Deer, caribou, bison and other similar animals are often infected with a range of internal parasites, including worms called helminths. Although many of these infections are not fatal, they can still impact animal health or behavior.
For example, infected animals may eat less grass or other vegetation than they normally would. In an interesting twist, this means that a world with more sublethal parasitic infections is a greener world.
A new study, led by Washington University in St. Louis and including a co-author from the University of Michigan, uses a mathematical model and comprehensive meta-analysis to highlight the cascading consequences of common parasitic infections in wild animals on terrestrial ecosystems.
“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 in the context of the larger ecosystems they inhabit,” said Amanda Koltz, biologist at the University of Washington and first author of the study published online May 9 in Proceedings of the National Academy of Sciences.
“In this study, we show that invasive parasitic infections reduce herbivory rates and therefore can trigger trophic cascades that impact plant communities,” Koltz said. “This work contributes to filling a recognized knowledge gap regarding the ecological consequences of parasitic infections in natural ecosystems.”
UM biologist Aimée Classen is co-author of the new study. She provided the context of the ecosystem to the working group and the modeling effort, and she collected data as part of the meta-analysis.
“While ecosystem ecologists typically think on a large scale, our work highlights how small things that may be invisible, like herbivorous parasites, can shape large-scale processes like plant biomass across landscapes,” said said Classen, a professor in the Department of Ecology and Evolution. Biology and director of the UM Biological Station. “As our climate warms and ecosystems become more stressed, these unseen interactions are likely to become even more important.”
The research effort was led by an interdisciplinary and international working group funded by the Living Earth Collaborative, a partnership between the University of Washington, the Missouri Botanical Garden and the Saint Louis Zoo. The team included biologists, veterinarians and wildlife epidemiologists, ecosystem ecologists, modellers and infectious disease specialists.
Deer, bison, giraffes, gazelles and antelopes are hoofed animals called ruminants: they are vegetarians whose eating habits have measurable impacts on local ecosystems. Just imagine the amount of greenery a grazing deer could consume in an unfenced yard.
But every wild ruminant is a world unto itself for the myriad of parasites that occupy its gut and tissues as it goes about its usual grass-munching activities.
While other recent studies suggest that the types of parasites that eventually kill their hosts can trigger cascading effects on ecosystems in somewhat similar ways to predators, this study also considered the impacts of non-lethal parasitic infections. .
The researchers used a mathematical model and global meta-analysis to test the potential of helminth parasites – any group of common parasitic worms – to trigger trophic cascades through their lethal and sublethal effects on ruminant hosts.
“The potential for lethal infections that dramatically increase host mortality to have ecosystem-level effects is somewhat intuitive, particularly when the hosts that are killed play key roles in the ecosystem. For example, the rinderpest, a virus that killed millions of ruminants in sub-Saharan Africa before the 1960s, has altered tree density in the Serengeti ecosystem through its effect on wildebeest mortality,” said Vanessa Ezenwa, Professor of Ecology and Evolutionary Biology at Yale University., lead author of the study.
“Our model suggests that sublethal infections, although more cryptic, can have equally important effects on ecosystems; and our meta-analysis shows how ubiquitous the sublethal effects of helminth parasites are in nature.
Modeling led by Dave Civitello at Emory University relied on well-documented examples of caribou and reindeer and their parasites. Caribou and their helminths are among the best-studied wild ruminant parasite systems, due to the ecological, economic, and cultural importance of caribou in tundra ecosystems and the continued disease threats to the system posed by changing climatic.
“Cool tundra ecosystems, where caribou live, are increasingly stressed as the climate warms,” said UM’s Classen. “Our models have allowed us to test how parasites, and in particular non-lethal parasites, alter the productivity of ecosystems. The results highlight that parasites reduce herbivore feeding rates, an impact that shapes plant and lichen biomass.
Next, the scientists turned to data from more than 59 published, peer-reviewed studies to investigate the links between helminth infections and these same key traits of free-ranging ruminant hosts in the wild.
Their analysis revealed that helminth infections significantly reduce host feeding rates but are not, on average, associated with host survival or fecundity.
“Taken together, the broader relevance of these findings is that there are likely widespread—but overlooked—ecological consequences of sublethal parasitic infections,” Koltz said.
“Because helminth parasites are ubiquitous in free-ranging ruminant populations, our results suggest that global rates of herbivory in ruminants are lower than they would otherwise be due to ubiquitous helminth infections. By reducing ruminant herbivory, these common infections can contribute to a greener world.
Written by Talia Ogliore, Washington University in St. Louis