Encyclopedia of Evolution.pdf - Online Reading Center

The number of species that can exist in any habitat is
large but not unlimited. The resources that a species uses, its
spatial limits, and the times at which it uses them, is called
the niche of a species. Two species cannot, in theory, have
exactly the same niche. They either use slightly different
resources, or in different places, or at different times. There
is no general rule for how different the niches of two species
must be (the limiting similarity of the species) in order
for them to coexist. Tropical species can apparently divide up
their environment into finer spatial niches, which may contribute
to the greater biodiversity of the tropics. Species that
live in seasonal climates must tolerate those seasonal changes
if they are to survive, or else they migrate great distances in
order to avoid these seasonal changes. Tropical species, in
contrast, can remain in and specialize on one location.
Keeping track even of the species that are discovered is
a difficult task. Sometimes new species turn out to have been
already described. Although duplicate descriptions occur
about 20 percent of the time, the other 80 percent of the
descriptions are of genuinely new species. Usually new species
are within previously known phyla, families, and genera (see
Linnaean system). There are occasional surprises. Whole
new phyla of animals have been found in recent decades in
places such as the deep ocean sediments.
If there are indeed 100 million species, it would take
at current rates 14,000 years for biologists to catalog them.
Some biologists and information scientists are designing
streamlined methods for cataloguing biodiversity that bypass
the slow process of academic publication and even allow
researchers in distant jungles to send their information via
satellite to databases, and which make information available
online for identifying known species. Examples include the
All Species Foundation, started in 2001 by Wired Magazine
cofounder Kevin Kelly, and the Encyclopedia of Life project
(see Wilson, Edward O.). It is also important to train
researchers native to each country to catalog their countries’
biodiversity. If there are not and will not be enough scientists,
then this research must rely on trained amateurs.
The simple count of species (called species richness) is
not always a sufficient measure of diversity. For example, the
74 species of terrestrial vertebrates from the Karoo Basin in
the late Permian period, contrasted with the 28 species that
lived in that location in the early Triassic period, indicate
that species richness decreased by about two-thirds. (The
extinction rate was far greater, since almost all of the 28 Triassic
species evolved after the Permian extinction event.) The
74 Permian species were more or less equal in abundance,
while in the early Triassic, one genus (Lystrosaurus) was
overwhelmingly common, not only in the Karoo but worldwide.
The post-extinction world had one-third the number of
terrestrial vertebrate species but was far less than one-third as
diverse in terms of ecological interactions, since most of the
higher animals were all members of one generalist species.
In ecological and evolutionary terms, scientists distinguish
between an ecosystem that contains, say, 50 equally
abundant species and one that contains 50 species, 49 of
which are rare. Ecologists have, accordingly, devised different
diversity indices (symbolized by H) rather than to rely
simply on species richness. In each of these indices, there are
s species, and the proportional importance of the ith species
is represented by pi. If one adds up all of these proportions,
the result is 1. The indices are calculated by adding up mathematical
derivatives of the proportions. One of these indices,
Simpson’s diversity index, emphasizes the dominant species
because it adds up the squares of the proportions:
H = ∑ s
p 2
i
i=1
The other, the Shannon-Wiener diversity index, which is based
upon information theory developed by mathematicians Claude
Shannon and Norbert Wiener, emphasizes the rare species
because it multiplies each proportion by its natural logarithm:
H = ∑ s
pilnpi i=1
Besides diversity, scientists can also quantify equitability.
In a community of species with high equitability, the species
are all about equally important; in a community with low
equitability, there are a few dominant species, and the rest
are rare. Equitability (E) is the diversity divided by the natural
logarithm of the number of species:
E = H/ln s
biodiversity
Within the United States, Appalachian cove forests not only
have many plant species but these species are equitable. In
contrast, a boreal forest has few species and low equitability.
Much of the Earth remains unexplored. The deep oceans,
for example, are nearly inaccessible. Until the 1970s, no one
even suspected the existence of entire complex communities of
species living at the deep-sea vents where ocean floor volcanic
eruptions occur. These deep-sea vents may have conditions
that resemble those of early life on Earth (see origin of life).
Not all unknown species hide in remote regions. New species
of plants are continually discovered, some of them very close
to scientific research centers. A new species of centipede was
discovered recently in New York City’s Central Park!
The diversity of microbes, especially bacteria, is particularly
difficult to quantify. Biologists typically grow bacteria
in media in order to estimate the number of species present
in a sample; but most media have been designed for medical
research. What about the bacterial species (especially the
archaebacteria) that require hot, acid, or salty conditions
not represented by standard bacterial growth protocols? Many
bacteria live off of minerals, and they may do so up to several
kilometers deep into the crust of the Earth (see bacteria, evolution
of). This indicates that there is a much greater number
of bacterial species than previously suspected. Bergey’s Manual,
the closest thing scientists have to a list of bacterial species,
includes about 4,000 species. But in 1990, Norwegian biologists
Jostein Goksøyr and Vigdis Torsvik took a random gram
of soil and determined that it contained four to five thousand
species of bacteria. In another sample, they found a largely
different set of 4,000 to 5,000 species. Bacterial species are
now often identified by their DNA sequences (see DNA [evidence
for evolution]). Biotechnologist Craig Venter used
DNA sequencing techniques to detect thousands of previously
unidentified microbial species in the Sargasso Sea in 2004.