Biology Reference
In-Depth Information
species lost or endangered due to human impact, as will be shown in
the following.
Recent and present extinctions
The discussion above shows that there has been a significant increase in
diversity over evolutionary time, although there were periods of stasis or
even mass extinctions leading to reduced diversity. All available evidence
indicates that a new era of species collapse began thousands of years ago as
the result of the evolution and spread of man on Earth, and that this
collapse is continuing at an accelerating rate. Freeman and Herron (
2004
)
have given an illuminating account of what is happening, underpinned
by many quantitative examples from various sources. They refer to
Steadman (
1995
) who estimated that 2000 species of birds have become
extinct in the Pacific region over the last 2000 years. In New Zealand, to
mention just one example, 44 bird species disappeared after the arrival of
man. The extant fauna of the sheep rangelands of Australia includes
about 30 species of amphibians, 220 species of reptiles, 200 species of
birds, and 80 species of (mainly small nocturnal) mammals (Robertson
et al.
1987
). However, the fossil record indicates that about 30 000 years
ago, before or at the beginning of human settlement, diversity of mam-
mals at least was much richer and included much larger species, such as
many large kangaroos, a rhinoceros-like marsupial, a large wombat, a
large koala, all of them now extinct. When modern Europeans moved
into the rangelands of western New SouthWales, 16 of 22 marsupials and
5 of 7 rodents disappeared. Freeman and Herron (
2004
) refer to data
compiled by Smith et al.(
1993
), who list the number of species from
various major groups that have become extinct in the last 400 years, and
those that are threatened by extinction. The list includes molluscs, crust-
aceans, insects, vertebrates, and among plants gymnosperms, dicotyledons,
monocotyledons, and palms. A total of 0.3% of plant species are already
extinct, and 9% are threatened, if current trends continue. However, the
authors caution that only estimates for birds and mammals are likely to be
approximately correct, whereas those for insects and plants are almost
certainly underestimates. If current trends are enhanced, which is likely in
view of the increasing effects of global warming, a much higher proportion
of species will be threatened. There can be no doubt that global warming
has occurred, although estimates on the degree of warming to be expected
in the future and its effects on species extinctions vary widely. Hughes
(
2003
) discusses climate change in Australia, including present trends,
