B io F actsheet www.curriculum-press.co.uk
Number 318
Measuring biodiversity Fig. 1
Species diversity Biodiversity is a word that is commonly used in the popular media although its meaning is not always made clear. In this Factsheet we will explain: the meaning of the word biodiversity. • • how we measure species diversity. • how a knowledge of species diversity is important in the conservation of unimproved chalk grassland.
12 10 8 Total number of 6 species
What is biodiversity? Before we consider ways of measuring biodiversity, it is important to be clear what the word actually means. If we start by looking up the term in a dictionary we are likely to nd a denition along the following lines:
4
“biodiversity: the existence of a wide variety of species of living organisms in their natural environment.” This is a rather simple definition. There is much more to biodiversity than this. Biologists consider that biodiversity has three main aspects: • Species diversity At its most simple this is species richness or the number of species present in a given area. •
Habitat diversity A wider variety of dierent habitats in an area allows for much greater species diversity.
•
Genetic diversity Genetic variation exists between members of a community. In species such as tigers that have been reduced to a number of isolated small populations, genetic diversity is often very low. This can result in the accumulation of harmful recessive alleles resulting from the breeding of closely related animals.
2 0
0
10
20
30 40 Time/minutes
50
60
You can see that the curve rises steeply at rst, then levels out. The commonest species are likely to be found in every sample and will therefore be found early on. As time passes, there is a greater probability that rarer species will be seen and added to the list. Eventually, there comes a time when the curve flattens out as no more new species are seen. What this means is that we are only justified in comparing the species richness of different communities if we put the same amount of effort into sampling in terms of time spent and area covered. Q1 Use Fig. 1 to suggest the total amount of time that should be spent in sampling this garden. Explain your answer.
In this Factsheet we will concentrate on species diversity.
A1 Approximately 40 minutes as this is when the curve levels out and no new species are seen.
Species richness As we pointed out in the previous section, the simplest way to determine the species diversity of a community is to count the number of species present. No calculations are involved and it allows us to make comparisons over a period of time and from place to place.
The second problem that arises with simply counting the number of species is that this completely ignores one important aspect of the composition of a community. It misses the point that dierent species differ in their abundance. Some are common and some are rare. We will look at another example; the number of species of plants found on a lawn. Suppose our total was 15. We could have obtained the same total from two entirely different sets of data. On the one hand, we could have equal numbers of each species and, on the other, most of the species concerned might have belonged to a one species with all the other 14 represented in each case by a single specimen. Common sense would seem to suggest that the diversity in the first scenario was higher than that in the second but the species richness would be exactly the same.
Although apparently simple, it is rather more dicult in practice because we are only sampling the particular community in which we are interested. We will illustrate this point with a specic example. Every year the RSPB runs its Big Garden Birdwatch. One of the aims of this is to count the number of species of birds present in gardens on a particular weekend in January. This should give a measure of species richness, but look at Fig. 1. This graph shows the cumulative number of species of birds over successive 10 minute periods for a total of one hour.
Species richness, then, is a simple way of measuring species diversity but: • the same sampling effort must be made if communities are to be compared. it ignores the differences in abundance that exist with different • species.
1
318 Measuring biodiversity
Bio Factsheet www.curriculum-press.co.uk
Q3 The scientists used a dierent diversity index from that in the worked example to process the data collected in the investigation. What is the evidence from Fig. 2 that a dierent index was used? (1 mark )
Indices of diversity An index of diversity is a mathematical way of expressing species diversity that involves both the number of species and their abundance. There are many dierent indices but one that is usually used at A-level is calculated from the formula;
A3 The lowest values of the diversity index on the graph were less than 1 / with the index shown in the worked example, the lowest value is 1; Mathematics plays an important part in A-level Biology questions. It is important to understand the underlying principles as well as to be able to perform the calculation. If you were able to answer Q2 satisfactorily, you should have no difficulty with this question.
N (N – 1) d= Σ n (n – 1) where N = the total number of organisms of all species and n = the total number of organisms of a particular species. The symbol Σ means the sum of. Worked example Table 1. shows the number of ladybird beetles of ve dierent species found on a birch tree. Table 1. Species of ladybird harlequin seven spot two spot ten spot cream spot All species
Q4 (a) The experimental plot had fertiliser added to it. Describe how the control plot should have been treated. (2 marks) (b) Use the results for 1913 to explain the importance of the control plot in this investigation. (3 marks)
Number of ladybirds 17 2 8 3 1 31
A4 (a) Should be treated exactly the same as the experimental plot; But no fertiliser added. (b) Allows the experimental plot to be compared with untreated plot; 1913 shows a decrease in diversity in both plots; Suggesting some factor other than the addition of fertiliser was involved. Control experiments are a very important part of the design of an investigation. Make sure that you understand their purpose.
The formula for the index of diversity, d, is N (N – 1) d= Σ n (n – 1) Substituting the values from the table in the formula, (31 × 30) d= (17 × 16) + (2 × 1) + (8 × 7) + (3 × 2) + (1 × 0) d=
Q5 The scientists who analysed the data collected in this investigation suggested that the nutrients added to the soil led to a few species dominating and out-competing others. Explain how Fig. 2 supports this suggestion. (2 marks)
930 272 + 2 + 56 + 6 + 0
930 d= = 2.8 336 On its own, the value in this worked example does not mean very much but it does allow comparisons of species diversity to be made.
A5 Decrease in the diversity of plants with the addition of fertiliser; Very low diversity suggests that few species are common; High diversity maintained in control plot / plot with no fertiliser added.
Q2 What would be the value of the index of diversity if only one species of ladybird were present? Explain how you arrived at your answer.
Conservation and chalk grassland Fig. 3 Chalk grassland
A2 The value would be one because the values of N and n would be identical.
Practice question Scientists investigated the eects of adding fertiliser on the diversity of plants growing on a grassland plot. Fig. 2 shows some of their results.
Fig. 2 3 Control plot
Index 2 of diversity
Q6 Describe how you could collect the data to calculate an index of diversity for the plants on an area of chalk grassland such as that shown in Fig. 3.
1
0 1860
1880
1900
1920
1940
A6 Sample by, for example using quadrats; In order to avoid bias, quadrats should be placed at random;
Experimental plot 1960
Large number of quadrats; Record number of plants of each species present.
Year
2
318 Measuring biodiversity
Bio Factsheet www.curriculum-press.co.uk
The photograph in Fig. 3 shows chalk grassland. This is one of the most species-rich habitats in Western Europe. It is also a habitat that is disappearing rapidly and such chalk grassland that remains is becoming increasingly fragmented. Until the early 1940s much of the chalk grassland in southern England was used for low intensity grazing, a pattern of farming that resulted in a botanically diverse patchwork of vegetation with up to 40 species of plant per square metre. After 1940, more ecient farming techniques involving drainage, the application of fertilisers and pesticides, and ploughing and planting arable crops led to a marked decline in ower-rich chalk grassland. The UK has approximately 50% of the world’s surviving chalk grassland so this is of major conservation importance. Table 2. lists two insects that have decreased in number with the decline in chalk grassland. Table 2. Species
Notes
Chalkhill blue butterfy
• • •
Bee wolf
• •
Larva (caterpillar) feeds only on horseshoe vetch. Adult feeds on nectar from the owers of a variety of species. Occurs on chalk heath. Occurs where sand lies over chalk. The bee wolf is a solitary wasp that nests in burrows in sandy soils. It captures bees which it stings and drags to its burrow providing food for its larvae .
Fig. 4 Chalkhill blue butterfy
Fig. 5 Bee wolf
Before you attempt Q7, read the information in Box 1 on answering questions involving environmental issues. Q7 Use information from Table 2 to suggest why populations of each of the organisms shown in the photographs have become uncommon. A7 Chalkhill blue buttery The food plant of the larva, horseshoe vetch, is a plant of chalk grassland. As the area of unimproved chalk grassland has decreased, so has the larval food plant.
Beewolf Chalk heath disappearing so fewer nest sites; Fewer owering plants so few bees on which bee wolf preys.
Box 1. Writing about environmental issues Articles in newspapers or on the internet about environmental issues often contain dramatic and eye-catching statements. If you look critically at such articles you will often nd exaggeration and biological inaccuracy. Remember that you are a scientist and when you are answering questions relating to environmental issues, you should write as a scientist. • Avoid dramatic statements that are not founded on secure biology. It might seem a good way to nish an account about conservation of chalk grassland with a statement that, “the remaining chalk grassland will disappear completely in the next 10 years” but statements like this cannot possibly be supported with appropriate evidence. General references to “animals” and “plants” should be avoided unless the point you are trying to make applies to all of them. Back up • statements with specic examples wherever you can. • Write in appropriate biological language. Remember, organisms compete for resources; they don’t “ght each other for food”. Ploughing an area of unimproved chalk grassland removes habitats for organisms. Don’t write about removing their “homes”. Q9 Providing that the area to be managed is large enough, grazing is preferred to mowing. Explain why grazing produces a greater diversity of habitats than mowing.
Maintaining species diversity We will now look at how an understanding of species diversity allows ecologists to conserve unimproved chalk grassland.
A9 Sheep and cattle do not graze an area of grassland uniformly. They are selective in the plants on which they feed and the areas where they feed. This creates a patchwork of vegetation leading to a greater variety of ecological niches and a greater species diversity. Mowing on the other hand is non-selective and cuts the vegetation to a uniform height.
Q8 Unimproved chalk grassland must be managed by grazing or mowing. Explain what would happen if this were not were not done. A8 Grassland represents an early stage in succession. Without grazing or mowing, shrubs such as hawthorn would soon become established. Ultimately these shrubs would be replaced by trees leading to a climax community of woodland.
Fig. 6 is a diagram summarising the times of year when grazing should be carried out.
3
318 Measuring biodiversity
Bio Factsheet www.curriculum-press.co.uk
Fig. 6
J
F
M
A
M
J
J
A
S
Spring grazing
O
N
D
Autumn grazing
Q10 Grazing should be avoided in June, July and August. Suggest why. A10 During the summer months, most of the herbaceous plants ower and set seed. It is important that this seed is dispersed before grazing takes place. Summer is also the time when most insects complete their life cycles. They are also vulnerable to grazing animals. The avoidance of grazing during these summer months therefore will help to maintain species diversity. Q11 Suggest why livestock are most likely to eat brambles and woody plants in the spring. A11 Over winter, a lot of the herbaceous plants have died down. With less food available, cattle are more likely to feed on other plants.
Acknowledgements: This Factsheet was researched and written by Bill Indge Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU. Bio Factsheets may be copied free of charge by teaching sta o r s tudents, provided t hat t heir s chool i s a registered subscriber. N o p art o f thes Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136
4
