When something's messing with your insides and you feel like you're going to hurl, the last thing you probably want to do is eat.
Deer, caribou, and other ungulates (hoofed animals) experience a similar problem when infected by non-deadly parasites. It utterly sucks for them, but it turns out infections that put them off their food have a wider benefit for the ecosystem.
"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 Washington University biologist Amanda Koltz.
Koltz and colleagues analyzed data from the well-studied plant, caribou and helminth (parasitic worm) system, using computer modeling and a global meta-analysis. They found that the non-lethal effects of some parasites, such as reduced feeding in hosts, had a more significant impact than lethal effects because they occur more commonly.
As these parasites and their impacts are so widespread, it all can add up to big consequences globally.
Obviously, when lethal parasites wipe out populations it can have knock-on impacts on the surrounding environment, similar to predators taking their prey out of the picture. Removing either can completely alter an ecosystem's dynamics.
For example, in the 19th century the rinderpest virus killed up to 90 percent of all domestic and wild cattle in sub-Saharan Africa, but a population increase after a successful vaccination campaign saw a decline in fire frequency – thanks to less undergrowth which the cattle ate – which in turn allowed more trees to grow.
This is an example of a trophic cascade – an ecological domino effect triggered by changes to one part of the food chain that end up having much broader ramifications. In this case, the change in the trophic cascade shifted the sub-Saharan region from being an overall carbon source to a carbon sink, thanks to its increase in tree density.
Most living things have non-lethal infections of all sorts of parasites, but how these ecological black holes impact wider ecology is not well understood.
We know that on an individual level parasites can have a huge impact on our bodies, from influencing the way we think to being unexpectedly helpful. What's more, parasites are estimated to compose up to half of all living species.
Yet there's so much we still don't know about these often unpleasant creatures, which could potentially be quite problematic when, as with most other areas of life, we're driving many parasitic species to extinction.
In the almost 60 studies the researchers analyzed, the helminth infections consistently put the caribou off their food, reducing their feeding rates (awesome for the plants they eat). In turn, this impacted the mammals' body condition and body mass, but on average did not impact their breeding or survival.
What's more, the team's modeling suggests that when the helminth impacted a caribou's survival or feeding rate, it had a stabilizing effect on the plant-herbivore cycle, but if the parasitic worm impacted the herbivore's ability to breed, it was more likely to destabilize the system.
"Given that helminth parasites are ubiquitous within free-living populations of ruminants, our findings suggest that global herbivory rates by ruminants are lower than they otherwise would be due to pervasive helminth infections," explains Koltz. "By reducing ruminant herbivory, these common infections may contribute to a greener world."
"In short, diseases of herbivores matter to plants," concluded Washington University disease ecologist Rachel Penczykowski.
Of course, this is just a single example in one system, and experimental fieldwork will be needed to establish the accuracy of the modeling and reveal the true scale of the trophic cascade impacts.
"Our work highlights how the little things that can be unseen, like herbivore parasites, can shape large-scale processes like plant biomass across landscapes," says Classen.
"As our climate warms and ecosystems become more stressed, these unseen interactions will become even more important."
Their research was published in PNAS.