OREANDA-NEWS Recently, scientists carried out the first large-scale study of what climate change may do to the world’s much-loathed parasites. The team came to a startling conclusion: as many as one in three parasite species may face extinction in the next century.

As global warming raises the planet’s temperature, the researchers found, many species will lose territory in which to survive. Some of their hosts will be lost, too.

“It still absolutely blows me away,” said Colin J. Carlson, lead author of the study and a graduate student at the University of California, Berkeley.

He knows many people may react to the news with a round of applause. “Parasites are obviously a hard sell,” Mr. Carlson said.

But as much as a tapeworm or a blood fluke may disgust us, parasites are crucial to the world’s ecosystems. Their extinction may effect entire food webs, perhaps even harming human health.

Parasites deserve some of the respect that top predators have earned in recent decades. Wolves were once considered vermin, for example — but as they disappeared, ecosystems changed.

Scientists realized that as top predators, wolves kept populations of prey in check, which allowed plants to thrive. When wolves were restored to places like Yellowstone, local ecosystems revived, as well.

Researchers have begun carefully studying the roles that parasites play. They make up the majority of the biomass in some ecosystems, outweighing predators sharing their environments by a factor of 20 to 1.

For decades, scientists who studied food webs drew lines between species — between wildebeest and the grass they grazed on, for example, and between the wildebeest and the lions that ate them.

In a major oversight, they didn’t factor in the extent to which parasites feed on hosts. As it turns out, as much as 80 percent of the lines in a given food web are links to parasites. They are big players in the food supply.

Parasites can control populations of their hosts. Some are killed outright; other hosts, once infected, cannot reproduce, which would divert resources that the parasite craves to eggs or sperm. Some parasites move from host to host by making prey species easier for predators to kill.

So if these horrendous pests are major players in ecosystems that we want to save — what then? “This view requires that parasites be protected alongside their hosts,” said Kevin D. Lafferty, an ecologist at the University of California, Santa Barbara, who was not involved in the new study.

A warming climate complicates the picture. Some researchers had already investigated the fate of a few parasite species, but Mr. Carlson and his colleagues wanted to get a global view of the impact of climate change.

They began their work with the National Parasite Collection, founded in 1892 and now maintained by the Smithsonian Institution. One of the world’s biggest, it includes 20 million specimens, some preserved in jars of alcohol and some mounted on slides.

By determining the present range of each parasite species, Mr. Carlson and his colleagues were able to estimate the kind of climate in which it can survive and how it might fare in a hotter world.