Caribou Intestinal Parasites Indirectly Create Greener Tundra

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Intestinal parasites in large plant eaters like caribou grow out of sight and somewhat out of mind. But these tiny belly tenants can have big impacts on the landscape their hosts traverse.

Parasites in the caribou digestive tract can reduce the amount of food their hosts eat, allowing them to more plant growth in the tundra where the animals livereport researchers in the May 17 Proceedings of the National Academy of Sciences. The finding reveals that even non-lethal infections can have ripple effects on ecosystems.

It has long been known that species interactions ripple through ecosystems, indirectly affecting other parts of the food web. When predators eat herbivores, for example, a reduction in plant-eating mouths leads to changes in the plant community. For example, sea otters can promote kelp growth by feeding on herbivorous sea urchins (NS: 03/29/21).

“Anytime you have a change in species interactions that alters what animals do on the landscape, that can influence their impact on the ecosystem,” says Amanda Koltz, an ecologist at Washington University in St. Louis. .

When parasites and pathogens kill their hosts, it can have an effect similar to predators on ecosystems. A prime example is the rinderpest virus which, at the end of the 19th century, devastated ruminant populations – buffaloes, antelopes, cattle – in sub-Saharan Africa. Once wildebeest populations in East Africa were spared further infection following the vaccination of livestock and the eradication of the virus, their explosive numbers cut the grass in the Serengeti and led to other landscape changes.

But unlike rinderpest, most infections are not fatal. Non-fatal parasitic infections are ubiquitous in ruminants – plant eaters that play a key role in the formation of terrestrial vegetation. Koltz and his team wondered if changes in a ruminant’s overall health or behavior following chronic parasite infection could also induce changes in the surrounding plant community.

SEM image of a brown stomach worm (Teladorsagia circumcincta)
Parasites like this brown stomach worm (Teladorsagia circumcincta), shown in an SEM image, are common residents of the guts of ruminants such as sheep, cattle, and deer.Dennis Kunkel Microscopy/Scientific Source

Researchers looked at caribou (Rangifer tarandus). Using data from published studies, Koltz and his team developed a series of mathematical simulations to test how caribou survival, reproduction, and feeding rate might be influenced by stomachworm (Ostertagia spp.) infections.

The scientists then calculated how these effects might alter total mass and population changes in caribou, parasites and plants. The simulations predict that not only could lethal infections trigger a cascade leading to more plant mass, but non-lethal infections would also have an equally large effect. Sick caribou that ate less or experienced a decline in reproductive rate resulted in increased plant mass compared to a parasite-free scenario.

The team also analyzed data from 59 studies of 18 ruminant species and their parasites, gathering information on the impact of parasites on feeding rates and host body mass. The analysis found that chronic parasitic infections typically cause many types of herbivores to eat less, which also reduces their body mass and fat stores.

Indirect ecological ramifications of parasitic infections could be common in ruminants around the world, the researchers conclude.

The study “highlights that there are widespread interactions that we don’t yet consider in ecosystem contexts, but we should,” says Koltz.

Globally, parasites face an uncertain future with rapid environmental changes – such as climate change and habitat loss due to land use changes – altering relationships with their hosts, potentially leading to many pest extinctions. “How such changes in host-parasite interactions might disrupt the structure and functioning of ecosystems is something we should think about,” says Koltz.

The results will also “change the way we think about what controls ecosystems,” says Oswald Schmitz, a population ecologist at Yale University who was not involved in the research. “Maybe it’s not necessarily the predators that control the ecosystem, maybe the parasites are more important,” he says. “And so what we really need to do is more research that unravels [this].”

Scientists are rapidly gaining a better understanding of parasite ubiquity and abundance, says Joshua Grinath, an ecologist at Idaho State University in Pocatello. “Now we are challenged to understand the roles of parasites within ecological communities and ecosystems.”

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