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belong to a constellation of demographic changes that have altered communities of
birds, reptiles and amphibians in the area and which are clearly linked to regional
climatic warming. Later work more clearly linked
Atelopus
population crashes with
climate change. There were some 110 species of
Atelopus
endemic to the New World
tropics, a number that made it possible to make a statistically valid local extinction
study.
The idea was put forward that temperatures at many highland habitats of
Atelopus
spp. were shifting towards the growth optimum of a chytrid fungus,
Batrachochytrium
dendrobatidis
, which is known to infect many species of frog. In short, in this instance
climate change was promoting infectious disease in highland areas. Looking at the
records of when species were last recorded in highland areas in the wild and compar-
ing these to climate records the researchers were able to show that most extinctions
(78-83%) occurred in years of higher tropical temperature and that this was statistic-
ally significant to 99.9%. Further, the extinctions varied with altitude (which is also
related to temperature) and it is thought that this reflected the optimal conditions for
Batrachochytrium
to flourish (Pounds et al., 2006). This means that not only does
global warming reduce individual mountains habitat islands as habitat zones migrate
upwards, but that the environmental conditions of the complex mosaic of habitats in
highland areas can change so that some areas see an increase in disease organisms.
There is a further twist to this story. Around a third (1856) of amphibian species
in the Global Amphibian Assessment are classified globally as threatened and 427
species as critically endangered. One question therefore is whether the
Atelopus
studies meaningfully relate to the global amphibian picture. The earliest known hosts
of
Batrachochytrium
were a continent away, the African clawed frog (
Xenopus
)of
South Africa. These frogs became economically important in the 1950s as their
tissue was used in some pregnancy-testing kits. Museum records also suggest that
Batrachochytrium
achieved a global distribution after the 1960s. The possibility exists
therefore that the expansion in one frog species due to commercial activity might have
led to the extinction of other amphibian species (Blaustein and Dobson, 2006).
The recent decline in amphibians is not just due to the chytrid fungus
B. dendroba-
tidis
and climate change (not to mention the interaction of the two), but also land-use
change leading to loss of habitat. And, of course, land-use change involving defor-
estation and drainage releases carbon and so is a factor in climate change. In 2011,
Christian Hof, Miguel Ara ujo, Walter Jetz and Carsten Rahbek attempted to map
these three factors of amphibian decline. They found that the areas harbouring the
richest amphibian faunas are disproportionally more affected by more than one of
these factors than areas of low amphibian diversity. They therefore predicted that
amphibian declines are likely to accelerate in the 21st century. (See also Alford,
2011.)
This is not the only interaction known to take place between climate and disease.
Similarly the warmer climate conditions in upland regions of the western USA favour
the mountain pine beetle (
Dendroctonus ponderosae
), allowing it to complete its life
cycle in 1 year rather than 2 years. These beetles act as a vector, enabling the
transmission of pine blister rust (
Cronartium ribicola
), and this is having a serious
impact on pine trees on some of the Rockies' highest mountains (Blaustein and
Dobson, 2006). The issue of economic activity, globalisation, human population
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