The Balance between Predators and Prey

Transcript from a lecture series by Professor Stephen Nowicki, Duke University

The balance of life between predators and their prey is complicated and fascinating. Read about two classic experiments on this interaction performed by Josh Van Buskirk.

dragonfly nymphs are voracious predators

You might imagine that increased mortality due to the presence of a predator might be so strong that it would lead to the complete elimination of a prey population in some local areas. The interaction between a prey and a predator would affect not only the abundance of the prey species but also the distribution of the prey species—whether the prey species could exist someplace or not because of the presence of a predator.

I should point out that we would expect this kind of effect to occur more frequently in communities where predators are generalists, as opposed to specialists. By “specialists” I mean a case where a predator or a parasite really relies on only a single kind of food resource, perhaps only a single species.

Commonly, predators are generalists who consume a variety of different kinds of species—although, generally, they will consume whatever is most available to them in the environment. It is here where the interaction between a predator and its prey may affect the distribution of prey species by causing the prey to go locally extinct.

Let me make this point clear by giving you an example: work done on populations of frogs by an ecologist named Josh Van Buskirk.

Tadpoles and Dragonfly Nymphs

As you probably know, frogs breed in water. Their juvenile stage—what we call “tadpoles”—must live in the water, so frogs lay their eggs in the water and tadpoles will spend some time in a pond or a lake or another body of water before they mature, turn into adult frogs, and then move out of the water.

photo of Western Chorus Frogs, the subject of a predator study by Van Buskirk
Western Chorus Frogs

Van Buskirk was working on a species of frog called the western chorus frog. This type of frog actually breeds throughout southeast Canada and down through the midwestern United States. Specifically, Van Buskirk was looking at patterns of the abundance and distribution of chorus frogs on islands in Lake Superior.

Van Buskirk stumbled upon an interesting pattern: He found that tadpoles of these frogs would be found only in a subset of the ponds occurring on these islands. But all of the ponds, at least at first impression, seemed identical to Van Buskirk. So the question was, “Why are frogs found in one pond and not another?”

photo of a Nymph of dragonfly under water - the suspected predator in Van Buskirk's study
Nymph of dragonfly under water

In other words, what Van Buskirk then became interested in asking was, “What is determining the distribution of this species of frog in these pond communities?” A closer look showed that the ponds without frogs also happened to include populations of dragonfly nymphs. Dragonflies lay their eggs in the water as well, and the dragonfly nymphs themselves are voracious, generalist predators.

What Van Buskirk found was that if dragonfly nymphs were present in a pond, the frogs were absent.

So what Van Buskirk found was that if dragonfly nymphs were present in a pond, the frogs were absent. This suggested that the presence of the predator—the dragonfly nymph—restricted the distribution of the prey species. In other words, the dragonfly’s distribution was driving the tadpoles’ distribution because of a predator-prey interaction. This pattern suggested a specific process—that the dragonfly nymphs was too strong a predator for the tadpoles. But this was a process that had to be tested experimentally.

This was exactly what Van Buskirk did.

Manipulating Pond Populations

Van Buskirk set up a classic experiment by using two ponds, one which he left as a control and the other which he manipulated.

What Van Buskirk did was manipulate the presence or absence of one or the other species and see how that affected the distribution of the other. Van Buskirk did two experiments, really.

In the first experiment, Van Buskirk selected two sets of ponds, both of which contained tadpoles but not any dragonfly nymphs. One of these sets he left unmanipulated as a control. In an identical set of ponds, starting out with tadpoles, Van Buskirk added the density of dragonfly nymphs that you would find in a normal pond. So what happened?

..in the pond to which he had added the dragonfly nymphs, within a couple of weeks all the tadpoles were gone.

In the ponds where Van Buskirk started out with tadpoles and left it unmanipulated, he found that some of the tadpoles died over the several weeks it took for them to mature, but many survived. On the other hand, in the pond to which he had added the dragonfly nymphs, within a couple of weeks all the tadpoles were gone.

The problem with this experiment was that Van Buskirk could have just been throwing a monkey wrench into ponds that had distributions of tadpoles determined for other reasons. It is not surprising if you know that dragonflies eat tadpoles that when you add dragonflies to a pond full of tadpoles, the tadpoles get eaten.

Van Buskirk’s Second Experiment

The more important experiment was his second experiment. What he did in this case was again to select two sets of ponds. But both of these ponds started out with no tadpoles present and had dragonflies. For one of these ponds, Van Buskirk removed the dragonflies and added tadpoles; so after the manipulation, we’ve got a pond where there are no dragonflies, even though there were at the start. Now there are a bunch of tadpoles.

For the second set of ponds, he threw in several hundred tadpoles and left the dragonflies there. Again, the experimental contrast here was that in one set of ponds we have tadpoles without dragonflies. In the other set of ponds we have tadpoles and dragonflies.

This was like the first experiment, but with a key difference: The starting point for these ponds was to not have any tadpoles present. So what happened?

In ponds where Van Buskirk had removed the nymphs and added tadpoles, again, some of the tadpoles died over several weeks, but many of them survived and emerged as adults. Again, not surprisingly, in the pond that had started out with dragonflies where hundreds of tadpoles were added, pretty quickly those tadpoles were eaten by the dragonflies.

The key point about this experiment is that it demonstrated that the pond where no tadpoles were found that had dragonfly nymphs could support populations of tadpoles. In other words, the lack of tadpoles in these ponds could be attributed more directly to the presence of the predator.

An alternative possibility is that there is some ecological condition—maybe the water chemistry—that differs between ponds, and based on water chemistry you will find tadpoles in some ponds and dragonflies in other ponds. But what Van Buskirk was able to show with his second experiment was that ponds that didn’t normally have tadpoles could sustain a population of tadpoles as long as the dragonflies were eliminated.

That allowed him to conclude relatively definitively that it was the presence of the predator that determined the distribution of the prey in this case.

From the lecture series Biology: The Science of Life
Taught by Professor Stephen Nowicki, Duke University

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