Page 297 - Environment: The Science Behind the Stories

(pp. 78, 98) or habitats (p. 79) within some specified area. In some groups, many species evolved rapidly as popu-
Scientists may also consider the geographic arrangement of lations spread across a variety of environments and adapted
habitats, communities, or ecosystems at the landscape level, to local conditions. Other groups diversified because of a
including the sizes and shapes of patches and the connections tendency to become subdivided by barriers that promote spe-
among them (pp. 131–133). Under any of these concepts, a ciation (pp. 71–72). Still other groups accumulated species
seashore of beaches, forested cliffs, offshore coral reefs, and through time because of low rates of extinction.
ocean waters would hold far more biodiversity than the same
acreage of a monocultural cornfield. A mountain slope whose
vegetation changes with elevation from desert to forest to Many species await discovery
alpine meadow would hold more biodiversity than a flat area Scientists often express biodiversity in terms of species rich-
the same size consisting of only desert, forest, or meadow. ness because that component is most easily measured and is
The Serengeti’s open plains are vast, but the region holds a good gauge for overall biodiversity. Yet we still are pro-
a diversity of habitats, including savanna (p. 115), grassland foundly ignorant of the number of species that exist. So far,
(p. 113), hilly woodlands, seasonal wetlands, and rock out- scientists have identified and described about 1.8 million spe-
croppings called kopjes. This habitat diversity contributes to cies of plants, animals, and microorganisms. However, most
the rich diversity of species in the region. of Earth’s species remain undiscovered. Estimates for the total
number that actually exist range from 3 million to 100 million,
Some groups hold more species than others with the most widely accepted estimates in the neighborhood
of 14 million.
Species are not evenly distributed among taxonomic groups. Our knowledge of species numbers is incomplete for
In number of species, insects show a staggering predomi- several reasons. First, many species are tiny and easily over-
nance over all other forms of life (Figure 11.3 and Figure 11.4). looked. These include bacteria, nematodes (roundworms),
Within insects, about 40% are beetles, and beetles alone out- fungi, protists, and soil-dwelling arthropods. Second, many
number all non-insect animals and all plants. No wonder the organisms are so difficult to identify that ones thought to
British biologist J.B.S. Haldane famously quipped that God be identical sometimes turn out, once biologists look more
must have had “an inordinate fondness for beetles.” closely, to be multiple species. This is frequently the case with

Plants
Plants

Fungi
Insects

Molluscs Figure 11.3 Some groups
Arachnids
Crustaceans contain more species
Birds than others.This illustration
shows organisms scaled
in size to the number of
Bacteria species known from each
Mammals group, giving a visual sense
Reptiles Protists of their disparity in species
richness. Because most spe-
cies are not yet discovered
Sponges Flatworms or described, some groups
Amphibians Archaea Jellyfish (such as bacteria, archaea,
insects, nematodes, protists,
and fungi) may contain far
more species than we now
Annelids
Annelids know. Data from Groombridge,
Fishes
B., and M.D. Jenkins, 2002. Global
Roundworms
Roundworms Echinoderms biodiversity: Earth’s living resources
in the 21st century. UNEP-World
Conservation Monitoring Centre.
Cambridge, U.K.: Hoechst
296 Foundation.

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