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Tropical rainforest is the hottest and
wettest biome and covers 7 per cent
of Earth’s surface. One of the oldest
biomes, it also contains far more animal
and plant species than any other biome.THE LIVING EARTH
Very few species have identical
climate requirements. Even among
varieties of the same plants, there
are variations. The sugar maple
of eastern North America, for
example, is slightly more tolerant
of winter cold than its cousin the
silver maple. Although the areas
where both trees grow overlap, the
sugar maple can be seen far over the
Canadian border, whereas the silver
maple flourishes as far south as
Texas. Since biomes give only an
approximate picture of plant and
animal distribution, ecologists are
constantly devising new systems
of classification.Rain, heat, and evolution
One of the most widely recognized
systems of classification is the life
zones system devised by American
botanist Leslie Holdridge in 1947,
and updated in 1967. His system is
based on the assumption that two
key factors, rain and heat, determine
vegetation type in each region. He
created a graphic depiction of 38 life
zones in a pyramid. The three sidesof the pyramid represent three
axes: rain, temperature, and
evapotranspiration (which depends
on both rain and temperature). Using
these axes, he could plot hexagons
showing regions that also reflect
humidity, latitude, and altitude.
American plant ecologist Robert
Whittaker devised a much simpler
graph, with average temperature
on one axis and annual rainfall on
the other. With these two variables
plotted against each other, he was
able to divide the graph into nine
biomes—from tropical rain forest
(the hottest and wettest) through
to tundra (the coldest and driest).
Underpinning all these systems
is the idea of convergent evolution,
which argues that species develop
similar traits as they adapt to similar
environments. Insects, birds, bats,
and pterosaurs all developed wings
independently to occupy air space.
Different biomes are therefore
assumed to develop corresponding
life forms in response to similar
environmental conditions. However,
in recent decades, it has been noted
that species can evolve differently
in the same biome and also that
different stable biomes can develop
in an identical climate. While central
to understanding life, biomes remain
a complex and elusive concept. ■Ecozones
Biomes are about identifying
the similar forms that life
takes in response to particular
regional conditions such as
climate, soil, and topography.
However, there are other
methods of dividing the world
in ecological terms. In 1973,
Hungarian biologist Miklos
Udvardy came up with the
concept of biogeographic
realms; this system was then
further developed in a scheme
by the World Wildlife Fund.
The BBC later replaced the
term “biogeographic realm”
with “ecozone.” Biogeographic
realms divide the whole planet
according to the evolutionary
history of plants and animals.
The ways in which continents
have split apart and drifted
means that species have
evolved variously in different
parts of the world. Ecozones
are therefore based on
identifying this diversification.
Australasia, for example, is
a single ecozone, because
marsupials evolved there in
isolation from other mammals
in the rest of the world.The short-beaked echidna
is one of the most widespread
native mammals in the Australasian
ecozone. They live in a range of
habitats from desert to rain forest.US_206-209_Biomes.indd 209 12/11/18 6:25 PM