The HEALTHY HERDS HYPOTHESIS stating that trophic predators keep herds of prey animals at levels that minimize their susceptibility to plagues of parasites and disease(as well as preventing undue stress from other predators) is a basic tenet wildlife ecology stemming from Aldo Leopold observations in the 1930's...............Georgia Institute of Technology and U. of Illinois Researchers have discovered that when the Water Flea(Daphnia dentifera) is attacked by one of the trophic predators that feeds on it (a Midge known as Chaoborus), the flea responds by expanding its body size as a means of making it harder to be eaten..........This "growth" makes the flea more susceptible to a parasite known as Metschnikowia..........The Flea's larger size causes it to ingest more of the deadly parasites but because the parasite is more of a periodic attacker versus year round vulnerability to the Midge, the flea defaults to the BIGGGER body size............This research calls into question the pros and cons of increasing predator densities to control the diseases of prey animals.........The results of this research suggests that it is important to consider the indirect effects of predators, such as this one in which trying to avoid one enemy increases the hosts vulnerability to another..............An interesting point to take into account in our continuing discussions about our suite of native trophic, terrestrial carnivores and the hoofed browsers that they prey on

Scientists Uncover an Unhealthy Herds Hypothesis

 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.

"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 University of Illinois at Urbana-Champaign and Indiana University, 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.

"Unfortunately for the Daphnia, this defense against predation makes them more vulnerable to parasitism," said 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.

This research was funded by the National Science Foundation  and appears online in the journal Functional Ecology.