By Stephen Nowicki, PhD, Duke University
The life-and-death balancing act between predator and prey is complicated and fascinating. Read about Ecologist Josh Van Buskirk’s classic experiments investigating this delicate relationship.
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 prey and predator would affect not only the abundance of the prey species, but also the distribution of the prey species—and whether the prey species could exist somewhere or not because of the presence of a predator.
We would expect this kind of effect to occur more frequently in communities where predators are generalists as opposed to specialists. By “specialists” we mean a case where a predator or a parasite truly relies on only a single kind of food resource, perhaps only a single species.
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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.
To make this point clear, consider the example of work done on populations of frogs by Ecologist Josh Van Buskirk.
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Tadpoles and Dragonfly Nymphs
As you probably know, frogs lay their eggs in water, and tadpoles—their juvenile stage—will spend some time in a pond, lake, or another body of water before they mature, turn into adult frogs, and then move out of the water.
Van Buskirk was working on a species of frog called the western chorus frog. This type of frog 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?”
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.
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 were 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.
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Manipulating Pond Populations
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.
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. 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 with tadpoles and left them 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 that if you know dragonflies eat tadpoles, so that when you add dragonflies to a pond full of tadpoles, the tadpoles get eaten.
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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; 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 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.
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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.
Common Questions About Predators and Prey
The impact of a predator(s) on populations of prey is manifold. They tend to “weed-out” the sick, weak, and feeble. This changes the genetic makeup of the survivor prey. Additionally, the smartest prey reproduces and thus carry on those beneficial habits.
The impact of a predator on an ecosystem is huge. They keep the number of prey down to a reasonable size, and when paired with another predator in terms of impact, they contribute in even deeper ways such as controlling the size of game that they both hunt.
Predator/prey relationship diversifies their particular ecosystem and keeps it functioning stably.
The impact of a predator on prey is balanced. If a predator is removed from the ecosystem, the population of prey tends to surge and have damaging effects on other areas of the ecosystem such as vegetation or other small game if the prey are carnivores.
