The world’s largest study on parasites has found something terrible. They die. : ScienceAlert

Parasites are not all bad, and in a rapidly changing world they need our protection, but they don’t seem to get it.

Indeed, in the second largest estuary in the United States, scientists have cataloged a mass die-off among marine organisms that depend on free-living hosts for survival.

Over the past 140 years, from 1880 to 2019, the number of parasites in Puget Sound has declined by 38 percent for every degree Celsius of warming in sea surface temperatures, University of Washington (UW) researchers have found.

The study is the largest and longest data set on parasite abundance collected anywhere in the world, and the results are even worse than some conservationists had feared.

Parasites are the invisible threads that help tie food webs together. How the ecosystems will cope without their influence is unclear.

“The findings are a real bummer if you care about biodiversity or you know anything about parasites,” UW parasitologist Chelsea Wood told ScienceAlert.

“The decline we observed shocked even me.”

If the same rate of loss were observed among mammals or birds, Wood says it would trigger conservation immediately.

Birds in North America, for example, have declined by just over 6 percent in a decade from 1970 to 2017, and already they are hard to find in conservation plans.

In comparison, no one really cares about parasites. A decreasing number of creatures that devour the lives of others is usually seen as a good thing. But it’s an outdated view that neglects the bigger picture.

Today, many scientists agree that climate change has caused Earth to rush towards a mass extinction event, but the scenario looks even worse when you consider that we haven’t really taken into account how heavily life forms on Earth depend on parasites (the vast majority of which is not described).

Currently, very few ecological studies consider parasites, and conservation efforts almost always overlook their binding role in a habitat, despite their widespread and essential role in maintaining ecological balance.

Only when parasites spread and become a problem do we tend to pay them any notice.

In 2020, for example, Wood’s lab at UW made headlines when it found a specific parasitic worm in raw seafood that had increased 280-fold since the 1970s.

But not all parasites fare so well. In fact, many of them are probably suffering in the current climate crisis. Like bubbles in a boiling pot, they disappear faster than we can count them.

In the latest findings from Puget Sound, parasites with three or more hosts (just over half of all samples) appeared to be particularly vulnerable to warming waters.

As to why, it is possible that higher temperatures may put parasites at direct physiological risk, or alternatively that warming the water may affect the availability and viability of their host or hosts.

Regardless, the more hosts a parasite has to bounce between, the more vulnerable it is likely to be to changes in climate.

Of the 10 parasites Wood identified that had been eradicated by 1980 in Puget Sound, nine of them had life cycles that depended on three or more hosts.

“What we expect when we look at a changing environment are winners and losers,” says Wood.

“But what we found here was a whole bunch more losers than we expected.”

If the Puget Sound is anything like other ecosystems in the world, Wood believes parasite loss could match or even exceed the mass extinctions taking place among free-living species.

But no one can say for sure if that is the case without other researchers following in Wood’s footsteps.

Wood believes the current view of parasites is similar to how people once viewed top predators, such as wolves or bears, in the 1960s and 1970s. For centuries, large carnivores were hunted by humans until they were almost extinct out of fear and anger.

It was not until the middle of the 20th century that it became clear to the researchers what had been done. The world had systematically removed some of the most profound movers and shakers in ecosystems to the detriment of habitats worldwide.

Apex predators, as it turns out, weren’t always disruptive pests; they were essential habitat stabilizers. Reintroducing them to habitats helped ecosystems flourish again.

“That’s where we are for parasites,” Wood says, “We’re at this moment when research is starting to come together to suggest how incredibly powerful parasites are in an ecosystem. But that information has yet to leak out to the public.”

In 2017, a study of 457 parasite species predicted that up to 10 percent could become extinct by 2070, including 30 percent of parasitic worms. Spurred by the results, the authors created the first endangered “red list” for parasites.

In 2020, Wood teamed up with like-minded scientists from around the world to detail a 12-goal parasite conservation plan for the future.

Colin Carlson, a co-author on the paper, said The Atlantic in 2015 that the starting point is to stop destroying parasites the moment we find them.

“The most basic idea, and it’s kind of silly that we’ve missed this, is that you don’t break something if it’s going well,” Carlson told reporter Ed Yong.

The next step is data collection and synthesis, and Wood is a leader in this field. Her lab at the UW is the very first to use museum specimens of fish to create a historical timeline of marine parasite abundance.

“No one has noticed anything like this,” says Wood. “And part of it is that no one is watching.”

Unlike apex predators, parasites are harder to see if you’re not actively looking for them. And finding them is not exactly glamorous work.

“Your fieldwork is sitting in the basement of a museum dissecting fish that are filled with nasty chemicals,” says Wood.

“It doesn’t have sex appeal. But it gives us the opportunity to travel in time. And if I get the chance to travel in time, I’ll sniff some formalin fumes.”

The parasites of the present and the past are there for us to count. Now we just have to plug our noses and dive.

The study was published in PNAS.

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