A study led by former Duffy Lab postdoc Laura Lopez in 2019 has just appeared in Ecology! The study set out to test for healthy herds predation — where predators can actually increase prey populations as a result of decreasing parasitism. The general idea that predation can benefit prey populations by reducing disease is really pervasive — Meghan’s favorite example comes from this wrapper from a chocolate bar that she bought back when she was a grad student:
This idea that predators can help their prey populations was formalized in a paper in 2003, leading to what has become known as the “healthy herds hypothesis”. As this UMich press release states:
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.
To test this, Laura led an experiment that manipulated the abundance of phantom midge larvae (which are important predators of zooplankton) and the presence and absence of a fungal parasite in populations of Daphnia dentifera. We found that high predation levels reduced parasitism in Daphnia, providing support for the first half of the healthy herds prediction. However, population sizes of Daphnia were often dramatically reduced.
Read more at this press release from UMich: https://news.umich.edu/a-healthy-but-depleted-herd-predators-decrease-prey-disease-levels-but-also-population-size/
and read the full article, which is open access, here: https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecy.4063
Data and code for the paper can be found here: https://datadryad.org/stash/dataset/doi:10.5061/dryad.w3r2280tm
In addition to Laura and Meghan, the authors are former Duffy Lab undergrad Bruce O’Brien, Mike Cortez of Florida State, Turner DeBlieux and Spencer Hall of Indiana University, and Ilona Menel and Carla Cáceres of U. Illinois.