The Ecology Book

280

See also: Biomes 206–209 ■ Pollution 230–235 ■ Endangered habitats 236–239

■ Deforestation 254–259 ■ Overfishing 266–269

S

ince the 1980s, hundreds of

species of amphibians have

suffered population crashes

and localized extinctions—at a rate

thought to be more than 200 times

the natural, “background” extinction

rate unaffected by modern human

activity. This alarming phenomenon

first attracted public attention in

1999, when American environmental

scientist Malcolm McCallum

published his findings about the

dramatic increase in deformities

in frogs. He went on to produce

landmark studies on amphibian

decline and extinction.

The causes of the problem are

wide-ranging, and include habitat

destruction and pollution, as well as

competition from nonnative species.

But one of the most devastating

causes is undoubtedly disease, with

two particularly lethal culprits.

Chytrid and ranavirus

Chytridiomycosis is a disease

caused by the chytrid fungus, and

it has ravaged populations of frogs

and toads in particular. The fungus

affects amphibians’ skin, such that

they are not able to breathe, hydrate,

or regulate their temperature. The

exact origin of the fungus is not

known, but the global trade in live

amphibians for various uses, be it

pets, food, fishing bait, or research,

has been a major factor in its spread.

Ranaviruses evolved from a fish

virus. They infect amphibians and

reptiles, and have caused mass

mortality in frogs since the 1980s.

The common midwife toad ranavirus

causes bleeding, skin sores, lethargy,

and emaciation. It is notably virulent

as it has the ability to “jump” from

one species to another. ■

The North American bullfrog is

resistant to the chytrid fungus, but

acts as a deadly carrier of the infection

to other species of amphibians.

IN CONTEXT

KEY FIGURE

Malcolm McCallum

(1968–)

BEFORE

1989 The formerly common

golden toad of Costa Rica is

declared extinct. Various

explanations are proposed.

1998 In the US, many poison-

dart frogs die at the National

Zoo in Washington DC. The

chytrid fungus is implicated

as a cause.

AFTER

2009 The Kihansi spray toad

of Tanzania is declared extinct

in the wild as a result of chytrid

infection.

2013 A second species of

chytrid fungus causes the near-

extinction of fire salamanders

in the Netherlands.

2015 The chytrid fungus is

detected in amphibians in 52

out of 82 countries sampled.

ONE OF THE MAIN

THREATS TO

BIODIVERSITY IS

INFECTIOUS DISEASES

AMPHIBIAN VIRUSES

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