Cambridge International A Level Biology Revision Guide

Chapter 18: Biodiversity, classification and conservation
100 smaller squares. You then decide approximately what
percentage of the area inside the quadrat is occupied by
each species. These percentages may not add up to 100%.
For example, there might be bare ground in the quadrat,
so the numbers will come to less than 100%. Or there
may be plants overlying one another, in which case the
numbers may add up to more than 100%. An alternative
to estimating percentage cover of each species is to use
an abundance scale, such as the Braun–Blanquet scale for
number and plant cover (Table 18.1).
QUESTIONS
18.4 A survey gave the following results for a species
of the red sea anemone, Isactinia tenebrosa
(Figure 18.10), on a rocky shore in New Zealand,
using a quadrat with an area of 0.25 m 2 .
Quadrat 1 2 3 4 5 6 7 8 9 10
Number
of red sea
anemones
0 3 0 1 0 0 5 2 0 1
a Calculate i the species frequency, and ii the
species density of I. tenebrosa from the results of
this survey.
b Suggest when it might be more appropriate to
use species frequency rather than species density
to record the abundance of a species.
18.5 A survey was made of Benghal dayflower,
Commelina benghalensis, growing on a lawn and in
a field of young soybean plants. Ten 1.0 m 2 quadrats
were placed randomly in each area, and the number
of dayflower plants in each quadrat was counted.
The results are shown in the table.
Quadrat 1 2 3 4 5 6 7 8 9 10
Number of
dayflowers
on lawn
0 0 4 3 0 1 2 4 0 3
429
Figure 18.9 Estimating percentage cover. This 1 m 2 quadrat
is divided into 100 small squares to make it easier to make the
estimation for each species.
Number of
dayflowers
in field
0 0 0 2 5 0 0 1 0 0
Braun–Blanquet Cover Scale
Description
Value
Very few plants, cover is less than 1% +
Many plants, but cover is 1–5% 1
Very many plants or cover is 6–25% 2
Any number of plants; cover is 26–50% 3
Any number of plants; cover is 51–75% 4
Cover is greater than 75% 5
a Calculate i the species frequency, and ii the
species density of dayflower plants in each of the
two areas.
b Explain why it is important to use randomly
placed quadrats.
c Suggest two disadvantages with calculating
percentage cover or using an abundance scale,
such as the Braun–Blanquet scale.
Table 18.1 The Braun–Blanquet scale for recording
vegetation within quadrats.
Figure 18.10 A red sea anemone that lives between the tides
on rocky shores.