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Unit Chapters
Proteins & Proteomics
Evolution & Phylogenetics
Microbial Diversity
Emerging Infectious Diseases
Genetics of Development
Cell Biology & Cancer
Human Evolution
Biology of Sex & Gender
What is Biodiversity and Why Should We Conserve It?
Global Species Diversity
The Erwin Study
Seven Kinds of Rarity
What Factors Determine Extinction Probability?
Keystone Species and the Diversity-Stability Hypothesis
Mass Extinctions
The Sixth Mass Extinction
Genetically Modified Organisms
Global Species Diversity

Biodiversity is copious and imperiled, yet it is difficult to measure. This feature makes it also difficult to quantify its loss as well: we know little about what we are losing. Despite its importance, knowledge about biodiversity lags behind that of other areas of science. The statement that opened this chapter echoes those made by several researchers in environmental biology, who have been frustrated by the lack of progress quantifying biodiversity. As we shall see, even the simple question "How many species of animals are on Earth?" has not been answered, even to within an order of magnitude.

Before discussing how scientists address the question "How many species of animals are on the planet Earth?" let's first ask, "How many species of animals have been described?" There is uncertainty even to the second question's answer. Some uncertainty reflects differences in opinion among taxonomists about whether different populations are indeed separate species. Some is due to inadequate centralized databases. While there are efforts underway to provide a centralized catalog of described species, none exist as of 2003. The most current estimates are that there are about 1.4 to
1.6 million described species of animals.

What are these 1.6 or so million species of animals? At least one million are insects. A quip from JBS Haldane, polymath and one of the founders of the evolutionary synthesis, illustrates the taxonomic concentration of biodiversity. When asked about what he could divine from nature about the Creator, Haldane replied that he must have had "an inordinate fondness for beetles." Haldane's quip was in reference to the sheer quantity of beetle diversity. There are roughly 450,000 different described species of beetles, representing about thirty to forty percent of the known species of insects (Fig. 1). There are about 200,000 described species of flies. In contrast, there are only about 9,000 species of birds and 4,000 species of mammals. Every year, about 2,400 new species of beetles and 1,200 species of flies are described. Thus, the number of species of beetles that scientists will describe in the next five years alone is greater than the total number of current bird species.

Figure 1. Pie chart of species
In addition to animals and plants, biodiversity also includes a vast number of unlabeled species of bacteria, fungi, and protists. These contribute to environmental homeostasis by degrading organic matter and by making the energy in inorganic matter available for growth. Although we often forget these organisms in our consideration of biodiversity, they are critical to the balance and resilience of the environment, especially with respect to their role in nutrient cycles. (See the Microbial Diversity unit.)

Much of the known biodiversity is located in the tropics. In general, species diversity greatly increases as one moves toward the equator: specific hotspots of biodiversity are located in tropical rain forests. Even though they account for only about seven percent of the land area on the planet, tropical rain forests are home to around half the known species of animals.

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