Positive parasites, how deer and ruminant pests help the overall ecosystem | The Optimist Daily
Today’s Solutions: July 12, 2024

Some pests and critters annoy us, but everything in nature has its purpose and its place. Wolves are the villains of fairy tales, but they are in fact keystone predators that control the overpopulation of their prey. Beavers chew on powerline poles, but they also help preserve wetlands. As it turns out, even certain parasites can help vegetation by infecting species like deer and caribou. 

A new study from Washington University in St. Louis shows how non-deadly parasites can actually make for a greener environment. 

Helpful helminths 

Those of us who have lived in the Northeast, and many other parts of the United States, can attest to the devasting effects deer can have on lawns and even meadows. Deer, bison, caribou, giraffes, gazelles, antelopes, and other similar, large, hoofed mammals are what’s known as ruminants: vegetarians whose populations and eating habits can have a drastic effect on their ecosystem. Each of these species has the potential to overpopulate and overeat the vegetation in their ecosystem, creating widespread effects in their ecosystem. 

These ruminants, however, often have whole hosts of parasites that do some overeating of their own. Unlike lethal parasites, which have a cascade ecological effect much the same as predators like wolves, non-lethal parasites, like helminths — any of a group of common parasitic worms — can reduce how much these ruminants eat, by affecting their appetite or actually subsisting on the food in the host’s stomach, while leaving them relatively unharmed. 

“Parasites are well known for their negative impacts on the physiology and behavior of individual hosts and host populations, but these effects are rarely considered within the context of the broader ecosystems they inhabit,” says Amanda Koltz, senior scientist in biology at Washington University in St. Louis, first author of the study in the Proceedings of the National Academy of Sciences. “In this study, we show that pervasive parasitic infections reduce herbivory rates and can therefore trigger trophic cascades that impact plant communities. This work helps fill a recognized knowledge gap regarding the ecological consequences of parasitic infections in natural ecosystems.”

The study examined existing, well-established data on deer and caribou and their known parasites. The team also examined the known prevalence of helminth parasites and the general behaviors of ruminants. 

“Our models allowed us to explore the consequences of different ways that parasites harm their hosts,” says Rachel Penczykowski, senior author of the study. “We used the models to test the effects of parasite infections on population densities of caribou hosts and, on their plant, and lichen food resources. “We discovered that any of three types of harm caused by parasitic infections—that is, harm to host survival, feeding rates, or reproduction—can cause a cascading effect.”

While there was a positive effect on the vegetation that ruminants would have otherwise eaten, there was not an overall negative effect on the ruminants themselves. With this knowledge and further study, conservationists might better be able to preserve habitats while not compromising the survivability of one species. All with the help of pesky little parasites. 

Source Study: PNAS https://doi.org/10.1073/pnas.2117381119

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