This SciShow video is supported by Linode!

Go to linode.com/scishow for a $100 60-day credit on a new Linode account. Listen, most people are  not huge fans of parasites.

Whether it’s the tongue-replacing kind, the live-in-your-gut-and-eat-your-food kind, or really, any of them, they tend  to inspire the heebie-jeebies. And more often than not, they do a  number on the health of their hosts, so it’s hard for us to think of them  as a positive influence on the world. But research from 2020 suggests  that thanks to climate change and other factors, overall parasite  populations are decreasing.

And that could actually be a really bad thing. [INTRO] Parasitism is a way of life, and not  limited to any one group of organisms. It’s a strategy, not a family. And it’s a pretty simple one.

If you’re living in, or on, a  host and getting your food from their bodies at their expense, you’re a parasite. Parasitism is one of the most common  life strategies to evolve across a huge range of organisms from the  microscopic to many meters long, making it one of the most abundant and hugely diverse forms of life on Earth. Since parasitism has evolved many times over, exactly how each kind does its thing varies.

Some parasites live inside their  hosts while others simply latch on. And it gets even more complicated, because many parasites require  a host in order to grow from an egg to an adult. Plus, some even need more  than one host, hopping from species to species depending on  the stage of their life cycle.

Some even need three separate hosts  through their various life stages! And while we may think of them as unwanted in pretty much every scenario, parasites are deeply interwoven  into food webs and ecosystems, and their presence can actually be beneficial. These benefits obviously  don’t apply to the individuals who have parasites, since a higher parasite load is strongly correlated with worse health.

But the pressure those  parasites put on populations can have really big impacts by  keeping ecosystems in check. Take invasive species. Invasive species are really  good at taking over ecosystems.

It’s kind of what they’re known for. But it’s possible that the  reason they don’t wreak havoc on the ecosystems where they came  from has to do with parasites. Parasites can help prevent any one  species from becoming too dominant, by keeping their host species’  growth at a reasonable level relative to the other organisms in the ecosystem.

And escaping their natural parasites  might be one of the reasons that some species become  invasive in the first place. For example, when left unhampered by the burden of their pesky parasites, European green crabs are able to grow larger and spread more aggressively. So we can kind of think of  parasites as helping to keep some species from becoming  overbearing in their own environments!

Some researchers have also  hypothesized that as hosts evolve to outfox their parasites, the selective  pressure created by those parasites causes more speciation and diversity, which is beneficial for the ecosystem overall. So the greater the diversity of parasites present, the more stable and resilient  ecosystems may become. The problem is, thousands or  even millions of parasite species may be at risk of extinction  without us even knowing it.

And, the impact of their  disappearance could radiate through their ecosystems in dramatic ways. See, unless the parasites are infecting us or domesticated animals like pets and livestock, we haven’t exactly been invested in studying them or their ecological importance. So we have almost no long term  data on parasite population trends, other than the ones that affect us.

Since most ecological studies tend not to include parasites on the species checklist, they’re not even on the radar of most scientists, even those actively studying biodiversity. So even if we wanted to rally  around saving the parasites, we’re lacking a full understanding  of their ecosystem roles and long term population trends  to even know where to start. But we’re starting to unravel the  mysteries of some of our parasitic pals.

A study published in 2023 looked  into the long-term changes in parasite populations thanks  to their accidental preservation in natural history collections. The study looked at 139 years’  worth of museum fish specimens from Puget Sound on the northwest  coast of the United States. Because the fishes had all been  preserved whole and un-dissected, their parasites were also  preserved alongside them, or more accurately inside them.

That meant the researchers  could measure the parasite load of the fish populations through time. Looking at 8 fish species, they  discovered that the majority of parasite species were  dwindling in numbers over time. Specifically, the pickled parasites  that required three or more hosts as part of their life cycle  were considerably worse off, declining in abundance at a rate  of over 10 percent every decade.

The study also looked at factors that might have most directly impacted these multi-host parasites and found a correlation with sea  surface temperature increases. And although other factors likely also influenced this drop in parasite populations, it was found that for every 1°C  increase of surface temperature, multi-host parasites were  diminishing by 38 percent! Since they can’t live without their hosts, if the host species suffers, so do the parasites.

And if they require multiple host  species for their life cycle, there’s a delicate balance to juggle  before the whole thing falls apart. Parasites with more complex  and specific host relationships appear to take the biggest hit when it comes to overall biodiversity loss  and climate change impact. This is bad news for parasites, given  that living through multiple hosts is actually a very common strategy for them.

They made up 76 percent of the  total parasite diversity found in the Puget Sound fish in the study. And even if a parasite species only has one host, it’s still likely to go down with the  ship if something happens to that host. One study predicted that if just  5 of North America’s carnivores went extinct, they’d take out 56  parasite species along with them.

And seeing as about a quarter of  the world’s mammals are already listed as threatened, that  could mean tens of thousands of parasite species are also threatened. Parasite ecologists have been trying  to sound the alarm for decades, and it’s about time the rest of us  start worrying about their decline, too. In 2020, a group of researchers developed a global conservation plan for parasites, to help guide efforts over the coming decade, from increasing education to pinpointing where to focus research efforts and legal protections.

And don’t worry, these  researchers are not advocating for saving our parasites, just  the species with non-human hosts. Parasites aren’t exactly built for cute logos or cheerful conservation slogans, so it’s tough to convince people  to champion this particular cause. But if we’re really going to take  biodiversity conservation efforts seriously, there’s some PR legwork ahead  of us to get people on board with protecting the less glamorous,  but equally important, species, too.

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