Environmental Science
FACT SHEET www.curriculum-press.co.uk
Number 065
Investigating Sand Dunes Sand dunes are important habitats. In the exam, questions on sand dunes may test your understanding of succession, zonation, conservation management strategies or ecological sampling. You may decide to investigate sand dunes for your coursework. If so, remember that Environmental Science investigations must always have human impact as a central theme. So, if you are planning to do your investigation on sand dunes, you should be focussing on impacts of tourism or management techniques, for example.
This Factsheet summarises: • • •
These dunes increase in height as the plants grow, thus further decreasing wind speed. Marram grass is a very tough grass and can withstand desiccation, high salt levels and even burial. It is therefore, an extremely important plant in the formation of dunes.
the key features of sand dunes the practical approaches you might use, or be tested on, in exam questions recent exam questions and the most common mistakes students make in doing their own investigations and in exam questions
Marram grass has xerophytic features ie features that allow it to withstand desiccation (Fig 1).
How dunes form When the tide is low, sand bars become exposed. The wind can blow sand off these bars on to the beach. Dunes will form behind any object that slows down the sand-bearing wind. The leaves of plants such as marram grass (Ammophila arenaria) are very effective at slowing the wind so that small mounds of sand build up around the base of the plant.
Succession As a community develops, it alters its physical environment eg. root growth changes the physical and chemical nature of the soil by aerating it and adding exudates etc. The changed environmental conditions which result actually favour the colonisation and growth of different species which eventually give rise to a new community. This process of change is called succession.
Fig 1. Marram grass hairs reduce air movement/traps water vapour/traps air/increases relative humidity so there is less of a diffusion gradient for water vapour
Within the dunes, the initial organisms provide organic material for soil development. As this slowly continues, it enables the establishment of more advanced plant species, which in turn provide more organic material and encourages further soil development. As nvironmental conditions become more favourable, particularly due to soil accumulation, the level of primary production and so biomass increases. Species diversity also increases rapidly, with each successional stage having a dominant species, which is either the largest or the most abundant plant.
Section of a leaf of marram grass.
leaf rolled –this reduces external (exposed) surface area/increases relative humidity
The decline in species diversity approaching climax occurs because the dominant species out-competes other species.For example, oak trees in deciduous woodlands out-compete other species for sunlight, water and nutrients.
thick cuticle reduces evaporation/ diffusion of water/water loss
Sand dune succession Various zones can be recognised in a set of sand dunes which may represent different stages of succession (Fig 2)
Fig 2. 1 Embryo dunes – small scattered patches of marram grass which are largely selfseeded or growing from rhizomes which are up to 3m long. Only a very few species -Pioneer species can cope with the adverse conditions -salty, dry, nutrient-poor, shifting sand, intensely hot in Summer, cold in Winter
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sea
2 Mobile dunes (yellow dunes) – some large areas of bare, moving sand but greater cover of marram grass.
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4 Fixed dunes – almost complete vegetation cover. Marram grass is sparse and is only found in isolated clumps before utlimately disappearing. Many other species of plants are present
5 Dune slacks – areas which develop where the sand becomes eroded so that the water table is reached. The sand forms a damp depression at low level and the area is prone to flooding in winter. Large numbers of rabbits keep the vegetation short but cause damage in ‘blow-outs’ where bare sand is exposed.
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4 3
6 Climax – sand dunes normally develop into scrub then woodland
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3 Semi-fixed dunes (or grey dunes) – smaller patches of bare sand with a greyish tinge. Many plants besides marram grass. increasing age of dunes
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Environmental Science
Factsheet 065 - Investigating Sand Dunes
Trampling
Typical Exam Question
Marram grass cannot, however, tolerate trampling and the tufts of marram grass can be damaged, exposing bare sand. Thus there is a reversal of succession and the ecosystem goes back a stage or two in its development. A large hollowed out depression of bare sand, a ‘blow-out’, can develop and the sand dune is in danger of being completely eroded.
Explain why the concentration of nitrates is much higher on the fixed dunes than at the embryo dunes (4 marks).
Answer The fixed dunes have more organisms/plants/animals; Dead plant material /humus is converted to nitrate by soil bacteria; Plant material decomposed by saprophytes/ saprobionts; Organic molecules containing nitrogen / protein converted to ammonia; Involving ammonifying bacteria; Ammonia to nitrite; nitrite to nitrate; Involving nitrifying bacteria;
Managing tourists Limiting access to the dunes by: • fencing off sensitive areas; • closing some paths/areas; • providing wooden ‘boardwalks’ along the most used paths through the dunes to encourage visitors to use these routes and avoid others; • charging for car parking and higher charges at peak times.
Gross primary productivity (GPP) – the total amount of organic matter produced or the solar energy fixed by photosynthesising plants per unit area per unit time – is very low initially as there are very few photosynthesizing organisms (Fig 3).
Education of the public and supervision: • Visitor Centres, information boards, leaflets, guide books, guided walks; • warden patrols in the summer months; • restricting the number of students working in the dunes and the area used for study.
But GPP increases rapidly through the pioneer stage as colonization by photosynthesizing plants increases. The structural complexity or number of layers increases and GPP is usually at its maximum at the climax stage. GPP is limited at climax by abiotic factors e.g. nutrients, sunlight, temperature and water. Sometimes, GPP declines slightly as older/more woody plants dominate.
Providing alternatives: advertising other attractions in the area to encourage the public to go elsewhere.
Similarly, species diversity may fall at the end of the succession as a result of one or a few species of large tree outcompeting most other plant (and therefore some herbivorous) species for light/ mineral ions etc.
It isn’t just the plant species that change as you move from the embryo dunes to the far end of the dunes – soil conditions and animal species change dramatically too. Your understanding of these changes is commonly tested in exam questions.
Fig 3 Productivity and diversity during succession
Table 1 shows the results of part of an investigation on sand dunes. The table shows how some soil factors change across the dunes.
Table 1 Changing soil conditions across the dunes Embryo Mobile Semi-fixed Fixed Dune slack 0.2
0.4
0.4
0.4
1.5
pH
9.0
8.8
8.8
8.8
7.5
17.0
18.0
18.0
18.0
16.5
0.7
0.8
0.8
0.8
0.3
130.0
25.0
25.0
25.0
16.0
Soil temperature /oC CaCO3 /% dry mass Conductivity /µS cm-1
primary production
Numbers
organic matter/ % dry mass
biomass
species diversity
(Pioneer)
Note the key points Organic matter increases because there is an increase in the number and biomass of organisms. Some of the animals excrete substances into the soil, but they all end up dying and decomposing –adding organic matter.
Time (climax) increasing age of dunes
Similarly, the concentration of other useful breakdown products such as nitrogen-containing compounds will also increase. The amount of detail that you go into needs to reflect the marks available. For example:
Many other abiotic factors will change over the course of the dunes:wind speed, soil moisture content, infiltration rates of soil, air temperature, mineral content, soil compaction, humidity, soil texture, gradient/slope angle etc. All of these could be asked about in the exam!
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Environmental Science
Factsheet 065 - Investigating Sand Dunes
You don’t even have to be able to identify each species, just identify how many different ones there are.
Practical skills: succession in dunes Belt transects should be used. You must be able to explain why this technique is the most appropriate: • A transect records changes (in vegetation)/zonation/ref to environmental gradient • It gives more information than a line transect • Random quadrat sampling might miss an entire part of the area or provide much more data about some areas than others
Typical data is shown in Table 2. A belt transect was used from the high tide line, inland across sand dunes, to a woodland behind the dunes. Quadrats were taken at five metre intervals. The dominant species in each quadrat were recorded. quadrat sand marram number couch grass grass
The transect can be continuous –each quadrat is placed immediately in front of the last one –or interrupted, if the distance to be covered is too great for continuous quadrats You must also be able to describe in detail how you would actually carry out the belt transect: Here is a recent Markscheme for an exam question on this: Mark out transect from beach/embryo dunes to fixed dunes/ slack; States suitable size of quadrat - minimum 0.25m2 quadrat position/sampling at regular intervals/interrupted belt transect; appropriate comments on number of sampling points; appropriate measurements of: distance from shore; % cover / frequency or density; number of species/species diversity; measurement of abiotic factors;
+ +++
3 4 5
++
6 7 8
+ ++ + +++ +++ ++
+
9 10 11
++ +++ +++
+ + +++
12 13 14
+++ +++
+++ +++ +++
15 16 17
Extract: Chief Examiner’s Report In very few cases was there any explanation and justification of the selection of the actual sites for dune transects. It isn’t enough to justify why you used a belt transect, we need to know why you did it in that particular stretch of dune. Typically, you will need to present plant data in the form of kite diagrams(Fig 4)
Fig 4 Distribution of species across dunes
+++ +++ +++
18 19 20
+ + ++
21 22
+
key 50
% of vegetation sampled at each station
1 2
lichens heather rye birch oak and grass mosses
+ + ++
+ ++ +++
increasing abundance of palnts concerned
0
Species A
This technique has both advantages and disadvantages Advantages It can be used with any species (irrespective of size); You don’t need to distinguish one individual from another; It is quick;
50 0
Species B
50 Species C
0
Disadvantages
50
It is subjective; The dominant species may be over-estimated;
0
Species D
Despite the subjective nature of the data, it is still possible to convert the data into useful kite diagrams.
50 3 4 5 6 7 8 9 10 station number along transect Seaward Landward 1
2
To do this you would need to: Convert your ACFOR scale into a numerical scale; Record each reading at each site on to graph paper; width of diagram related to ACFOR (maybe shown on diagram); points from each site joined together; repeat for each species found present;
Sometimes, in order to save time/money, a quick technique known as ACFOR is used. This involves making a quick, subjective estimate of the abundance of each species in each quadrat. You don’t actually measure percentage cover or count the individual plant species.
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Environmental Science
Factsheet 065 - Investigating Sand Dunes
Beetles were sampled at point D using the mark-release-recapture method (Lincoln Index). 30 beetles were trapped on the first occasion and marked. These were released into the study area and a further sample collected 24 hours later. The second sample yielded 20 beetles of which 4 bore marks from the first capture.
Typical Practical technique exam questions Describe a method used to investigate (i) % soil organic matter • Mass of sample recorded (W1); • Heat at 1200C to remove water • Reweigh • Continue until to constant weight achieved (W2); • Heat in oven (to burn off organic matter); to constant mass (W3); • W3 - W2 × 100 W1 (ii) soil pH • stated soil volume / mass; • standardised soil depth; • suitable technique for measuring pH (universal indicator/ pH meter / electronic probe); • appropriate method of use;
Question 1 Estimate the number of beetles in the area using the formula n1 × n2 population = nm where n1 = number of marked indivuals released n2 = number in second sample nm = number marked in second sample Answer: 30×20/4 =150
Question 2 Describe precautions that should be taken in carrying out this technique to ensure a reliable estimate.
Calculations You must practice using two formulae: species diversity index and Lincoln Index (mark-release-recapture technique). In both cases, you will be given the formulae but you need to be able to put the numbers in the right place.
Answer Marking non-toxic / not conspicuous / doesn’t alter behaviour/ will not rub off; allow enough time for sample to mix randomly; interval not too long to allow significant births/deaths/migration; exactly same method used for collecting 2 samples; over same time period;
Species Diversity Index Fig 5 shows a transect across a sand-dune ecosystem and the number of plants found in samples taken at point B.
Question 3
Fig 5
Suggest why the Lincoln Index may be unreliable as a method of estimating populations of small mammals.
D A
B
Species at point B
Answers
C
Mammals too intelligent to get trapped twice/’trap shy’; may actively seek out traps as source of food and shelter/’trap happy’; difficulties of marking procedure; migration possible/cover large territories;
Number of plants in sample
Marram grass
36
Ling
4
Bell heather
2
Gorse
1
Sometimes the natural progression of plant communities is interrupted and the climax community is not reached; succession is said to have been deflected. You may be asked to suggest what could have caused this deflection: • •
The index of diversity may be calculated from the formula
• • • •
N(N-1) d= Σ n(n-1) where N = total number of organisms of all species and n = total number of organisms of each species.
Human recreation –trampling/building etc climate change – higher average temperatures altering species composition or more violent storms, for example rise in sea level/flooding grazing/mowing burning conservation management techniques such as removal of certain species
Know your limitations and problems!
Question
At A level, examiners do not expect you to be a skilled, professional ecologist. They do expect you to be able to identify reasons why your investigation might be flawed. It does not matter that it is flawed, you actually get a lot of marks for telling us why it’s flawed.
Use the data in the table to calculate the index of diversity for this sample.
Answer 43×42 (36×35)+(4×3)+(2×1)+(1×0) = 1.41
Imagine you had carried out a belt transect across the dunes. You have carefully recorded and analysed all your data. You have drawn lovely kite diagrams and done species diversity calculations. Why might your results/conclusions be flawed/misleading?
d=
Often, you will be asked to do this at two different areas of the dune and then explain the difference.
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Environmental Science
Factsheet 065 - Investigating Sand Dunes
Here’s some possible reasons: • You have insufficient replicates/you needed more transects. • The transect you did might just have coincided with an underground stream of pollution that no-one knows about. • Other areas of dunes may have different patterns of vegetation. • Your data is not representative of any other area of the dunes! • Your transect was not continuous and you missed key data • Your estimation of % cover is subjective. If you looked at the same quadrat tomorrow even you might disagree with yourself! • It’s difficult to count individual plants, especially of species such as marram grass • It’s difficult to measure fluctuating environmental conditions (e.g. wind) • You estimated rabbit populations by counting droppings. • The dunes may have been subjected to recent, but unrepresentative pressures from large numbers of visitors • You couldn’t survey some areas because of protective designations eg NNR; • The limitations of using Lincoln index/pitfall traps • Seasonal changes in animal/plant populations • Seasonal changes in abiotic factors • Inaccuracy/calibration of apparatus used to measure abiotic factors • Explanation of these factors – if they were true of your work- will gain marks.
2. The kite diagram shows the results of a transect across a sand dune system in Wales.
% of vegetation sampled at each station
50
No. of plant species per unit area
Woodland
25
90
100
100
12
36
95
140
92
50 Species C
0 50 0
Species D
50 3 4 5 6 7 8 9 10 station number along transect Seaward Landward 1
2
1. (a) Woodland; (b) growth of large trees / tall producers; allows them to outcompete other plant species; for light/mineral ions; reduced range of niches/habitat; fewer/smaller herbaceous plants can grow (c) 50×83/13 = 319; [A 319.23]
1. The results of a student’s investigation across sand dunes, from dunes near the sea-shore to woodland, are shown in the table.
2
0
Species B
Answers
Practice Questions
Mean % plant cover
50
(a) Describe the changes in the vegetation across the dunes. (3 marks) (b) Suggest reasons for these changes (4 marks)
Extract from Chief Examiner’s report However, limitations should be developed from a genuine, constructive reflection on a thorough and rigorous investigation and not as a means of redemption by offering a review of weaknesses or omissions that should have been considered and overcome at the piloting or main study stage.
Mobile Fixed Heath Dunes dunes dunes & scrub near sea-shore
0
Species A
2. (a) Only species A and B abundant close to sea; as distance inland increases the range of species increases; Species C dominant inland; (b) Species A probably pioneer species; Can tolerate harsh conditions; Salty/low in OM; Primary succession; are harsh, soils are Species diversity increases inland; Soil depth, structure/OM content/mineral content improving by death and decomposition of plants; More shelter;
(a) Identify the climax community (1 mark) (b) Suggest why the number of plant species per unit area in the woodland is less than that in the heath and scrub.(2 marks) (c) The student used pitfall traps to carry out a mark-releaserecapture investigation on the woodlice population (small invertebrates) in the woodland. Number of woodlice Number of woodlice Number of marked orginally marked (n1) second sample (n2) woodlice recaptured in second sample (nr) 50
83
13
Acknowledgments: This Factsheet was researched and written by Kevin Byrne. Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU Environmental Science Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. ISSN 1351-5136
Use the student's results to estimate the population of woodlice on the dunes using the formula: n1 × n2 Show your working. population = nm
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