14.2 Applications and skills 14.2.1 Using models: Limits of tolerance and the niche 14.2.2 Pyramids, energy and nutrient diagrams 14.2.3 Case studies: Human impacts on ecosystems 14.2.4 Field techniques: Using Transects 14.2.5 Lab: Modelling population growth 14.2.6 Lab: Nutrient content of soils (HL) Site: Philpot Education Course: Biology Support Site Book: 14.2 Applications and skills Printed by: Guest user Date: Thursday, 22 November 2018, 9:22 AM In this unit 14.2.1 Using models: Limits of tolerance and the niche 14.2.2 Pyramids, energy and nutrient diagrams 14.2.3 Case studies: Human impacts on ecosystems 14.2.4 Field techniques: Using Transects 14.2.5 Lab: Modelling population growth 14.2.6 Lab: Nutrient content of soils (HL) 14.2.1 Using models: Limits of tolerance and the niche Students often find it difficult to understand what is meant by ‘ecological niche’. A niche is not a physical place or role. Rather, a niche considers the sum of the habitat requirements, both biotic and abiotic, for each species. Limits of tolerance and zones of stress A single limiting factor influences the distribution and success of species. An example is shown in Figure 14.2.1a. Here, temperature affects the metabolic activity of three species of mussel of the genus Mytilus. There is more energy available for growth (scope for growth) at different temperatures. The shape of the graph is characteristic of the zone of stress model. Figure 14.2.1a – Limit of tolerance examples Source: www.asnailsodyssey.com Activity 1: Analysing data Study Figure 14.2.1a and answer the following: Which species has the highest scope for growth at 25°C? Which species has the lowest overall scope for growth? Does the data show evidence of niche overlap? Explain. Compare the shapes on the graph to Figure 14.1.1a. How useful is the model in explaining the phenomenon? A niche considers all limiting factors Imagine that we now put together all of the tolerance data for all of the limiting factors in a single graph. It would have too many dimensions to represent on a flat surface, but it would represent a species’ niche more realistically. Figure 14.2.1b – A three-dimensional niche Figure 14.2.1b shows the distribution of one plant species based on three limiting factors: latitude, elevation and mean annual temperature. A niche tells us about species distribution based on n limiting factors. Fundamental vs Realised niche Removal experiments demonstrate that most species are limited in natural environments by interspecific competition for resources. An example is shown in Figures 14.1.2c and d. Figure 14.1.2c – Paramecium cultured separately When three species of Paramecium are cultured separately, they show a normal pattern of sigmoid population growth. Figure 14.2.1d – Paramecium cultured together When different species of Paramecium are cultured together, one species is limited. Because competition excludes species from their full range of habitat, the realised niche is smaller than the fundamental niche. We use the terms ‘narrow’ and ‘broad’ to describe a species niche. Niches are narrowed as a result of competition. Figure 14.2.1e – Waterfowl populations Distribution of waterfowl according to foraging habits. Foraging area has been partitioned as a natural consequence of competition. Activity 2: Analysing data Study Figures 14.2.1e and 14.2.1f and answer the following questions: Which species of waterfowl show niche overlap? Consider pairs of species and match each pair to the level of resource partitioning shown in Figure 14.2.1f. Extension: How does resource partitioning relate to evolution? Figure 14.2.1f – Levels of resource partitioning Figure 14.2.1g – Woodland duck The woodland duck's (Cairinini sp.) foraging range is limited by competitive exclusion. Key questions Define niche Distinguish between fundamental and realised niches Explain how competitive exclusion leads to narrowing of realised niches Figure 14.2.1h – Mussel In the genus Mytilus, metabolism and growth are limited by temperature. Figure 14.2.1i – P. bursaria All three species of protists exploit the same type of resources and environment. Figure 14.2.1j – P. caudatum limits the realised niche of the other paramecium species. Figure 14.2.1k – P. aurelia has a wider realised niche when it is not in competition with other protists. Nature of Science A good model explains phenomena and has predictive power. Discuss ways in which the limits of tolerance model can predict interactions between species in a community. 14.2.2 Pyramids, energy and nutrient diagrams The shape of a pyramid of net productivity is shown in Figure 14.2.2a. The amount of energy stored as biomass is very high at the lowest trophic level, because a lot of producers are needed to support consumers. Figure 14.2.2a – General shape for all pyramids of productivity Since consumers depend on producers for energy, the distribution and abundance of life in any ecosystem is determined by its net primary productivity (NPP). The average NPP of each of the world’s major biomes is shown in Figure 14.2.2b. Figure 14.2.2b – Net primary productivity of the Earth’s major ecosystems Activity 1: Interpreting energy diagrams Explain how the shape of all energy pyramids illustrates the second law of thermodynamics. Predict which of the ecosystems shown in Figure 14.2.2b can support the longest food chains. Explain your prediction. Discuss the relationship between NPP and climate conditions in the major biomes. Refer to [link] Figure 4.1.2d [end link] and Figure 14.2.2b in your answer. Application: Sustainable food At each trophic level, energy is used in respiration, so the rates at which food is converted to biomass differ. Food conversion rates are expressed as ratios of feed input to protein output, and are important considerations in agriculture. Figure 14.2.2c – Food conversion ratios Amount of plant feed necessary to produce different types of food for human consumption. Cattle have the least efficient food conversion rates: approximately 7kg of feed is needed to produce 1kg of cow protein. Representing nutrient flows in Gersmehl diagrams Nutrients such as carbon, nitrogen and phosphorus flow between three major stores in an ecosystem. The nutrient stores are: biomass (B) litter (L) soil (S) Each ecosystem has a unique profile for the amount of nutrients in each of the stores, as well as the rate of flow, or flux, between the stores. Some of the processes that contribute to rate of flux are shown in the following table. Natural processes Leaching, erosion, runoff, sedimentation, weathering, precipitation, decomposition, death, precipitation, feeding/assimilation Human processes Fertiliser use, sewage (eutrophication), harvesting, clear cutting (removal of nutrients) Table 1: Processes that contribute to nutrient flow Gersmehl diagrams are useful in visualising the inter-relationships between nutrient stores and flows. In a Gersmehl diagram: Stores (B, L, S) are represented by circles. The size of the circle represents the amount of biomass held there. Fluxes are represented by arrows. The thickness of the arrow represents the rate of flux, but the arrows themselves may represent many processes. The Gersmehl diagrams for three different ecosystems are shown in Figures 14.2.2d–f. Figure 14.2.2d – Nutrient flow in a rainforest: the largest store is biomass (B) There is high precipitation (P) all year round, so rapid transfer between stores and environment because moisture and heat are ideal for decomposers. Figure 14.2.2e – Nutrient flow in the taiga (boreal forest): the largest store is litter (L) There is very little transfer between stores. Figure 14.2.2f – Nutrient flow in a desert: the largest store is soil (S) There is a high supply of litter but little transfer between stores. Key skills Compare pyramids of energy from different ecosystems Construct of Gersmehl diagrams to show relationships between nutrient stores and flows Concept help Depending on the discipline, an energy pyramid may be called a pyramid of ‘biomass’ or ‘productivity’ or simply ‘energy’. The general shape of all of these pyramids is the same when the following units are used: Pyramid type Unit Biomass g m-2yr-1 Productivity J m-2yr-1 Energy J Language tools Litter refers to organic matter in and on the soil, including detritus and humus. Soil refers to disintegrated rock particles and water. Soil is composed of inorganic nutrients only. Science and social responsibility (Aim 8) Forests are often cleared to make room for grazing livestock, which then emit carbon dioxide, a greenhouse gas, as a product of respiration. Eating less meat significantly reduces a person’s carbon footprint. Is a vegetarian diet more socially responsible? Figure 14.2.2g – Rainforest In a rainforest, high precipitation and moisture lead to high productivity and biomass. Figure 14.2.2h – Taiga In a boreal forest, or taiga, the main store of nutrients is litter – mostly leaf needles. Figure 14.2.2i – Desert In a desert, biomass does not decompose due to dry conditions. The main nutrient store is soil. Activity 2 On the Gersmehl diagrams, identify which arrows represent the processes in the table. Course link We have already learned about carbon flux in 4.1.3 Carbon cycling. 14.2.3 Case studies: Human impacts on ecosystems We have already encountered numerous examples of how humans have an impact on ecosystems. Case studies are often used in ecology to study general principles because controlled experiments are not always possible in the field. As you go through this page, consider: how the causes and consequences of these examples illustrate basic ecological concepts ways that conservation/control programmes can be evaluated. Invasive species: Cane toads in Australia The poisonous cane toad, Bufo marinus, is native to central and south America and was introduced to Australia in 1935 in order to control a common sugar cane pest, the cane beetle, Dermolepida albohirtum (Figure 14.2.3d). Cane toads have no natural predators in Australia, and are poisonous during all stages of their life. Any small predator that attempts to eat a cane toad is almost always lethally poisoned, although some species are not susceptible to the toxin. The Australian government suggests that the toads are advancing at a rate of about 45 to 60km per year westward from their current distribution. Figure 14.2.3a – Distribution of cane toads in Australia Source: Kearney et al., WP 2008, 'Modelling species distributions without using species distributions: the cane toad in Australia under current and future climates’, Ecography, vol. 31, pp. 423-434 Scientists and policymakers are uncertain how to proceed in dealing with cane toads. When evaluating what type of biological control is necessary, scientists must consider: The life cycle of the invasive species. Cane toads disperse quickly because cannibalism is common. A tadpole has a better chance of survival if it is far away from its more mature (and hungry) cousins. The effect on local flora and fauna. Setting up traps has been unsuccessful since other amphibians often get caught. Egg and tadpole collection is difficult because cane toads look very much like native amphibian species during these stages. Biomagnification: Mercury in fish Mercury released by burning coal and incinerating wastes eventually settles in freshwater ecosystems. Figure 14.2.3b – Location of mercury deposition in the United States (1mg = 10-6g) Once in the water supply, elemental mercury is converted by specific bacteria into methylmercury, a persistent organic neurotoxin that accumulates in fatty tissues and is biologically magnified through aquatic food chains. Figure 14.2.3c – Methylmercury concentrations in organisms collected from Lake Washington. Activity Using the information from Figures 14.2.3b and 14.2.3c and your own knowledge: Explain the causes of mercury biomagnification. Draw a food web for Lake Washington species. Name species that occupy more than one trophic level. Categorise the species as having a high (>100mm/kg), medium (20–100mm/kg) or low (<20mm/kg) concentration of methylmercury. Explain the categories. Estimate the concentration of methylmercury in a smallmouth bass from Lake Norman. Justify your prediction. Course link The cane toad on discussion here, Bufo marinus, is the same species that we encountered in Page 5.2.2. Redbellied black snakes in Australia have evolved mechanisms to avoid the poisonous toad. Often, the effect of a disturbance on an ecosystem is impossible to predict! Figure 14.2.3d – The cane beetle, Dermolepida albohirtum Cane toads were originally brought to Australia to eat the beetles’ eggs during the breeding season. The effectiveness of this programme remains in question today. (Photo credits: ©Alexander Dudley 2005) Figure 14.2.3e – Cannibal cane toad This cane toad was found with these immature cane toads in its stomach! Did you know? If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically appropriate language, on an internet search. It’s like a reverse glossary search! Figure 14.2.3f – Lake Washington, Washington State, USA Figure 14.2.3g – Lake Norman, North Carolina, USA 14.2.4 Field techniques: Using Transects Transects are often used to sample the distribution of populations. The most common type is a line transect. A piece of tape or string of fixed length is randomly placed in a straight line in a test site. The number of organisms that fall on the line and their locations are noted. The process is repeated until a representative sample has been taken. Figure 14.2.4a – Placement of a line transect The number of organisms that fall on the line, or (as shown) within a fixed distance from the line can be sampled. A belt transect involves laying two lines at a fixed distance from each other and counting the organisms within the two lines. Sometimes this is done using a quadrat, so that an estimate not only of distribution but also of relative abundance can be calculated. Figure 14.2.4b – Belt transect In this case, counting organisms within a 1 x 1m quadrat along a line transect is equivalent to using a belt transect with two lines 1m apart. A point transect is useful for sampling organisms in tall trees (e.g. birds and bats) or to determine canopy cover. The researcher stands at a fixed point and measures the number of organisms detected within a certain radius of that point. Sometimes a point is painted on the researcher’s boot for accuracy. Figure 14.2.4c – Point count In all cases, care must be taken to ensure that transects are done in random locations through the test sites. Application: Primary succession in sand dunes Sand dunes develop in coastal areas where there are high winds and little vegetation. High tides deposit sand, which becomes an effective wind (and wave) breaker, leading to more sand deposition. Grasses may eventually colonise the dunes if they provide enough wind cover. Sand dune ecosystems demonstrate primary succession in action because the youngest dunes are closest to the strand line (i.e. the beach) and the oldest dunes are furthest away. Figure 14.2.4d – Primary succession in sand dune ecosystems The diagram shows the successional gradient from the newest sand dunes (east) to the oldest sand dunes (west) with respect to various abiotic factors. Figure 14.2.4e – Data collected from a sand dune ecosystem near Sefton, UK Activity 1: Analysing data Referring to the data in Figure 14.2.4e: 1. Name the pioneer species. 2. Describe the trends in abiotic factors that are typical of primary succession. 3. Identify which data was collected using transects and describe what type of transect was used. 4. Explain the trend in species number from early to late stages of primary succession. 5. Suggest a reason why marram grass does not appear in sites 3, 4 or 5. Activity 2: Investigate the effect of disturbance on an ecosystem Disturbances affect living and non-living parts of the ecosystem, including, but not limited to: species diversity nutrient cycling water movement erosion leaf area index percentage cover relative abundance edge effects (e.g. narrowing of realised niche, or defended territories) For this task, work in groups of two or three. You can use any of the skills you have learned in this course, including quadrat samples, transects, mark-recapture (HL), Simpson’s reciprocal index of diversity, and statistical analysis such as the t-test and chi-square test. Have your plan approved by your teacher before you begin your investigation. Key skills Perform a transect. Analyse data showing primary succession. Investigate the effect of disturbance on an ecosystem. Course link If you don’t know the name of something, it helps to describe it as accurately as possible, using scientifically appropriate language, on an internet search. It’s like a reverse glossary search! Nature of Science Avoiding bias: Be specific in your reporting about how you decided on the length and placement of your transects. It’s best to follow an arbitrary rule or use a random number generator. Extended Essay ideas Get to know your local ecosystem: Combine your knowledge of statistics with your knowledge of population sampling methods to investigate a limiting factor. Perform a monitoring study of local indicator species. Figure 14.2.4f – Sand dunes Sand dunes on Schiermonnikoog, Netherlands (above) and Lake Huron, Canada. Sand dune ecosystems occur in both saltwater and freshwater environments. Investigation hints What constitutes a disturbance? A paved path through a park reduces connectivity for small insects (island/edge effects). Transects should be laid out in a way that you can collect a reasonable amount of data (minimum 40 organisms). It takes practice to determine how many samples is enough. Keep track of the total length of your transect lines and the distance from the line in order to improve precision of your method. 14.2.5 Lab: Modelling population growth Do populations show logistic or exponential growth? What are ideal conditions for growth? Use these two techniques to investigate. Technique 1: Counting yeast with a haemocytometer Yeast, Saccharomyces cerevisiae, is a eukaryotic microorganism. You can monitor the growth of populations of yeast over a period of days or weeks by taking population samples using serial dilutions and a haemocytometer, which is a specialised tool for counting cells under a microscope. You will need: One packet dry brewers’ yeast Erlenmeyer flask Haemocytometer (and specialised cover slips) 0.1ml pipette Distilled water Test tubes 10ml graduated cylinder Instructions: 1. Activate the yeast according to the package instructions (usually 15g in 100g water at 37°C and 1g sugar is enough). This is your original population. 2. Once yeast is activated, after about 10 minutes, dilute a sample of the suspension with distilled water. 3. Add 0.1ml of the suspension into 9.9ml of distilled water in a test tube. Repeat the dilution two more times as shown in Figure 14.2.5a. You have just performed a serial dilution with a dilution factor of 10-6. Figure 14.2.5a – Serial dilution The number of yeast cells in the original population is too large to count. Serial dilutions are a very useful technique to learn for biological analysis. 4. Take a sample of the last dilution and mount it on the haemocytometer. First, place the cover slip over the counting grid area. Then drop the sample at the edge of the cover slip, allowing it to move by capillary action into the counting chamber. Figure 14.2.5b – Side view of a haemocytometer The sample is loaded at the end of the cover slip and pulled into the counting chamber by capillary action. The cover slip is 0.1mm in height from the counting surface, so that the concentration of cells can be calculated accurately. 5. Mount the slide under high power and count the number of yeast cells that are visible within 10 randomly selected boxes. This will give you the population from a total volume of 0.25mm3. Figure 14.2.5c – Counting grid Counting is normally done in the smallest boxes (shown in blue), in the central area of the grid. Each of the boxes is 0.5 x 0.5 x 0.1mm (height from coverslip). 6. Calculate the concentration of yeast in your original sample using the formula: Concentration of sample = Number of cells counted Volume of squares counted x Dilution factor where the volume of squares counted is 0.25 mm3, and the dilution factor is 10-6. The population should be expressed as number of cells/mm3. 7. Keeping your original population at room temperature, repeat steps 3–6 every day for two weeks. Graph your results. Technique 2: Modelling population growth in duckweed Unlike most plant species, duckweed (Lemna sp.) grows continuously. It reproduces asexually by producing thalli which break loose from the parent plant when its roots are large enough to support itself. Thalli are easy to count. Figure 14.2.5d – Duckweeds are small freshwater autotrophs that consist of thalli (leaves) and long roots You will need: Glass beaker Healthy Lemna Lamp or other continuous light source Forceps Pond water (or dechlorinated tap water with added nutrient salts) Instructions: 1. Place two healthy plants in a beaker with a set volume of pond water (e.g. 200ml). Count and record the number of thalli on both plants. 2. Continue to count thalli every other day for 3–4 weeks. Make sure to replenish any volume of pondwater lost to evaporation. Graph your results. Variation: Compare the rates of population growth when the starting population in the same-volume beaker is much higher. In the lab Make sure your 0.1ml pipette is not contaminated when you do your serial dilutions. More data is usually more reliable. Why not pool class data? Yeast: How many times more concentrated is your original suspension compared with your diluted sample? Duckweed: Tap water can be dechlorinated by leaving it in a shallow pan overnight. Duckweed: Make sure you specify a minimum size for counting thalli. Figure 14.2.6e – Micrograph Counting yeast is tricky! Take your time. Use a tally sheet or a handheld tally counter (below) so you don’t have to look away from the microscope. Figure 14.2.6f – Handheld tally counter Discussion Do yeast and duckweed populations grow exponentially or logistically? Discuss the shape of the graphs. Suggest variations to the main method that would (i) increase the rate of population growth, (ii) decrease the rate of population growth and (iii) change the carrying capacity. IA: Exploration Does competitive exclusion occur when two microorganisms are cultured together? Figure 14.2.6g – A spectrophotometer as lab variation You can also use a spectrophotometer to estimate population sizes in your samples of yeast (above). A sample dilution is placed in a small cuvette and the optical density is read (below). Darker samples have more yeast. Figure 14.2.5h – Optical density principle Figure 14.2.5i – Duckweed pond Duckweeds are food for frogs and ducks and respond quickly to eutrophication. 14.2.6 Lab: Nutrient content of soils (HL) Soils are dynamic systems which shape and are shaped by their environments. Soil structure and nutrient content have an impact on the type and distribution of plants growing on them and, in turn, the organisms affect the structure of soils through death and decomposition. Soil structure is generally determined by particle size. Clay particles have the smallest diameter, followed by silt and sand. Most soil is composed of a combination of these three types of particle, as shown in Figure 14.2.6a. Figure 14.2.6a – Soil classification system. All soils are composed of a percentage of small (clay), medium (silt) and large (sand) particles. Different plants grow best in different types of soil depending on their oxygen and nutrient requirements. Application: Effects of waterlogging on the nitrogen cycle Soils become waterlogged when water saturation prevents oxygen from reaching plant roots. Waterlogging can occur because of excessive irrigation or poor drainage, often in flat clay soils. Waterlogging leaves anaerobic conditions in the soil, so the plant roots are not able to respire. In addition, carbon dioxide accumulates in the soil. The soil condition is said to be ‘anoxic’. This affects the nitrogen cycle because anaerobic conditions favour denitrification over nitrification. More nitrogen is released into the atmosphere. Increased denitrification leaves less nitrogen available for plants. This also affects the nitrogen cycle because waterlogging can cause runoff. Nitrogen in runoff water can also lead to eutrophication in other ecosystems. Figure 14.2.6b – Well aerated vs waterlogged soils Waterlogged soils affect the nitrogen cycle by encouraging denitrification. Plant growth is also affected. Concept help Process Summary Conditions Bacterial species Nitrification NH3 → NO2- and NO3- Aerobic Nitrobacter Nitrosomonas Denitrification NO3- → N2 Anaerobic Pseudomonas Lab activity: Determining soil texture by feel Here is a quick activity you can do to help you understand the different structures of soils and why certain soils are more prone to waterlogging than others. All you need to do is obtain samples of different types of soil, then follow the instructions given below. Download Figure 14.2.6c as PDF > Figure 14.2.6c – Determining soil texture by feel Skill: Assessing nutrient content of a soil sample 1. Nutrient content of a soil sample can be assessed indirectly by examining the plants growing there. Some examples are given in the table below. Plant appearance Nutrient deficiency Green glossy leaves None Yellow leaves, especially older leaves, new leaves lighter in colour Nitrogen Leaf tips appear burnt, older leaves may be purple Phosphorus Interveinal chlorosis from edges moving inwards Potassium New leaves are distorted or irregularly shaped Calcium Figure 14.2.6d – Interveinal chlorosis: yellowing between the veins and the edges of the leaf 2. Testing the pH of your soil sample using universal indicator is another indirect method of indicating which nutrients are present. Figure 14.2.6e – Soil pH and nutrient availability 3. Electrical conductivity can be measured to indicate the concentration of dissolved salts in the soil. In general, the higher the salinity, the more nutrients are dissolved in the soil Figure 14.6.2f – Electrical conductivity Nitrates and dissolved salts increase conductivity of a soil sample. 4. The easiest way to test for nutrient content of soils is to use a commercial soil testing kit like the one shown in Figure 14.6.2g. The chemicals in each of the tests indicate levels of nitrogen, potassium and phosphate. Figure 14.2.6g – Commercial soil test kit Chemical indicators change colour when nutrients are present. Key skills Explain the impact of waterlogging on the nitrogen cycle. Assess the nutrient content of a soil sample. Figure 14.6.2h – Particle size All soils are made of three sizes of particles Data analysis From Figure 14.2.6a: Identify the type of soil that is composed of 40% silt, 30% sand and 30% clay. What is the minimum amount of clay necessary for a soil to be considered at least partially ‘loamy’? Figure 14.6.2.i – Cassia obtusifolia The sicklepod, Cassia obtusifolia, grows where soils are well aerated and is used as an indicator plant for soil quality. IA: Exploration Which soil conditions are best for plant growth? What soil texture is most appropriate for different crops? How does soil structure affect respiration rates? Figure 14.6.2j – Raised beds Gardeners use raised beds to prevent waterlogging Figure 14.6.2.k – Fertiliser showing NPK ratings Fertilisers show NPK ratings to indicate how much nitrogen, phosphorus and potassium they contain. Different crops grow best with different ratios. Downloads PDF of Figure 14.2.6c - Determining soil texture by feel Data analysis At which pH is the availability of calcium highest? At which pH range is the NPK rating highest? Suggest the characteristics of plants grown in soils at pH4.5.