Nature documentaries will tell you that lions, cheetahs, wolves and other top predators target the weakest or slowest animals and that this culling benefits prey herds, whether it’s antelope in Africa or elk in Wyoming.

This idea has been widely accepted by biologists for many years and was formalized in 2003 as the healthy herds hypothesis. It proposes that predators can help prey populations by picking off the sick and injured and leaving healthy, strong animals to reproduce.

The healthy herds hypothesis has even been used to suggest that manipulating predator numbers to protect prey might be a useful conservation strategy. Even so, hard evidence supporting the hypothesis is scarce, and in recent years many of its assumptions and predictions have been questioned.

In a study published online April 26 in the journal Ecology, a University of Michigan-led research team used a pint-sized predator-prey-parasite system inside 20-gallon water tanks to test the healthy herds hypothesis.

Their study system consisted of predatory fly larvae that feed on the water flea Daphnia dentifera, which hosts a virulent fungal parasite.

The researchers found that while high predation levels reduced parasitism in Daphnia—providing partial support for the healthy herds hypothesis—populations of those poppy seed-sized crustaceans were often dramatically reduced, as well. In some cases, Daphnia populations were nearly wiped out by predation.

The findings may have implications for conservation efforts involving much larger animals, according to the study authors. Specifically, the results suggest that caution is warranted when wildlife managers manipulate predator numbers in the hopes of promoting healthy herds of prey.

“The appeal of the healthy herds hypothesis lies in the alignment of multiple conservation goals—simultaneous conservation of predators, reduction of parasitism, and protection of vulnerable populations—as well as the potential to reduce spillover risk to other populations, including humans,” said U-M aquatic and disease ecologist Meghan Duffy.