Hint Note the use of the cex argument in the plot() function: cex is used to define
the size of an item in a graph. Here its value is a vector belonging to the spe
data frame, i.e., the abundances of a given species (e.g., cex = spe$Satr).
The result is a series of bubbles whose diameter at each site is proportional to the
species abundance. Also, since the object spa contains only two variables x and y,
the list of arguments has been simplified by replacing the first two arguments for
horizontal and vertical axes by the name of the spa data frame.
From these maps you can understand why Verneaux chose these four species as
ecological indicators. More about the environmental conditions later.
At how many sites does each species occur? Calculate the relative occurrences of
the species (proportions of the number of sites) and plot histograms (Fig. 2.4):
Species Occurrences
Number of occurrences
Number of species
0
5 10 15 20 25 30
0
2
4
6
8
10
12
Species Relative Frequencies
Frequency of occurrences (%)
Number of species
0
20
40
60
80 100
0
1
2
3
4
5
6
7
Fig. 2.4 Frequency histograms: species occurrences and relative frequencies at the 30 sites
18
2 Exploratory Data Analysis