Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
430
Estimating numbers of mobile animals
Quadrats are obviously no use for finding or counting
mobile animals, so different methods have to be used
for these.
Small mammals, such as mice and voles, can be caught
in traps that are filled with hay for bedding and suitable
food as bait. Insects and other invertebrates, such as
spiders, can be captured by sweep netting. Pond nets are
used for sampling aquatic organisms. The techniques for
this vary according to the size of the body of water, and
whether it is still or moving. Single birds can be counted
quite easily, although this does become more difficult in
dense forest. Flocks of birds are much more difficult to
count, although it can be done by counting a group of ten
birds and estimating how many such groups there are.
A good method of estimating the population size of
mobile organisms, if used with care, is the mark–release–
recapture technique. First, as many individuals as possible
are caught. Each individual is marked, in a way that
will not affect its future chance of survival. The marked
individuals are counted, returned to their habitat and left
to mix randomly with the rest of the population.
When enough time has elapsed for the mixing to
take place, another large sample is captured. The number
of marked and unmarked individuals is counted. The
proportion of marked to unmarked individuals is then
used to calculate an estimate of the total number in the
population (Worked example 1). For example, if you find
that one-tenth of the second sample was marked, then
you presume that you originally caught one-tenth of the
population in your first sample. Your best estimate is
therefore that the number in the population is ten times
the number you caught and marked in your first sample.
WORKED EXAMPLE 1
The mark–release–recapture technique
Brown planthoppers are a serious insect pest of rice.
Some students used sweep nets to catch a large sample
of planthoppers in a field of rice. Each animal was marked
with a very small spot of non-toxic waterproof paint and
then they were released across the field. The next day, a
second large sample was caught.
Number caught and marked in first sample = 247
Number caught in second sample = 259
Number in the second sample that had
been marked = 16
247 × 259
So the estimated number in the population =
16
= 3998
Simpson’s Index of Diversity
When you have collected information about the
abundance of the species in the area you are studying, you
can use your results to calculate a value for the species
diversity in that area. We can do this using Simpson’s
Index of Diversity, D. One formula for this is:
n
D = 1 − ∑
2
N
where n is the total number of organisms of a particular
species, and N is the total number of organisms of all
species (Worked example 2).
Values of D range from 0 to 1. A value near 0 represents
WORKED EXAMPLE 2
Simpsonʼs Index of Diversity
A sample was made of the animals living on two rocky
shores. 10 quadrats were placed on each shore, and the
number of animals of each species in each quadrat was
counted. The results are shown in the table.
Species
Number of individuals, n
Shore A Shore B
painted topshells 24 51
limpets 367 125
dogwhelks 192 63
snakelocks anemones 14 0
beadlet anemones 83 22
barnacles 112 391
mussels 207 116
periwinkles 108 93
total number of
individuals, N
1107 861
To determine Simpson’s Index for shore A, calculate
n
for each species, square each value, add them up and
N
subtract from 1. Repeat the procedure for shore B.
Species
Shore A
n
n
N
n
2
N
painted topshells 24 0.022 0.000
limpets 367 0.332 0.110
dogwhelks 192 0.173 0.030
snakelocks anemones 14 0.013 0.000
beadlet anemones 83 0.075 0.006
barnacles 112 0.101 0.010
mussels 207 0.187 0.035
periwinkles 108 0.098 0.010
total number of
individuals, N
1107
∑
n 2
N
For shore A, Simpson’s Index of Diversity (D)
= 1 − 0.201 = 0.799
= 0.201