Let’s think about the meaning of biodiversity. Most people
understand that biodiversity includes the great heterogeneous assemblage
of living organisms. This aspect of biodiversity is also known as
"species diversity." Biodiversity includes two other components as well-
genetic diversity and ecosystem diversity.
Currently the planet is inhabited by several million species in about
100 different phyla (Dirzo & Raven 2003). About 1.8 million have
been described by scientists (Hilton-Taylor
et al. 2008), but
conservative estimates suggest that there are 5–15 million species alive
today (May, 2000), since many groups of organisms remain poorly
studied. Over 15,000 new species are described each year (Dirzo &
Raven 2003), and new species are evolving during our lifetimes. However,
modern extinction rates are high, at 100 to 1000 times greater than
background extinction rates calculated over the eras (Hambler 2004).
Although new species appear, existing species go extinct at a rate 1000
times that of species formation (Wilson 2003). Many biologists agree
that we are in the midst of a mass extinction, a time when 75% or more
of species are lost over a short geological time scale (Raup 1994). The
last great mass extinction was 65 million years ago, at the end of the
Cretaceous, when the dinosaurs went extinct. The International Union for the Conservation of Nature
estimates that 22% of known mammals, 32% of amphibians, 14% of birds,
and 32% of gymnosperms (all well-studied groups) are threatened with
extinction (Hilton-Taylor
et al. 2008). Species that were
abundant within the last 200 years have gone extinct. For example,
passenger pigeons, which numbered three to five billion in the mid 1800s
(Ellsworth & McComb 2003), are now extinct.
Species diversity
The 1.8 million species described by science are incredibly diverse. They range from tiny, single-
Nanoarchaeum equitans, 400 nm in diameter living as parasites on other microbes in thermal vents at temperatures of 70–98°C (Huber
et al. 2002), to giant organisms like
Sequoias,
blue whales, the "humungous fungus," and "Pando" (Figure 1). "Pando" is
the name given to a clonal stand of aspen trees, all genetically
identical and attached to each other by the roots (Grant
et al. 1992). The stand covers 106 acres and weighs 13 million pounds. The "humungous fungus," a giant individual of the species
Armillaria oysterae is found in the state of Oregon, and covers 1,500 acres (USDA Forest Service 2003).
celled microbes like
While people are generally most familiar with multicellular organisms
such as plants and animals, these organisms form only small branches on
the tree of life. The greatest metabolic diversity is found among the
prokaryotic organisms of the Eubacteria and Archaea. Although some of
these microbes use oxygen for respiration, or photosynthesize like
plants, others have the extraordinary ability to derive energy from
inorganic chemicals such as hydrogen sulfide or ammonia, and they use
carbon dioxide as their only source of carbon for producing organic
molecules. Organisms that we consider extremophiles can survive in
saturated salt concentrations (36% compared to approximately 3% for
seawater), or in superheated water in deep-sea vents and geysers.
Genetic Diversity
Genes are responsible for the traits exhibited by organisms and, as
populations of species decrease in size or go extinct, unique genetic
variants are lost. Since genes reside within species, why should we
consider genetic diversity as a separate category? Because they hold
"genetic potential." For example, many of the crops that we grow for
food are grown in monocultures of genetically homogeneous individuals.
Because all individuals are the same, a disease, insect pest, or
environmental change that can kill one individual can extirpate an
entire crop. Most of our high-yield varieties show significant
reductions in yield within about 5 years, as pests overcome the crops’
natural defenses. Plant breeders look to wild plant relatives and to
locally grown landraces to find new genetic varieties. They can then
introduce these genes into crops to renew their vigor. However,
according to the UN Food and Agriculture Organization, 96% of the 7,098
US apple varieties cultivated prior to 1904, 95% of the US cabbage
varieties, and 81% of tomato varieties, are extinct, and the genes that
made these varieties unique are gone.
Genetic variation allows species to evolve in response to diseases,
predators, parasites, pollution, and climate change. The Red Queen
Hypothesis, named for Lewis Carroll’s character who runs continually in
order to stay in the same place, states that organisms must continually
evolve, or succumb to their predators and parasites that
will continue to evolve.
In addition to traditional breeding, advances in genetic engineering
have allowed scientists to introduce beneficial genes from one species
to another. For example, diabetics used to depend on insulin from human
cadavers, or from cows or pigs. Human insulin was expensive, and
non-human insulin could cause allergic reactions. Now we can isolate the
gene that codes for human insulin, insert it into bacterial cells, and
let the bacteria produce large quantities of human insulin. Other
notable feats in genetic engineering include the introduction of genes
that enhance the nutritive value of food, create crop resistance to
insect pests, induce sheep to produce a protein for treating cystic
fibrosis disease, and alter bacteria so that they can clean up toxic
mine wastes through their metabolic activities. Many other genetic
manipulations are currently in development.