42 | New Scientist | 6 March 2021
such as covid-19 will just keep on recurring.
That certainty comes not least because
we do now know that a lot of candidate
diseases are out there. “The last 15 years has
seen a real explosion in the understanding
of how many potential pathogens there are,”
says Walzer. A UN biodiversity panel report
last year estimated that there are 1.7 million
undiscovered viruses in animals. Not all
will acquire the traits they need to infect us
(see “How diseases jump to humans”, right).
But about five new infectious diseases in
people are identified every year, and 70 per
cent of emerging diseases are caused by
microbes of animal origin.
One major factor driving these
developments is the growing human
population and rising living standards.
Both of these fuel our need for land, and so
our encroachments into nature’s habitats.
Every new road or mine, or area of forest
cleared for agriculture, increases the chance
that people come into contact with species
carrying potential spillover diseases. “We have
these edges of destruction, the disturbed
areas, often only several hundred metres in.
You trap for food, hunt, you collect firewood,
you may be driving livestock in,” says Walzer.
“You’re creating this interface, and that does
increase the contact rate.” In that sense, the
increasing risk of zoonoses is essentially just
a numbers game.
According to research by Daszak, who works
at the EcoHealth Alliance in New York, and his
colleagues, highly biodiverse tropical regions,
where land use is often changing rapidly,
are hotspots for emerging zoonotic diseases.The most frequent spillover occurs around
people’s homes and fields. For some diseases,
we know in some detail how our activities
helped it occur. One of the best described
instances is the Nipah virus. Its jump from
bats to pigs to people in 1998 followed years
of the intensification of pig farming and
deforestation in Malaysia that increased the
interface between the species. There is still no
vaccine for Nipah, which kills up to 75 per cent
of people it infects. Another example is the
Hendra virus in Australia, where deforestation
is linked to bats carrying the virus reaching
people and horses. SARS-CoV-2, originally
also thought to have arisen in a bat, may one
day be added to the list.
The growing interface with other animals
created by our expanding cities, farms and
extractive industries is just one aspect of the
story, however. Another, perhaps even more
important one, is how human encroachment
is changing the composition of ecosystems.
“It’s this idea of how habitat degradation is
leading to sick landscapes,” says David Redding
at the Zoological Society of London.Disturbed habitats
Case studies suggest that as we disturb
habitats, we alter the community of animals
in ways that increase disease risk. Take Lyme
disease, which is carried by ticks found across
the northern hemisphere. As forests become
degraded, they support fewer larger-bodied
species that are better at removing ticks, while
smaller tick-friendly species such as rodents
thrive. More hosts means more ticks to bite us,
and more risk of the disease spreading.
Despite such compelling evidence,
Redding was initially sceptical that the
sick-landscape hypothesis was generally
applicable. But last year, he and his colleagues
examined data on disturbed and undisturbed
ecosystems across the world, gathered as
part of a project looking at emerging diseases
called PREDICT. What they found dispelled
his doubts: disturbed landscapes such as
cities and plantations that have replaced
natural forest have both a higher proportion
of disease-carrying species and greaterThis feature is the second in
a series of five produced in
association with the United
Nations Environment
Programme and UNEP partner
agency GRID-Arendal. New
Scientist retains full editorial
control over, and responsibility
for, the content. Part three of
the series next week will look
at how developing a rescue
plan for nature post-covid-19
means developing a rescue
plan for the people who
depend on itAbout this featureThe deadly Nipah
virus jumped
from bats to pigs
to humans in 1998GERR
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NA
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PLJAMES
CAVALLINI/
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PHOT
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