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Worksheet 4.2
Evaluating biodiversity and vulnerability
Answers
1.
There have been five mass extinctions in the past.
2.
See Figure 4.12 in the textbook, page 191.
3.
The Cretaceous-Tertiary extinction occurred about 65 million years ago and was probably caused
by the impact of the several-mile-wide asteroid that created the Chicxulub crater (which is hidden
on the Yucatan Peninsula and beneath the Gulf of Mexico). Other possible causes are gradual
climate change and flood-like volcanic eruptions of basalt lava from India’s Deccan Traps. The
extinction killed 16% of marine families, 47% of marine genera (the classification above species)
and 18% of land vertebrate families, including the dinosaurs. Overall, ca. 17% of all families were
lost (a family contains up to 1000 species) and all large animals including the dinosaurs.
The End Triassic extinction occurred roughly 199 million to 214 million years ago and was most
likely caused by massive floods of lava erupting from an opening in the Atlantic Ocean (which
may have led to a deadly global warming). Rocks from the eruptions can be found in the Eastern
United States, Eastern Brazil, North Africa and Spain. The extinction killed 22% of marine
families, 52% of marine genera. Land vertebrate deaths are unclear. Overall ca. 23% of all families
were lost.
The Permian-Triassic extinction occurred about 251 million years ago. It is suspected to have
been caused by a comet or asteroid impact, although direct evidence has not been found. Others
believe the cause was flood volcanism (as with the End Triassic extinction) from the Siberian
Traps, which destroyed algae and plants, reducing oxygen levels in the sea. It is also believed that
the impact of the comet or asteroid triggered the volcanism. This extinction was the Earth’s worst
mass extinction, killing 95% of all species, 53% of marine families, 84% of marine genera and an
estimated 70% of land species such as plants, insects and vertebrate animals. Ca. 54% of all
families were lost.
The Late Devonian extinction occurred about 364 million years ago for an unknown cause. It
killed 22% of marine families and 57% of marine genera. Very little is known about land
organisms at that time. Overall ca. 19% of families were lost.
The Ordovician-Silurian extinction, which occurred about 439 million years ago, was caused by a
drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. It killed 25% of
marine families and 60% of marine genera. Very little is known about land organisms at that time.
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4.
The National Science Foundation estimates that there could be anywhere from 5 million to
100 million species on the planet, but science has only identified about 1.8 million. It is impossible
to get an accurate count on the number of species, because the majority of the species that have yet
to be discovered and described are in the realm of the very small: insects, bacteria and other
microbes. Please read this article on biodiversity for your reference.
5.
There is little doubt that humans are the direct cause of ecosystem stress and species destruction in
the modern world (resulting in the sixth great extinction) through such activities as the
transformation of the landscape, the over-exploitation of species, pollution and the introduction of
exotic (non-native) species. The sixth extinction would seem to be the first recorded global
extinction event that has a biotic, rather than a physical, cause.
6.
The background level of extinction known from the fossil record is about one species per million
species per year, or between 10 and 100 species per year (counting all organisms such as insects,
bacteria, and fungi, not just the large vertebrates we are most familiar with). In contrast, estimates
based on the rate at which the area of tropical forests is being reduced, and their large numbers of
specialized species, are that we may now be losing 27 000 species per year to extinction from
those habitats alone. The typical rate of extinction differs for different groups of organisms.
Mammals for instance, have an average species ‘lifespan’ from origination to extinction of about
1 million years, although some species persist for as long as 10 million years. There are about 5000
known mammalian species alive at present. Given the average species lifespan for mammals, the
background extinction rate for this group would be approximately one species lost every 200 years.
Yet the past 400 years have seen 89 mammalian extinctions, almost 45 times the predicted rate,
and another 169 mammal species are listed as critically endangered.
7.
The sixth extinction would seem to be the first recorded global extinction event that has a biotic,
rather than a physical, cause. Some mass extinctions were caused by the impact of asteroids, floods
of lava and changes in water levels (when glaciers formed, the water levels dropped, and then
when the glaciers melted, the water levels rose). This next mass extinction would be caused by
ecosystem stress and species destruction in the modern world through such activities as the
transformation of the landscape, the overexploitation of species, pollution and the introduction of
exotic species.
8.
The average time between mass extinctions seems to be around 100 million years. The exception
of this is between the Permian-Triassic and the End Triassic extinctions, which were
approximately 50 million years apart. The last mass extinction occurred 65 million years ago.
9.
Scientists believe that the rate of extinction may be greater than in the past and happening over a
shorter period of time. One estimate is that 25% of all plants and animal species will become
extinct between 1985 and 2015. The rate will depend on where habitat destruction is occurring
most – if areas of high biodiversity (diversity ‘hotspots’) are lost, e.g. rainforest and coral reef,
extinction rates will be higher.
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10. It has taken 5 to 10 million years to recover biodiversity after past mass extinctions. After each of
the previous five mass extinctions, new species evolved that occupied similar niches to the ones
that had gone extinct. This period of ‘adaptive radiation’ took place over a geological timescale of
tens of millions of years. Similarly, to recover from current extinction events will take millions of
years (the time-frame over which evolution takes place).
11. Examples of case studies can be found on pages 196–199 of the textbook.
12. See pages 192–194 and page 196 of the textbook.
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