Chapter 3 Populations and interactions

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1 Chapter 3 Populations and interactions Worksheet 1: Definitions Worksheet 2: Succession *Practical 1: Investigating the distribution of plants using random sampling *Practical 2: Investigating the distribution of plants along an environmental gradient, using a transect End-of-chapter worksheet Marking scheme: Worksheet 1 Marking scheme: Worksheet 2 Marking scheme: End-of-chapter worksheet *includes Guidance for teachers

2 Worksheet 1 Definitions Standard level Match each term (1 15), relating to ecological sampling methods, with the best definition (A O). Term Definition 1 Species cover A Systematic linear sampling method 2 Sample B Measure of recording the relative abundance of organisms 3 Quadrant C A useful type of sampling method for estimating percentage cover 4 Interrupted belt transect D Sampling method involving recording the organisms touching a tape across a sampling area 5 Quadrat E Frame with pins for sampling vegetation cover 6 Divided quadrat F Quadrats laid along a line without any gaps between them 7 Biomass G Quadrats laid along a line, at intervals 8 Random number coordinates H Mean percentage of whole sample area covered by a particular type of organism 9 Point quadrat I Sampling area of known size 10 Transect J Method used to avoid bias in selecting a sampling position 11 Continuous belt transect K A quarter of a circle; nothing to do with ecological sampling! 12 Population density L Percentage of quadrat samples containing a particular species 13 Line transect M Mass of organisms in a defined area 14 ACFOR scale N A count of organisms in small fraction of an area, used to estimate the total in the area 15 Species frequency O Mean numbers of a particular species per unit area Total: 15 Score: % 3 Populations and interactions Cambridge University Press

3 Worksheet 2 Succession Standard level Read the following account of ecological succession (see also Biology 2, pages 34 36) and answer the questions that follow. Introduction An ecological community may remain stable over long periods of time. These established flora and fauna constitute an ecological climax or climax community. Whenever a climax community is partially or wholly destroyed by some environmental disaster, the climax may be restored through a series of seral stages, together known as a sere. Each seral stage is characterised by a particular community of plants and animals. The whole process is known as succession. This is defined as the gradual, directional change in the community with time, leading to the development of a climax community. Primary succession occurs when the starting point is bare ground with no living organisms present. Secondary succession occurs when soil, or even some vegetation, is present. Over time, in a suitable climate, non-woody herbaceous plant communities give way to woody perennial plant communities. Causes of succession Succession is complex and the process differs in different situations: the events after a woodland fire are not the same as those after a receding glacier. Long-term weather conditions (climatic factors) are important in all cases of succession, as is the presence of different plant species in the vicinity. An important point is that successional change is brought about by the effects of the plants themselves. One plant community changes the habitat so that it better suits another community, which then replaces the first. The principle of interspecific competition and its effects are seen during a succession. The change in the composition and properties of the soil (edaphic factors) play a key role, and the presence of soil animals, such as earthworms, will be important. On bare rock, the first colonisers will be lichens and certain mosses; these organisms are the pioneer species. Acids secreted by lichens attack the rock substratum in a process called weathering, which, along with dead organic matter from the lichens, starts the formation of a primitive soil. Soil may also be carried into crevices in the rock surface by the wind. The next colonisers to arrive will be larger species of moss, small ferns and herbaceous plants such as grasses. These in their turn will affect the habitat, causing further weathering of the rock surface and further development of the soil, so that larger herbaceous plants can become established. The later stages of succession, when a mature soil is present, involve colonisation by woody perennial species (shrubs and trees). On bare soil, such as a ploughed field, the first plant colonisers are likely to be herbaceous annual plants, or weeds. These have efficient methods of seed dispersal and are fast-growing but short-lived. At first, the only animal populations present will be those of the soil community. Climax communities The climax community of each region depends primarily on the climate, but the character of the climax is strongly influenced by the edaphic and other factors, such as exposure to fire, wind and grazing pressure. Succession will continue until the establishment of any new species no longer changes the environment for the community, at which point a stable form of the ecosystem, incorporating the climax community, is formed. At this stage, the death of old trees is counterbalanced by the growth of younger ones. As with each seral stage, the climax community contains one or more dominant species ( dominant often meaning the most abundant). In tropical regions, climax forests often include hundreds of different species, without conspicuously abundant forms. The climate of arid and semi-arid regions does not permit the development of forests. Here, climax communities consist of grasses or shrubby vegetation. 42 Cambridge University Press Populations and interactions

4 Succession without climax communities There are a number of situations in which succession can take place without leading to a climax community. Human intervention is often a cause of this. For example, in farmed areas, a natural succession can be prevented by ploughing or by keeping grazing animals. In this case, the result is known as a plagioclimax. 1 What is meant by flora and fauna? [1] 2 Define the term community. [2] 3 At which stage of the succession is the climax community? [1] 4 What is the name given to the complex sequence of communities in a succession? [1] 5 What is the main difference between a primary and a secondary succession? [1] 6 Which type of succession, primary or secondary, will generally take longer to lead to a climax community? [1] 7 Which type of succession will take place after the following events? Explain why. a A woodland fire that destroys all vegetation b A volcanic eruption covering an area with lava and ash [4] 8 State two examples of climatic factors. [2] 9 What is meant by a pioneer species? [1] 10 A lichen is an example of an association between a fungus and an alga, in which both organisms benefit from the association. What term is used to describe this relationship? [1] 11 What is the name given to dead organic matter in the soil? [1] 12 Give two examples of edaphic factors. [2] 13 In a succession starting from colonisation of bare rock, what type of plants usually replace lichens? [1] 14 Define the term interspecific competition. [1] 15 Which stage in a succession is the most stable? [1] 16 Compare the climax community of a tropical rain forest with that of a semi-arid region. [2] 17 What effect will excessive grazing of an area have on succession? [1] 18 Which of the following are herbaceous plants: grass, shrub, tree? [1] 19 In the table below, match each seral stage with the correct time scale. Seral stage Time scale (years) i Young broadleaved woodland A ii Closed tall herb community B 150+ iii Scrub vegetation C iv Open pioneer community D 3 15 v Mature mixed deciduous woodland E Pioneer plants on sand dunes show xeromorphic features. What is the meaning of this term? [1] [1] Total: 27 Score: % 3 Populations and interactions Cambridge University Press

5 Practical 1 Investigating the distribution of plants using random sampling Apparatus and materials two 10 metre measuring tapes, or string with knots at 1 metre intervals stout pegs (if using string) 0.5m 0.5 m (0.25 m 2 ) frame quadrat random number table (or calculator with random number function) plant keys or identification sheets (optional) Introduction In a habitat that is relatively uniform, such as a school field, random sampling can be used to investigate the abundance of plant species present. Quadrats are placed randomly in the area to be sampled. This is done by using random numbers to generate coordinates. Throwing quadrats is not random! Safety The nature of the areas to be sampled should be considered in the risk assessment for this investigation. Procedure 1 Mark out an area of ground 10 m 10 m in the chosen habitat, using measuring tapes or string. 2 Generate a pair of random numbers (from 0 to 9) to supply the coordinates for placing the first quadrat. 3 Locate the coordinates in the sampling area and place the quadrat in position. 4 Count the number of different species of plants within each quadrat. It helps if you can identify the species (use a plant key) but, if not, just call them species A, species B etc. There is no need to count the total numbers of each species. 5 Select a common species for the next part of the investigation, such as daisy, dandelion or plantain (not grass!). Let s call the species that you have selected X. 6 Record the total numbers of individual plants of species X within the quadrat. 7 Estimate the percentage cover of species X within the quadrat and convert the percentage to the Braun Blanquet scale using the following table. % cover Braun Blanquet scale < Cambridge University Press Populations and interactions

6 8 Repeat steps 2 6 another nine times and record your results in a table like this one. Quadrat Number of Presence/absence Number of Cover on the species of species X individuals of Braun Blanquet in quadrat species X scale (1 6) If there is enough time, repeat the investigation in a different area, for example a different part of the school field. Try to select areas with visible differences in vegetation. 10 Calculate the mean species richness (average number of species per quadrat). 11 For species X, calculate the species frequency, the mean species density and the mean cover on the Braun Blanquet scale, using the following equations: Species frequency = % of quadrats in which the species was found sum of number of individuals in quadrats Mean species density (per quadrat) = number of quadrats cover per quadrat Mean cover on the Braun Blanquet scale = number of quadrats 12 If you have had time to repeat the sampling in two areas, compare the differences in plant distribution between the two habitats. Can you think of any reasons why the two areas might be different? Consider both biotic and abiotic factors. 13 You may be able to use a statistical test to compare the species richness in each sampling area. A suitable test to use is the Mann Whitney U-test. 3 Populations and interactions Cambridge University Press

7 Guidance for teachers Teachers should visit the field sites to identify suitable species to record and to select two areas for comparison. Keys or identification sheets should be prepared that show all the common plant species found in the area. A simple identification sheet can be made by photocopying leaves of the common species. The names can then be written onto the photocopy paper and the sheets laminated. The Field Studies Council also publishes identification charts ( The Braun Blanquet scale is a relatively simple abundance scale with only a few, large categories. It is particularly suitable for use in species-rich communities. The Mann Whitney U-test is suitable for comparison of the species richness in the two habitats because it compares the median values of small samples of data that do not fit a normal distribution. Student s t-test is not suitable. Free Merlin statistics software (using Excel) is available from: If the total number of plants of each species is recorded, the two areas could be compared using a diversity index. Always refer to the departmental risk assessment before carrying out any practical work. See the Notes on practical activities, found in the Additional resources section, for additional guidance. 46 Cambridge University Press Populations and interactions

8 Practical 2 Investigating the distribution of plants along an environmental gradient, using a transect Apparatus and materials 20 metre measuring tape (or length suitable for area of study) point quadrat plant keys or identification sheets Introduction A transect is a systematic sampling method used to study the distribution of organisms across an environmental gradient, where there is a change from one habitat to another. An example of an environmental gradient is the change from a fully terrestrial habitat, such as a field, to an aquatic one, such as a pond. Here, the habitat gradually changes, passing through an intermediate marshy area. Another example is across a path, where the habitat changes from grass to a trampled area and back to grass again. Safety The nature of the area to be sampled should be considered in the risk assessment for this investigation. Procedure 1 Select a suitable area for investigation and place the measuring tape across the area. 2 Insert the point quadrat in the ground at the start of the tape, at right angles to the tape. 3 Drop a pin through the first hole in the point quadrat. Record any plant species that the pin touches, using a suitable table. For each pin drop, record only one hit per species, regardless of the number of times a pin touches that species. 4 Place the pin in the second hole and repeat step 3. Repeat this procedure until all ten holes have been used. For each species, you will have a maximum of ten hits. 5 Move the quadrat further along the tape, to the next sampling station (the distance will depend on the size of the sampling area to be investigated). 6 Repeat steps 2 5 at regular intervals along the tape. 7 Combine your results with those of other groups in order to get a bigger sample size. 8 Present the group results as bar charts to show the distribution of representative species across the line of the transect. 9 Answer the following questions. a b c d e Describe any change in the plant community along the environmental gradient. Can you explain any trends that are apparent? Explain the difference between a continuous transect and an interrupted transect. What are the advantages of using a point quadrat, rather than an area (frame) quadrat, when carrying out a transect? What are the disadvantages of using a point quadrat? 3 Populations and interactions Cambridge University Press

9 Guidance for teachers Teachers should identify a suitable field site, such as a trampled path through the school field or a gradient from dry to marshy ground. Keys or identification sheets should be prepared that show all the common plant species found in the area. A simple identification sheet can be made by photocopying leaves of the common species. The names can then be written onto the photocopy paper, and the sheets laminated. The Field Studies Council also publishes identification charts ( Always refer to the departmental risk assessment before carrying out any practical work. See the Notes on practical activities, found in the Additional resources section, for additional guidance. 48 Cambridge University Press Populations and interactions

10 End-of-chapter worksheet 1 The diagram below shows the growth curve for a population of bacteria in a flask containing a nutrient broth C Number of cells ( 10 6 cm 3 ) 200 D 100 A B Time (hours) a Name the four phases of growth labelled A, B, C and D. [4] b Explain why there is no increase in population during growth phase A. [1] c What is happening to bacterial numbers during phase B? [1] d During phase C, bacterial numbers are constant. State two reasons for this. [2] 2 In relation to factors affecting population size, explain what is meant by the following terms: a a limiting factor; [1] b the carrying capacity of a habitat; [2] c a predator prey relationship. [2] 3 Competition between organisms may be intraspecific or interspecific. Interspecific competition only occurs when the ecological niches of the two species overlap. a Explain the difference between intraspecific and interspecific competition. [2] b Define the term ecological niche. [1] c Explain why the niches of two species must overlap if interspecific competition is to occur. [2] 3 Populations and interactions Cambridge University Press

11 4 The diagram below shows a primary succession on sand dunes. dune scrub woodland sea pioneer species marram grass community stable dune heathland a Explain the meaning of primary succession. [2] b What is a pioneer species? [1] c Marram grass (Ammophila arenaria) has an extensive root system and shows xeromorphic features. i Suggest two advantages of the extensive root system. [2] ii Explain the meaning of the term xeromorphic features. [1] d One species found growing in the stable dune heathland is restharrow (Ononis repens). This is a legume with root nodules containing nitrogen-fixing bacteria. Explain why this is an advantage: i to the restharrow; [2] ii to other plants growing in the stable dune heathland. [2] e Describe how you could carry out an interrupted belt transect to sample the distribution and abundance of plants across the area shown in the diagram of primary succession. [5] f What is the name given to the final stage of the succession, represented here by the woodland community? [1] 5 Overuse of nitrate fertilisers may result in the eutrophication of rivers and lakes, with harmful effects upon the communities living there. Explain the sequence of events that takes place resulting from such overuse of fertiliser. [5] 6 Coppicing is a traditional woodland management system, allowing sustainable timber production. Trees such as hazel are coppiced over a rotational cycle of several years. a Describe how a tree is coppiced. [2] b What is meant by sustainable timber production? [2] c Explain why a rotational cycle of coppicing increases woodland biodiversity. [2] Total: 45 Score: % 50 Cambridge University Press Populations and interactions

12 Marking scheme Worksheet 1: Definitions 1 H 2 N 3 K 4 G 5 I 6 C 7 M 8 J 9 E 10 A 11 F 12 O 13 D 14 B 15 L [15] 3 Populations and interactions Cambridge University Press

13 Marking scheme Worksheet 2: Succession 1 Plants and animals [1] 2 All the organisms; living in a (particular) habitat [2] 3 The last stage [1] 4 Sere [1] 5 A secondary succession starts on soil, a primary succession on bare rock / no soil [1] 6 Primary [1] 7 a Secondary; soil is present after the fire / seeds or roots present in soil b Primary; no soil is present / soil covered with ash and lava/rock [4] 8 Precipitation/rainfall/snow; wind / air movements; temperature; humidity[max. 2] 9 An organism that is first to establish itself in an area [1] 10 Mutualism/mutualistic (accept symbiosis) [1] 11 Humus [1] 12 (soil) ph/water/temperature/air/minerals/clay/humus (must relate to soil) [max 2] 13 Mosses [1] 14 Competition between members of different species [1] 15 Climax community[1] 16 Rainforest has great species diversity / more than one dominant (tree) species / trees are dominant species. Semi-arid region has lower diversity / one or few dominant species / grasses or shrubs are dominant species. [2] 17 Interrupt succession / prevent climax community forming [1] 18 Grass [1] 19 i C ii D iii A iv E v B [1] 20 Adaptations to low water availability[1] 52 Cambridge University Press Populations and interactions

14 Marking scheme End-of-chapter worksheet 1 a A = lag phase; B = log/exponential phase; C = stationary phase; D = decline/death phase [4] b Bacteria are adjusting to conditions in the broth / synthesising enzymes to metabolise the nutrients [1] c They are doubling in equal time intervals / etc. [1] d Number of bacteria produced is equal to number of bacteria dying; nutrients used up; waste products accumulating [max. 2] 2 a A factor that stops a population from increasing in size [1] b The upper limit to a population size; that a habitat can support [2] c The size of a predator population depends on the size of its prey population; and vice versa [2] 3 a Intraspecific is competition between organisms of the same species; interspecific competition occurs between organisms of different species [2] b The role of a species in an ecosystem [1] c They must compete for the same resources/food etc.; so must partly share the same role in the ecosystem [2] 4 a Succession starting on bare ground/rock; with no soil present [2] b The first species to colonise an area [1] c i Allows plant to collect more water; roots stabilise sand (for plant growth) [2] ii Adaptations that allow a plant to grow in habitats where water is in short supply[1] d i Bacteria fix nitrogen gas into ammonia; which plant uses for making amino acids/proteins/for growth [2] e f ii When restharrow dies; its organic matter will enter the N cycle / produce nitrates for other plants [2] Use quadrats; suitable size of quadrat (e.g. 0.5 m 0.5m or 1m 1 m) / use point quadrat; placed along a line (through the area); at intervals; identify plant species in quadrat/touching pins of point quadrat; count numbers of each species; estimate percentage cover of each species [max. 5] Climax community[1] 5 Nitrate run-off / washes into rivers or lakes / reference to leaching; causes growth of algae / aquatic plants / algal bloom; layer of algae blocks light from plants/algae below; plants cannot photosynthesise / die; dead plants decomposed by (aerobic) bacteria; bacteria use up oxygen in water / increase BOD; animals / named animals / fish die. [max. 5] 6 a Cut down near base / forming stool; shoots/poles grow from stool/stump [2] b Timber is removed from the forest at the same rate as the trees are re-grown; over long periods of time / for (many) years; maintains forest ecosystem. [max. 2] c Opens up woodland / allows more light in; allows growth of ground layer / herbaceous plants / or equivalent; rotation means trees at different stages of growth, so produces variety of habitats. [max. 2] 3 Populations and interactions Cambridge University Press