The predatory phantom midge larva, Chaoborus, hunts down the small aquatic crustacean, Daphnia dentifera. Photo: Alan J. Tessier |
Biologists worldwide subscribe to the healthy herds hypothesis, the idea
that predators can keep packs of prey healthy by removing the weak and the
sick. This reduces the chance disease will wipe out the whole herd, but could
it be that predators can also make prey populations more susceptible to other
predators or even parasites? Biologists at the Georgia Institute of Technology
have discovered at least one animal whose defenses against a predator make it a
good target for one opportunistic parasite. The research appears online in Functional
Ecology.
“Unfortunately for the Daphnia, this defense against predation
makes them more vulnerable to parasitism,” said Duffy.
“We found that strategies that prey use to defend
themselves against predators can increase their susceptibility to infection by
parasites,” said Meghan Duffy, assistant professor in Georgia Tech’s School of Biology.
Duffy, along with colleagues at the Univ.
of Illinois at Urbana-Champaign and Indiana Univ., took a look at a small aquatic
crustacean, Daphnia dentifera, a water flea known to be an important
part of freshwater ecosystems. They exposed the crustacean to chemicals emitted
by one of its predators, a phantom midge larva known as Chaoborus,
known to feed on it. When the Daphnia detected those chemicals it grew
larger, making it harder for its predator to get its mouth around it.
This image shows an aquatic crustacean, Daphnia dentifera, that’s infected with the yeast parasite Metschnikowia (on the left) next to one that’s not infected (on the right). The parasite, looking like small puffs of cotton, can be seen under the eye and along its back. Photo: Meghan Duffy |
That’s because while growing larger keeps Daphnia safe from Chaoborus,
it actually makes it more susceptible to a virulent yeast parasite, known as Metschnikowia.
When Daphnia senses a threat from its predator and grows
larger, it ends up consuming more of these parasitic yeasts than it does when
normal size. When the yeast infects the crustacean, it kills it, causing the
dead animal to release yeast spores as it decomposes. The larger the host, the
more spores it releases back into the water to prey on other Daphnia.
“Since they need to grow larger to defend themselves against the predator
but the opposite to defend against the parasite, they’re sort of stuck between
a rock and a hard place,” she added.
Duffy reasons that this occurs because the predators are common year-round,
while the parasites are more episodic in nature, with their populations
expanding in epidemics only in the fall and not even yearly. This results in
long periods of predation in the absence of the parasite, which probably
explains why they respond so strongly to defend themselves against the predator
even though it decreases their defenses against the yeast, she added.
“While some have argued for increasing predator densities to control
disease, our results suggest that it is important to consider the indirect
effects of predators, such as the one we found in which trying to avoid one
enemy increases the hosts vulnerability to another,” said Duffy.
