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comprises a
huge variety of unique animals; some 83% of mammals,
89% of reptiles, 90%
of fish and insects and 93% of amphibians
that inhabit the continent are found nowhere else. This high
level of endemism can be attributed to the continent's long geological
isolation, tectonic stability, and the effects of climate change
on the soil and flora over geological time. A unique feature of
Australia's fauna is the relative scarcity of native placental mammals.
Consequently, marsupials mostly fill the ecological niches that
elsewhere are occupied by placental mammals. Australia
is home to the two of the four extant monotremes (egg-laying mammals),
and has numerous venomous species, which include the monotremes,
spiders, scorpions, octopuses, jellyfish, molluscs, stonefish, stingrays.
Uniquely, Australia has more venomous than non-venomous species
of snakes.
The
settlement of Australia by people from the north of the continent
more than 40,000 years ago, and by Europeans from 1788, has had
a significant impact on the fauna. Hunting, the introduction of
non-native species, and land-management practices involving the
modification or destruction of habitats, have led to numerous extinctions;
unsustainable land use continues to threaten the survival of many
species. In recognition of the threats to the survival of its fauna,
Australia has passed wide-ranging federal and state legislation
and has established numerous protected areas to protect the fauna.
However, many people believe that these measures fail to address
the immanent threats to the survival of many species.
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Several reasons for the uniqueness of Australia's fauna have been
advanced. Australia was once part of the southern super-continent
Gondwana, which also included South America, Africa, India and Antarctica.
Gondwana began to break up 140 million years ago (MYA); 50 MYA Australia
separated from Antarctica, and was relatively isolated until the
collision of the Indo-Australian Plate with Asia in the Miocene
era 5.3 MYA. The establishment and speciation of the present-day
fauna was apparently shaped by the unique climate and the geology
of the continent. As it drifted, Australia was, to some extent,
isolated from the effects of global climate change, and species
that were present on Gondwana, such as the marsupials, spread throughout
the continent. After the Miocene, fauna of Asian origin were able
to establish themselves in Australia. The Wallace Line—the
hypothetical line separating the zoogeographical regions of Asia
and Australasia—also marks the tectonic boundary between the
Eurasian and Indo-Australian plates. This boundary prevented the
formation of land-bridges, which explains the distinct zoological
distribution and limited overlap of most Asian and Australian fauna,
with the exception of birds. Following the emergence of the circumpolar
current in the mid-Oligocene era (some 15 MYA), the Australian climate
became increasingly arid, giving rise to a diverse group of arid-specialised
flora and fauna, just as the wet tropical and seasonally wet areas
have produced their own adapted species.
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