Biodiversity
Name:
Introduction: You will learn two techniques scientists use to estimate the biodiversity of an
area. You will look at biodiversity patterns and try to determine how various factors influence
species distributions.
Key concepts:
• How may we estimate biodiversity?
• Most species are rare (see definition of abundance, p. 7).
• What kinds of species are likely to be missed in censuses of diversity (definition p. 7)?
This workshop homework is due at the beginning of class on the Monday after the Friday
workshop.
(10 pts)
ESTIMATING PLANT DIVERSITY WITH QUADRATS
Imagine that the Nature Conservancy has purchased a new forest preserve. How might you go
about estimating the tree species diversity in this undisturbed forest? The area you are studying
is 10 km2, and you can’t count ALL the trees in that area. Instead of sampling the entire area,
you count the tree species in several small plots called “quadrats” distributed throughout the
study area. Then, you extrapolate from this subsample to the diversity of the entire study site.
The technique you use is the quadrat method, in which you identify and count all the individuals
in all species within a given area. The size of the quadrats in this study is 100 m long by 100 m
wide, about twice as big as a football field. In the diagram below, there are three species in the
quadrat, one species has two individuals and the other two species have only one individual.
1. Can you think of some kinds of trees that would be difficult to count using this
method? (1 pt)
1
2. The figure below shows a map of your study area with 10 vegetation-sampling quadrats
marked. A river runs through it. How might the quadrats near the river differ from the other
quadrats, in terms of diversity and species composition? List the locations that were
sampled near the river. (1 pt)
3. The side of each square quadrat is 100 m (0.1 km) long. What is the area of each quadrat?
What is the total area you sampled? What percentage of the entire study area did you
actually sample? (1 pt. Show all work in the space below.)
Complete the data sheet on page 3 by summing up the number of individuals from each species
(columns), the number of species in each quadrat (rows), and the cumulative number of species
(last [right-most] column). Complete the table and graph on p. 4, using the data from p. 3.
2
species name
Quadrat
total individuals
6
8
10
4
1
7
1
7
9
1
1
7
1
3
25
3
5
1
15
3
8
2
15
4
9
15
5
10
17
6
18
5
1
1
1
2
7
1
3
3
1
5
3
3
5
2
1
3
1
1
2
1
9
1
13
1
11
2
9
1
6
6
1
1
1
6
1
1
2
1
cumulative
total species
turpentine bush
ironwood
barrell cactus
mesquite
blue paloverde
jumping cholla
canyon ragweed
Fremont cottonwood
prickly pear
staghorn cholla
saguaro
spurge
desert hackberry
desert broom
velvet ash
golden aster
triangle-leaf bursage
creosote bush
ocotillo
foothills paloverde
Table 1. Data Sheet
3
3
7
3
4
1
9
7
7
7
9
Table 2. Species-Area curves
Complete the table below by adding in the new information for each quadrat. For example, with the first quadrat
you sampled an area of 10,000 m2 and it contained 3 species. When you add in the second quadrat, you have
sampled 20,000 m2, and you have a total of 4 different species. Plot these data on the graph.
Area
Cumulative # of
species
10,000
3
29,000
4
30,000
7
40,000
9
When you have plotted all your data points, draw a smoothed curved line through the data. Your line should begin
at the origin (0,0) and level out after a certain point.
4
4. Species distribution and abundance (definition, p. 7). (2 pts)
(refer to table 1)
How many of the species occurred in only one quadrat? _______
What percent of the quadrats is that? _________
How many of the species occurred in over half of the quadrats? _______
What percent of the quadrats is that? _________
Which species is most abundant (has the largest number of individuals)?
Return to question 1. Considering the tree species’ abundances and distributions (this
question), which kind of tree is more likely to be difficult to count, using quadrats? Had you
considered this in your original answer of question 1.
5. Compare the river-side quadrats with the upland quadrats. (1 pt)
(see the list of river quadrats in question 2)
List the plant species found only by the river.
Which habitat is more diverse (i.e., has the greater number of species unique to it), the
river-side or the upland habitat?
5
6. Based on the graph on p. 4, what is the total number of tree species are in the
study area? Estimate from the trend on the graph to the total area of ten square
kilometers (100,000,000 m2). (2 pts)
5. Estimating biodiversity for insects (1 pt)
What if you were estimating the diversity of something with many more species than
trees? The quadrat methods would be prohibitively time-consuming!. In the study you just
did, you estimated the total number of tree species using the species-area curve.
Scientists have developed other ways of estimating the number of insect species based
on known relationships. One of the best examples of this is the work by Terry Erwin (Erwin
1982) on beetles in the rainforests. He places sheets around a tree and then fogs the tree
with an insecticide, which causes all the insects in the canopy (upper level) of the tree to
fall onto the sheets, after which he collects and identifies all the canopy beetles on the
sheets. He then uses the relationships among insect abundances and ecological niches
to estimate how many insects there might be. Complete the necessary calculations using
the space below to show all your work.
160 beetle species were specialized to the canopy of the tropical tree Luehea seemannii,
and were not found on any other tree. Beetles make up about 40% of all insect species,
so a total of ___________ insect species are probably specialized to the canopy of
Luehea seemannii.
Specialized insects, found only in the canopy of one tree species represent only 2/3 of the
total number of insects found in the trees of the rainforests, so there must be a total of
___________ insect species on Luehea seemannii trees.
There are about 50,000 species of rainforest trees. If each tree species has its own
specialized fauna, like Luehea seemannii, there are a total of __________________insect
species in rainforest trees.
6
Reference cited:
Erwin, T. L... 1982. Tropical forests: Their richness in Coleoptera and other arthropod
species. The Coleopterists Bulletin, 36: 74-75.
GLOSSARY
Abundance: the number of individuals. For example, the number of plants within a given
species in the Tucson Basin.
Diversity: for a given area, or within a given category, both the number of species and their
relative abundances. For example: among two areas with equal numbers of species and
equal numbers of individuals, the area with 99 individuals in one species, and 9 species
each with only one individual is less diverse than an area with 10 species each containing
10 individuals.
7