Their study system consisted of predatory fly larvae that feed on the water flea Daphnia dentifera, which hosts a virulent fungal parasite. 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. Even so, hard evidence supporting the hypothesis is scarce, and in recent years many of its assumptions and predictions have been questioned. The healthy herds hypothesis has even been used to suggest that manipulating predator numbers to protect prey might be a useful conservation strategy. Each tank also contained nutrients and green algae. Each 20-gallon tank contained populations of tiny crustaceans called Daphnia dentifera, along with predatory fly larvae and a virulent fungal parasite. The experimental setup used to test the healthy herds hypothesis involved 48 tanks called mesocosms. It proposes that predators can help prey populations by picking off the sick and injured and leaving healthy, strong animals to reproduce. This idea has been widely accepted by biologists for many years and was formalized in 2003 as the healthy herds hypothesis. 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. Study: A healthy but depleted herd: Predators decrease prey disease and density (DOI: 10.1002/ecy.4063)
Image credit: Meghan Duffy, University of Michigan. Chaoborus is a fierce predator with a complex “catching basket” on its head for quickly trapping small crustaceans like water fleas. Microscope image showing a phantom midge larva (genus Chaoborus), top left, preying on a Daphnia dentifera water flea, bottom right.
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