In Parasites

Researchers have found that climate change could prove disastrous for the world’s parasites; up to 30 percent could go extinct by 2070. The September 6 issue of the journal “Science Advances” described a global study led by Colin Carlson, a graduate student at the University of California, Berkeley, who works in Dr. Wayne Getz’s laboratory.

Carlson had done earlier research on the climate change’s impact on species extinction. During those studies, he noticed that most of the research was focused on crop species or large mammals. There wasn’t much on how climate change might be affecting the world’s parasites.

world's parasites

A parasite is an organism that depends on another organism, the host, for its livelihood. Parasites can be found among all of the kingdoms of organisms. Parasitism is very common; scientists estimate that about half of the 7.7 million known species are parasitic. They also believe that parasitism has evolved independently hundreds of times.

Carlson and his team based their predictions on a 2004 study published in the science journal “Nature” that linked extinction rates to the amount of habitat a given species is expected to lose. Unfortunately, as Carlson pointed out, “We don’t know very much about where parasites live.”

Carlson and his team turned to the Smithsonian, which maintains the National Parasite Collection, a 125-year-old accumulation. The Collection contains over 20 million specimens of parasites representing several thousand species. While most of the specimens had been collected in North America, there were enough specimens from the other continents to enable the researchers to put together a historical database they could use to determine a given parasite’s geographic range over the years. The world’s parasites truly come from everywhere.

world's parasites
world's parasites

The first step was to sort through old paper records describing where the parasites had been found. As the collection’s curator, research zoologist Dr. Anna Phillips, said, “Many of these [records] still used a precise locality written out, such as ‘this stream at this crossing of this highway, 10 miles down east of this town.” Modern researchers prefer to use GPS coordinates, and the team had to convert the paper records into a form that could be stored on an online database.

The researchers then used the new database to determine how the distributions and habitats of 450 parasite species had changed over the past two centuries. They then used computer modeling to predict how climate change would affect those species.

The results of that modeling indicated that climate change would have dire impacts on parasitic species. Under even the best scenarios, 10 percent of parasites would go extinct by 2070. In the worst-case scenarios, about a third could vanish.

While many people would cheer at the idea of a world with no more mosquitoes, the researchers point out that a mass extinction of parasitic species could have grave impacts on the ecosystem and human health.

For instance, parasites often help control the populations of their host. The nematode Trichostrongylus tenuis, which parasitizes the red grouse, causes affected birds to emit a stronger scent that make it easier prey for predators. The nematode thus helps keep the birds’ numbers under control.

Parasites can have subtler impacts. The trematode Cryptocotyle lingua parasitizes periwinkle snails and weakens their digestive tracts in the process. Consequently, snails carrying the parasitic flatworms eat less than do healthy snails. That leaves more food available to other creatures.

Even if parasites survive, climate change will force them to shift their range – and that is bad news for humans and other potential hosts. Since parasites need their hosts in order to survive, they generally do not kill them. That, however, is only true of parasites that have evolved to parasitize specific hosts. If they try to feed on an unfamiliar host, things can go wrong for both host and parasite. In 2014, a Chinese man was hospitalized with seizures, severe headaches and inflammation of the brain. While the symptoms suggested a brain tumor, the cause proved to be a tapeworm that had taken up residence in his brain. The worm belonged to a rare Asian species that usually parasitized dogs and cats, not people. Being trapped in an unfamiliar host, the worm had wandered into the man’s brain, instead of his gut where tapeworms usually live.

world's parasites

A balance generally develops between parasites and the hosts they use. As they evolve together, the host develops a greater tolerance for the parasite, while the parasite develops the ability to feed on the host without killing them. In addition, many parasites go through a series of hosts during their life cycle. The presence of parasites in an ecosystem is often an indicator of a healthy, diverse and stable ecosystem.

By contrast, the extinction of a parasite species can throw the ecosystem into disarray. When a parasitic species goes extinct, other parasites will seek to fill that niche, and they will end up trying to feed on hosts that may not be able to sustain them. Similarly, if a host species goes extinct, the parasites that relied on it will also probably die out, and invasive species will swarm in trying to fill the gaps. Given parasites’ roles in maintaining a healthy ecosystem, they need to be studied and protected along with their hosts. When it comes to the world’s parasites, we don’t necessarily want to eradicate them…

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