many sharp teeth. They attach to a fish, bore through its skin, and take a meal of blood and body fluids. They don’t qualify as a typical predator because the taking of this blood meal usually does not kill the fish. They also don’t qualify as a typical parasite because they can attack many fishes throughout their lifetimes, while parasites typically have only one host. So what are they?
Lafferty and Kuris (2002) propose a useful system for categorizing natural enemies based on a set of 4 dichotomous questions. The first question divides natural enemies into the broad categories of predator and parasite while the subsequent questions provide greater resolution.
Here are the dichotomous questions that they posed and the categories that arise as answers:
1) Does the natural enemy attack one or many individuals during its lifetime?
Predators typically attack more than one prey individual over the course of their lives.
Parasites typically attack only one host individual over the course of their lives.
OK, nothing surprising here. Next question.
2) Does attack by this natural enemy eliminate the fitness of the attacked? This question creates categories which will group some of the more ambiguous natural enemies, such as the lamprey mentioned above. It is essentially asking if the natural enemy either kills its host/prey or if eliminates the reproductive potential of the attacked individual. This distinction leaves us with predation, typical parasitism and these two additional categories:
Micropredators: These natural enemies attack many individuals throughout the course of their lifetimes, but do not kill the organisms they attack (i.e., they do not eliminate the fitness of their “prey”). This category includes mosquitos and lampreys.
Facultative micropredators: Some natural enemies will behave as micropredators towards certain prey and as true predators towards others. For example, leeches suck blood (behaving as micropredators) and will also consume entire invertebrates (behaving as predators). Other examples of facultative micropredators listed by Lafferty and Kuris include cookie-cutter sharks, vampire bats, and many herbivores (think of plants as prey).
Parasitoids and parasitic castrators: Parasitoids eliminate their host’s fitness by killing it while parasitic castrators eliminate their host’s fitness by ensuring that they’ll never breed. The next dichotomy will split these two groups, so I’ll discuss them further below.
3) Is it necessary to the natural enemy that the victim die? This is a redundant distinction when thinking about predators, but it is useful for classifying parasites.
Parasitoids: These parasites live in their host for a period of time and kill the host when they are done with it. Parasitoids are typically an insect species that develops inside of another insect host. When the parasitic insect has completed development inside of its host and is ready to be free-living, it bursts out of its host which subsequently kills it. This is an economically important classification of parasites because they are used to biologically control insect pests. For your viewing pleasure, here is a cool video about parasitoids:
Parasitic castrators: These parasites prevent their host from reproducing. For example, barnacles of the genus Sacculina castrate their crab hosts which then care for the parasite’s eggs as if they were their own.
Trophically transmitted parasites (TTP’s): TTP’s have life cycles that include species from multiple trophic levels. The parasite does not directly kill its host, but its host must be consumed by the appropriate predator (and therefore die) in order for the parasite to complete its life cycle. For example, the trematode Euhaplorchis californiensis requires California horn snails, California killifish and predatory birds to complete its life cycle. Thought it does not directly kill California killifish, it requires that the fish be consumed by a predatory bird in order to make it to its definitive host. The parasite is able to manipulate the killifish’s behavior to increase the likelihood that the killifish is consumed by a predatory bird, but it doesn’t kill the killifish directly (here is a previous post on this host/parasite system).
4) Does the intensity of the natural enemies’ attack have an increasing impact on the victim’s fitness? For example, the intensity of an infection by a parasitic castrator does not affect the degree to which the host is castrated. The host is either castrated or not - there aren’t varying degrees of castration. Determining if the natural enemy is intensity independent (like parasitic castrators) or intensity dependent subdivides the following categories:
Pathogens and typical parasites (attack one victim and do not kill or castrate their host)
Pathogen: Their effects are intensity independent. Consider, for example, the virus that causes the flu. It only takes one virus particle to get into your body and multiply to give you the flu. Whether you initially come into contact with 1 particle or 10 particles, the result is the same in that you get the flu either way.
Typical parasite: The effects of typical parasites on their hosts increase as the number of parasites increase. Viruses typically produce offspring that attack the current host, while parasitic worms, for example, often produce offspring that leave their “parents’” host in search of new hosts. This means that an increase in the number of reproducing adults is what leads to increased pathology, making the effects of typical parasites intensity dependent.
Trophically transmitted parasites and pathogens (attack one victim and eventually require the death of their host for transmission to their next host)
Trophically transmitted typical parasite: As described earlier, these parasites infect one host and require their host’s death at the hands of the predator that is the next host in the parasite’s life cycle. Their effects are intensity dependent. For example, the more Euhaplorchis californiensis that infect a California killifish the more successful these parasites are at getting the host to behave conspicuously. Conspicuous behavior attracts the parasite’s definitive host, which is predatory birds (Lafferty and Morris 1996).
Trophically transmitted pathogen: These are pathogens that infect one host and require the host’s death in order to be transmitted to the next trophic level. Their effects are intensity independent for the same reason described for typical pathogens. Trophically transmitted pathogens are less common than trophically transmitted parasites (and quite honestly I can’t come up with an example of one).
Parasitoids and trophically transmitted castrators (attack one victim and either castrate or directly kill their host)
Trophically transmitted parasitic castrators: These parasites castrate their hosts and are intensity-dependent because a more intense infection results in more efficient modification of the host’s behavior in order to increase transmission to the next trophic level (i.e., in order to get the host eaten by a predator that is the parasite’s next host).
Parasitoids: Because the final effect will always be the host’s death, their effect is intensity-independent.
Social and solitary predators (attack more than one individual and kill it)
Social predators: If more than one predator is required to take down a particular prey item, then social predators are intensity dependent. For example, one lion may not pose a threat to a large elephant, but an entire pride of lions would have a good chance of killing it.
Facultative social predators: Most social predators are facultative social predators, meaning that they are capable of killing some prey while hunting alone and also of hunting socially to take down larger prey.
Solitary predators: Solitary predators are intensity independent.
OK, now that I’m done with that long list of definitions and classifications, it’s important to step back and ask why this is important. Is this all just an exercise in semantics or are these divisions actually useful in some way?
Ecologists (and scientists in general) spend a lot of time arguing about definitions. The primary reason that we do this is to avoid confusion. These divisions also allow us to make more accurate and useful ecological models. Typical parasites that are transmitted between members of the same species will have very different ecological consequences than trophically transmitted parasites that are transferred up the food chain. Being more specific also allows us to be clear about which groups of organisms are appropriate to apply the results of theoretical models.
Well, that was the first half of that paper.