38 | New Scientist | 2 November 2019Take the camel cricket. Like many crickets, it
can be infected by a parasite called a horsehair
worm. After the worm matures in the cricket’s
body, it needs to reach fresh water to reproduce.
Instead of waiting for the cricket to take an
accidental bath, the worm compels its host to
make a suicidal leap into a stream or pond,
where it can then bore a hole and slither into the
water. In doing so, the worm provides a crucial
delivery service: crickets account for more
than half the food eaten by trout in some areas,
according to a study by Lafferty and others.Extinction threat
If parasites are as valuable as other
components of ecosystems, they are also
as threatened. In 2017, Cizauskas was part
of an international team that tried, for the
first time, to assess how global warming
will affect parasites. Although climate change
has been linked with the spread of diseases,
the researchers found no indication that
pathogenic parasites will be able to increase
their ranges. Instead, the evidence points to
between 5 and 10 per cent of parasites going
extinct by 2070 as a result of climate-related
habitat loss alone. Add in the loss of host
species and changing environmental
conditions, and as many as 30 per cent of
parasitic worms are facing extinction within
the next half century. The forecast is even
worse for parasites such as lice and ticks
that live on the outside of their host.
For all this, only two animal parasites are
listed on the International Union for
Conservation of Nature’s Red List of
Threatened Species: the pygmy hog-sucking
louse and the European medicinal leech, and
knowledge about the threat facing parasites is
woefully lacking. “We have no idea how many
parasites we’ve lost. They’re not cute and
cuddly, so we’re not tracking them,” says
Skylar Hopkins, an ecologist at Virginia
Tech who has become a leading voice in the
movement to protect parasites. Nor are
scientists sure just how the loss of parasites
will affect ecosystems as a whole. One small
study, however, provides some worrying clues.
A clam called the New Zealand cockle lives
in many of the nation’s mudflats, using its
muscular foot to burrow into the ooze. A fluke
known as Curtuteria australis likes to embed
itself in the cockle’s foot. If they become
heavily infested, cockles lose their ability to
hide in the mud and find themselves stranded.
Research reveals that mudflats with more
parasitised cockles supported a higher level
NATURE PICTURE LIBRARY of animal diversity. Organisms living in the
Two parasites are
better than one
Woylies are cute, rat-faced,
rabbit-sized marsupials
endemic to Australia. In the first
half of the 20th century, their
numbers crashed, largely as
a result of habitat destruction
and predation by foxes and feral
cats. By 2001, conservation
efforts had boosted the species
back up to 40,000 individuals,
but then there was a sudden and
mysterious decline to fewer
than 6000. The finger of blame
pointed towards parasites.
Biologists noticed that woylies
infected with Trypanosoma
copemani, a single-celled,
corkscrew-shaped parasite,
had lower body weights than
uninfected animals – a sign
that their health was impaired.
At first glance, these results
supported a scorched earth
approach to parasite control:
any parasite could harm this
endangered marsupial, so they
all had to go. Captured woylies
destined for captive breeding
programmes were dosed with
anti-parasitic medication.
However, when parasitologists
Stephanie Godfrey and Andrew
Thompson from MurdochUniversity in Australia looked
more closely in 2017, they found
that the relationship between
woylies and their parasites
wasn’t as straightforward as
had been thought. Animals
infected with both T. copemani
and the closely related
Trypanosoma vegrandis were
much healthier than those that
hosted just one of the parasites.
“This co-infection allowed the
animals to survive better,” says
Godfrey. Far from needing more
parasite control, she realised
that the woylies needed less.
One possible explanation is
that under normal conditions
most parasites don’t cause
disease. “Parasites have evolved
with their wildlife hosts, who
have evolved good ways to
cope,” says Amanda Ash at
Murdoch University, who also
worked on the woylie project.
In fact, having a variety of
less-harmful or better-tolerated
parasites might help keep the
more virulent ones in check –
just as a healthy gut microbiome
can prevent colonisation by
nasties like Staphylococcus
aureus in humans.A duo of
parasites is
helping
conservationists
bring back
woylies from
the brink of
extinction