2 November 2019 | New Scientist | 37Less than a century ago, many countries
established programmes to exterminate
apex predators, arguing that everyone
would be better off if they didn’t exist.
Wolves were extirpated from most of
the continental US, and populations of
big cats and bears around the world plunged.
However, ecosystems lacking top predators
weren’t the peacenik utopias some had
hoped for. Populations of prey animals, from
antelope to zebra, exploded and then starved
as their food sources ran out. In the 1960s,
conservation biologists began a small but
forceful movement to reshape how society
views carnivores in a bid to return them to
their rightful habitat. This quest is succeeding.
Nichols saw similarities with parasites.
Because they have been portrayed in such
a negative way, no one had thought to ask
whether they might have a beneficial
role in the environment. Her own work on
dung beetles had told her about nature’s
interconnectedness, and she believes that
conservation biologists have overlooked some
of the strongest links in the natural world.
Andrew Dobson at Princeton University
agrees. “Parasites are as fundamental to an
ecosystem as predators are,” he says. In nature,
parasitism is the most common way to get
food, shelter and other resources – so common
that parasites far outnumber free-living
organisms. For example, the 45,000 known
vertebrates are home to more than 75,000
parasites.
“There are more animals being eaten
from the inside out than the outside in,”
says Dobson. At least 70 per cent of food-web
interactions are between a parasite and its
host, says Carrie Cizauskas at the University
of California, Berkeley. “Parasites are the glue
that holds an ecosystem together.”I
N 1987, officials from the US Fish and
Wildlife Service captured all 22 of the
world’s remaining California condors in
a last-ditch effort to save the species. Back at
San Diego Wild Animal Park and Los Angeles
Zoo, biologists liberally dusted them with
an insecticide called carbaryl to remove any
parasites. The condors emerged from the
blizzard unscathed, and the captive breeding
programme was so successful that, by 1991,
conservationists began returning birds to the
wild. The California condor louse didn’t fare so
well. With a few shakes of powdery pesticide,
the parasite quietly went extinct. Few people
noticed and even fewer cared. If saving this
iconic bird meant losing a louse, so be it.
Such attitudes still prevail. But perhaps not
for much longer. While parasites can harm
and even kill their hosts, there is also a growing
appreciation of their value. Far from being
organisms that must be eradicated at any
cost, it is emerging that they play important
and beneficial roles in ecosystems. They
can even keep their hosts healthy. In fact,
in a modern parallel to the California condor
story, conservationists trying to save an
endangered marsupial called the woylie have
discovered that parasites can actually help.
Despite this, many parasites are themselves
endangered and their plight has been almost
completely overlooked. Now a group of
pioneering biologists aims to change that.
They see parasites as an essential component
of life on Earth and believe we should be
protecting them.
There is no doubt that parasites have a
bad reputation. For most people, they are
synonymous with disease. This reflects our
daily interactions with them. We treat our
pets for heartworm and ringworm or take
precautions for ourselves against hookworm. >
We douse our crops in torrents of chemicals
to prevent parasites from ruining our food.
It is almost dogma that parasites are bad, end
of story, says Kevin Lafferty at the University
of California, Santa Barbara. Even biology
textbooks focus on the negative.
This is something conservation biologist
Liz Nichols discovered as a graduate student.
She is currently on sabbatical at the US State
Department, but a decade ago while at
Columbia University in New York she decided
to investigate what students are taught about
parasites. Working in the basement of the
American Museum of Natural History with
fellow postdoc Andrés Gómez, she searched
77 English-language textbooks published
between 1970 and 2009 and found that what
little was mentioned about parasites portrayed
them in a poor light.
History reveals the danger of such blanket
prejudice against a group of living organisms.“ More animals
are being
eaten from
the inside
out than the
outside in”