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distributed among four species. In Community 1, each the birds or small mammals you might see in your neigh-
species is represented by 5 individuals. In Community 2, borhood. Others are not so easy to find. Some species are
one species is represented by 14 individuals and each of the active only at night, live in inaccessible locations such as
other three species is represented by 2 individuals. Although the deep ocean, or cannot be seen without a microscope.
the species richness of the two forests is identical, the four To date, scientists have named approximately 2 million
species are more evenly represented in Community 1. species, which means the total must be larger than that.
That forest therefore has greater species evenness and is While 2 million described species is an impressive num-
considered to be more diverse. Because species rich- ber, it is equally impressive that the rate of discovering
ness or evenness often declines after a human distur- new species remains very high. Currently, a staggering
bance, knowing the species richness and species evenness 15,000 to 18,000 new species are discovered each year
of an ecosystem gives environmental scientists a baseline from around the world. This highlights the fact that we
they can use to determine how much that ecosystem has still have a long way to go in knowing the biodiversity of
changed. our planet.
Because insects contain more species than most other
groups, scientists reason that if we could get a good esti-
®
AP Exam Tip mate for the number of insect species in the world, we
would have a much better sense of the total number of
®
When taking the AP Environmental Science Exam species. In one study, researchers fumigated the canopies
of a single tree species in the tropical rainforest and then
make certain you describe vocabulary words. Some- collected all the dead insects that fell from the trees onto
times using the word isn’t enough. For example, you a tarp on the ground. From this collection, they counted
could write “Species richness, or the number of dif- the number of beetle species that fed on only the one
ferent species in an area, is one way to measure bio- tree species they fumigated. By multiplying this number
diversity.” Note: If you can’t remember the vocabulary of beetle species by the total number of tropical tree spe-
term, you can receive credit by describing the word cies, they estimated that in the tropics there were perhaps
instead.
8 million species of beetles that feed on a single species
of tree. Because beetles make up about 40 percent of
all insect species, and because insect species in the for-
est canopy tend to be about twice as numerous as insect
8-4 How can we estimate the number of species species on the forest floor, the researchers suggested that
living on Earth? a reasonable estimate for the total number of tropical
insect species might be 30 million. More recent work has
indicated that this number is probably too high. Current
It is difficult to estimate the number estimates for the total number of species on Earth range
of species on Earth between 5 million and 100 million, but most scientists

estimate that there are about 10 million total species on
A short walk through the woods, a corner lot, or a city park Earth.
makes one thing clear: Life comes in many forms. A small
plot of untended land or a tiny pond contains dozens, per- In this module, we have explored the important role
haps hundreds, of different kinds of plants and animals visi- that biodiversity plays in providing resilience from the
ble to the naked eye as well as thousands of different kinds of levels of genes to ecosystems. In the next module, we
microscopic organisms. In contrast, a carefully tended lawn or will examine how all of this biodiversity works together
a commercial timber plantation usually supports only a few to provide life-supporting resources such as clean water,
types of grasses or trees. The total number of organisms in timber, fisheries, and agricultural crops that are critical to
the plantation or lawn may be the same as the number in the humans.
pond or in the untended plot, but the number of species will
be far smaller.

As we have noted, estimating the number of species Species richness The number of different species in a given
in any given place is the most common measure of bio- area.

diversity, but estimating the total number of species on Species evenness The relative proportion of individuals

within the different species in a given area.
Earth is a challenge. Many species are easy to find, such as

ModUle 8 ■ Introduction to Biodiversity 103
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