Non-deadly parasites can bring amazing benefits to the world’s ecosystems

(ORDO NEWS) — When something is rummaging around in your insides and you feel like you’re about to vomit, the last thing you probably want to do is eat.

Deer, caribou and other ungulates (ungulates) face a similar problem when they become infected with non-lethal parasites.

For them, this is extremely unpleasant, but it turns out that infections that distract them from food bring great benefits to the ecosystem.

“Parasites are well known for their negative effects on the physiology and behavior of individual hosts and host populations, but these effects are rarely considered in the context of the broader ecosystems they inhabit,” says University of Washington biologist Amanda Koltz.

Koltz and colleagues analyzed data from well-studied plant, caribou, and helminth (parasitic worm) systems using computer modeling and a global meta-analysis.

They found that the non-lethal effects of some parasites, such as reduced host nutrition, were more significant than the lethal effects because they were more common.

Because these parasites and their effects are so widespread, all of this could lead to big consequences on a global scale.

Clearly, when deadly parasites decimate populations, it can have environmental consequences, much like predators take their prey out of sight. Removing any of them can completely change the dynamics of an ecosystem.

For example, in the 19th century, the rinderpest virus wiped out up to 90 percent of domestic and wild livestock in sub-Saharan Africa, but after a successful vaccination campaign, the population increased, resulting in a reduction in the frequency of fires – thanks to a decrease in the undergrowth that the cattle ate – which in turn allowed more trees to be grown.

This is an example of a trophic cascade – an ecological domino effect caused by changes in one of the links in the food chain, which ultimately have much wider consequences.

In this case, a change in the trophic cascade has shifted sub-Saharan Africa from a general carbon source to a carbon sink, thanks to an increase in tree density.

Most living things are infected with all sorts of parasites without lethal outcome, but how these ecological black holes affect the ecology as a whole is not well understood.

We know that, on an individual level, parasites can have a huge impact on our bodies, from affecting the way we think to being unexpectedly useful. Moreover, parasites are estimated to make up to half of all living species.

However, there is still a lot we don’t know about these often nasty creatures, which can be very problematic because, as in most other areas of life, we are driving many species of parasites to extinction.

In nearly 60 studies analyzed by the researchers, helminth infestations consistently distracted caribou from their food, slowing down their feeding rate (surprising for the plants they eat). In turn, this affected the body condition and body weight of mammals, but on average did not affect their reproduction or survival.

What’s more, the team’s simulations showed that when a helminth affected the survival or feeding rate of a caribou, it had a stabilizing effect on the plant-herbivore cycle, but if a parasitic worm affected the herbivore’s ability to reproduce, it most likely destabilized the system.

“Given that helminth parasites are ubiquitous in free-living ruminant populations, our results suggest that global ruminant herbivory rates are lower than they would be due to the ubiquity of helminthiasis,” explains Koltz. “By reducing ruminant herbivory, these common infections could contribute to a greener world.”

“In short, herbivore diseases matter to plants,” concludes University of Washington disease ecologist Rachel Penczykowski.

Of course, this is only a single example in one system, and experimental field studies will be required to establish the accuracy of the modeling and reveal the true scale of the impact of the trophic cascade.

But as our world moves towards an increasingly unstable climate, understanding these interactions can help develop predictive models and mitigation strategies.

“Our work highlights how small things, such as herbivore parasites, can shape large-scale processes, such as plant biomass in landscapes,” Klassen says.

“As the climate warms and pressures on ecosystems increase, these invisible interactions will become even more important.”

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