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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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[OUTRO]