Page 298 - Environment: The Science Behind the Stories

Archaea (0.03%) 500
Bacteria (0.2%) 4000
Roundworms (1.9%) 25,000
Fungi (4.1%) 72,000
Protists (4.6%) 80,000 Crustaceans (3.0%) 40,000
Vertebrates (3.9%) 52,000 Mammals (9.0%) 4680
Plants
(15.4%) Molluscs (5.3%) 70,000
270,000 Arachnids (5.7%) 75,000 Amphibians (10.0%) 5200
Other animal groups
(7.5%) 99,000
Reptiles (14.0%) 7280

Birds (19.0%) 9880
Animals
(75.7%)
1,324,000 Insects
(72.7%)
963,000

Fishes
(48.0%)
24,900

All life
(~1,750,000 known species) Animals
Vertebrates
Figure 11.4 Insects predominate in number of species. Of all known species, three-quarters are animals.
Among animals, nearly three-quarters are insects, whereas vertebrates comprise only 3.9%. Among verte-
brates, nearly half are fishes, and mammals comprise only 9%. Data from Groombridge, B., and M.D. Jenkins, 2002.
Global biodiversity: Earth’s living resources in the 21st century. Cambridge, UK: Hoechst Foundation.
What percentage of the world’s total species do mammals comprise?

microbes, fungi, and small insects, but also sometimes with Biodiversity is unevenly distributed
organisms as large as birds, trees, and whales. Third, some
areas of Earth remain little explored. We have barely sam- Numbers of species tell only part of the story of Earth’s biodiver-
pled the ocean depths, hydrothermal vents (p. 51), or the tree sity. Living things are distributed across our planet unevenly. For
canopies and soils of tropical forests. As one example, a 2005 instance, species richness generally increases as one approaches
expedition to the remote Foja Mountains of New Guinea dis- the equator (Figure 11.5a). This pattern of variation with latitude,
covered over 40 new species of vertebrates, plants, and but- called the latitudinal gradient, is one of the most obvious patterns
terflies in less than a month, while research in marine waters in ecology, yet one of the most difficult for scientists to explain.
nearby turned up another 50 new species. Hypotheses abound for the cause of the latitudinal gra-
Smithsonian Institution entomologist Terry Erwin pio- dient in species richness, but it seems likely that plant pro-
neered one method of estimating species numbers. In 1982, ductivity and climate stability play key roles (Figure 11.5b). CHAPTER 11 • Bi odiv ER si T y A nd Cons ER vAT i on Bi ology
Erwin’s crews fogged rainforest trees in Central America Greater amounts of solar energy, heat, and humidity at tropi-
with clouds of insecticide and then collected insects, spi- cal latitudes lead to more plant growth, making areas nearer
ders, and other arthropods as they fell from the treetops. the equator more productive and able to support larger num-
Erwin concluded that 163 beetle species specialized on the bers of animals. The relatively stable climates of equatorial
tree species Luehea seemannii. If this were typical, he fig- regions—their similar temperatures and rainfall from day to
ured, then the world’s 50,000 tropical tree species would day and season to season—discourage single species from
hold 8,150,000 beetle species and—because beetles repre- dominating ecosystems and instead allow numerous species
sent 40% of all arthropods—20 million arthropod species. to coexist. Whereas variable environmental conditions favor
If canopies hold two-thirds of all arthropods, then arthropod generalists (species that can tolerate a wide range of circum-
species in tropical forests alone would number 30 million. stances), stable conditions favor specialists (species highly
Many assumptions were involved in this calculation, and adapted to particular circumstances) (p. 79). Additionally,
follow-up studies have revised Erwin’s estimate downward, polar and temperate regions may be relatively species-poor
but it remains one of the better efforts at estimating species because glaciation events repeatedly forced organisms from
numbers. these regions toward tropical latitudes. 297

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