Unit 8_ Ecology Teaching Notes PDF

B4.2 Ecological Niches C4.1 Populations and Communities Page 1 of 64 Topic: B4.1 Ecological Niches Subtopic: Guiding Question What are the advantages of specialised modes of nutrition to living organisms? How are the adaptations of a species related to its ni...

B4.2 Ecological Niches C4.1 Populations and Communities Page 1 of 64 Topic: B4.1 Ecological Niches Subtopic: Guiding Question What are the advantages of specialised modes of nutrition to living organisms? How are the adaptations of a species related to its niche in an ecosystem? Linking Questions What are the relative advantages of specificity and versatility? For each form of nutrition, what are the unique inputs, processes and outputs? Understandings □ B4.2.1 Ecological niche as the role of a species in an ecosystem: include the biotic and abiotic interactions that influence growth, survival and reproduction, including how a species obtains food. □ B4.2.2 Differences between organisms that are obligate anaerobes, facultative anaerobes and obligate aerobes: limit to the tolerance of these groups of organisms to the presence or absence of oxygen gas in their environments. □ B4.2.3 Photosynthesis as the mode of nutrition of plants, algae and several groups of photosynthetic prokaryotes: details of different types of photosynthesis in prokaryotes are not required. □ B4.2.4 Holozoic nutrition in animals: understand that all animals are heterotrophic. In holozoic nutrition food is ingested, digested internally, absorbed and assimilated. □ B4.2.5 Mixotrophic nutrition in some protists: Euglena is a well-known freshwater example of a protist that is both autotrophic and heterotrophic, but many other mixotrophic species are part of oceanic plankton. Understand that some mixotrophs are obligate and others are facultative. □ B4.2.6 Saprotrophic nutrition in some fungi and bacteria: fungi and bacteria with this mode of heterotrophic nutrition can be referred to as decomposers □ B4.2.7 Diversity of nutrition in archaea: understand that archaea are one of the three domains of life and appreciate that they are metabolically very diverse. Archaea species use either light, oxidation of inorganic chemicals or oxidation of carbon compounds to provide energy for ATP production. Students are not required to name examples. □ B4.2.8 Relationship between dentition and the diet of omnivores and herbivores representative members of the family Hominidae: Application of skills: examine models of digital collections of skulls to infer diet from the anatomical features. Examples may include Homo sapiens (humans), Homo floresiensis and Paranthropus robustus. □ B4.2.9 Adaptations of herbivores for feeding on plants and of plants for resisting herbivory: for herbivore adaptations, include piercing and chewing mouthparts of leaf-feeding insects. Plants resist herbivory using thorns and other physical structures. Plants also produce toxic secondary compounds in seeds and leaves. Some animals have metabolic adaptations for detoxifying these toxins. □ B4.2.10 Adaptations of predators for finding, catching and killing and of prey animals for resisting predation: be aware of chemical, physical and behavioural adaptations in predators and prey. □ B4.2.11 Adaptations of plant form for harvesting light: include examples from forest ecosystem to illustrate how plants in forests use different strategies to reach light sources, including trees that reach the canopy, lianas, epiphytes growing on branches of trees, strangler epiphytes, shade-tolerant shrubs and herbs growing on the forest floor. □ B4.2.12 Fundamental and realised niches: appreciate that fundamental niche is the potential of a species based on adaptations and tolerance limits and that realised niche is the actual extent of a species niche when in competition with other species. □ B4.2.13 Competitive exclusion and the uniqueness of ecological niches: include the elimination of one of the competing species or the restriction of both to a part of their fundamental niche as possible outcomes of competition between two species. Page 2 of 64 GLOSSARY Page 3 of 64 Ecological Niches B4.2.1 Ecological niche as the role of a species in an ecosystem: include the biotic and abiotic interactions that influence growth, survival and reproduction, including how a species obtains food. Ecological niche refers to the role of a species in an ecosystem. It encompasses the biotic and abiotic interactions that influence the growth, survival and reproduction of a species. These roles include: ○ What it eats ○ Which other species depend on it for food ○ What time of day a species is active ○ Exactly where in a habitat a species lives ○ Exactly where in a habitat a species feeds Biotic factors are the living parts of the environment, such as: competition, disease, predators, and parasites. ➔ Plants must have enough light for photosynthesis in order to produce carbohydrates ➔ Aquatic organisms must be able to absorb enough oxygen from the surrounding water for respiration Abiotic factors are the non-living parts of the environment, including temperature, precipitation, sunlight, and soil. ➔ A prey organism being camouflaged to avoid predation ➔ A plant growing fast enough to outcompete nearby plants for sunlight Example: In a forest ecosystem, the ecological niche of a squirrel may involve gathering and storing nuts, competing with other squirrels for resources, mates, a place to live, avoiding predators such as hawks, and adapting to seasonal changes in temperature and food availability. Page 4 of 64 Differences between obligate anaerobes, facultative anaerobes, and obligate aerobes B4.2.2—Differences between organisms that are obligate anaerobes, facultative anaerobes and obligate aerobes. Limit the tolerance of these groups of organisms to the presence or absence of oxygen gas in their environment. Obligate Anaerobes Facultative Anaerobes Obligate Aerobes Obligate anaerobes Facultative Anaerobes Obligate Aerobes Explanation: Explanation: Explanation: Unable to survive in the Can switch between aerobic Unable to survive without presence of oxygen and use respiration (using oxygen) and oxygen. Require oxygen as the other compounds as electron fermentation (without oxygen) final electron acceptor for acceptors for respiration. depending on the availability of respiration. oxygen. Examples: Examples: Examples: Bifidobacterium species have probiotic properties that help Yeast can carry out fermentation Humans and many other its host and benefit digestive in the absence of oxygen but animals rely on oxygen to health. can also respire aerobically produce energy through aerobic when oxygen is present. respiration. Clostridium perfringens, which is found in the soil, may cause a life-threatening infection called gas gangrene Page 5 of 64 Photosynthesis as the mode of nutrition in plants, algae, and photosynthetic prokaryotes B4.2.3—Photosynthesis as the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes. Details of different types of photosynthesis in prokaryotes are not required. Plants Plants use sunlight to convert carbon dioxide and water into glucose and oxygen. This occurs in the chloroplasts, where the pigment chlorophyll located in the thylakoid membranes absorbs light energy. Photosynthetic algae Algae, which can be found in various environments (freshwater, marine, etc.). Also use chlorophyll to capture light energy and convert carbon dioxide and water into glucose and oxygen. They use different types of chlorophyll and accessory pigments (e.g., carotenoids, phycobilins) that allow them to absorb a wider range of light wavelengths. Photosynthetic prokaryotes Photosynthetic prokaryotes, like cyanobacteria, t perform oxygenic photosynthesis similar to plants and algae, using chlorophyll to produce oxygen as a byproduct. Page 6 of 64 Holozoic nutrition in animals B4.2.4—Holozoic nutrition in animals. Understand that all animals are heterotrophic. In holozoic nutrition food is ingested, digested internally, absorbed and assimilated. Unlike autotrophs, animals are heterotrophic and obtain their nutrition by ingesting and digesting food internally. Holozoic nutrition involves the ingestion, digestion, absorption, and assimilation of food. Example: Lions are carnivores They obtain their nutrition by hunting and consuming other animals. They have specialised teeth for tearing flesh A digestive system that can break down and extract nutrients from the meat they consume. In the space provided give your own example of holozoic nutrition in an organism. Use the example of lions above to help you construct your response. Page 7 of 64 Mixotrophic nutrition in some protists B4.2.5—Mixotrophic nutrition in some protists. Euglena is a well-known freshwater example of a protist that is both autotrophic and heterotrophic, but many other mixotrophic species are part of oceanic plankton. Understand that some mixotrophs are obligate and others are facultative. Euglena Some protists can obtain nutrition through both autotrophic and heterotrophic means. This is known as mixotrophic nutrition. Example: Euglena are protists that live in freshwater systems They have chloroplasts that allow them to carry out photosynthesis and produce their own food. They can also consume other organisms, such as bacteria or smaller protists, when sunlight is limited or nutrients are scarce. Understand that some mixotrophs are obligate and others are facultative. Obligate Aerobes (Requires oxygen) Facultative Anaerobes Coccolithophores Diatoms Are mostly photosynthetic but can also ingest Diatoms can perform photosynthesis but can also particulate organic matter. switch to heterotrophic metabolism in low-oxygen conditions. Page 8 of 64 Saprotrophic nutrition in fungi and bacteria B4.2.6—Saprotrophic nutrition in some fungi and bacteria. Fungi and bacteria with this mode of heterotrophic nutrition can be referred to as decomposers. https://www.biologyonline.com/dictionary/decomposer for videos Saprotrophs obtain nutrition by secreting digestive enzymes to break down dead organic matter. Fungi and bacteria are examples of saprotrophs. Mode of action: Saprotrophs secrete extracellular enzymes, such as cellulases, ligninases, and proteases, which break down cellulose, lignin, and proteins in the organic matter. The smaller molecules, like sugars and amino acids, are absorbed by the fungal or bacterial cells. Examples: Fungi Mushrooms (common mushroom): decompose leaf litter, wood, and other organic materials in forests. Moulds (Aspergillus spp. or bread mould): grow on decaying food and plant debris. Page 9 of 64  Bacteria Bacillus spp.: common in soil and decomposing plant material Pseudomonas spp.: Found in various environments, including soil and water, can degrade complex organic pollutants as well as natural organic matter. Streptomyces spp.: Found in soil, they also produce antibiotics. Diversity of nutrition in archaea B4.2.7—Diversity of nutrition in archaea. Understand that archaea are one of the three domains of life and appreciate that they are metabolically very diverse. Archaea species use either light, oxidation of inorganic chemicals or oxidation of carbon compounds to provide energy for ATP production. Students are not required to name examples. All living things are grouped into three domains, bacteria, archaea and eukarya (descended from LUCA). Archaea are thought to be the most ancient organisms on the planet. They are unicellular They have no true nucleus Many can thrive in extreme environments (extremophiles). Halophiles can tolerate very low or very high pH. Thermophiles can tolerate high temperatures (hydrothermal vents/hot springs) Page 10 of 64  They exhibit diverse metabolic capabilities and can obtain energy (ATP) through various means. You will not be required to identify any named examples. Light Some archaea can utilise light energy to produce ATP for energy. Use a protein called bacteriorhodopsin to capture light energy. Bacteriorhodopsin acts as a proton pump, creating a proton gradient across the membrane, Used to produce ATP through chemiosmosis. (Not to be confused with photosynthesis in plants and algae as it does not involve the use of chlorophyll or the fixation of carbon dioxide.). Sulfolobus acidocaldarius: Oxidation of inorganic chemicals Some archaea can produce their own carbon compounds using chemosynthesis They use energy released from chemicals in the environment to produce their own carbon compounds Chemosynthesis releases energy from chemicals which is transferred to carbon compounds to make ATP Inorganic chemicals that can act as energy sources for chemosynthetic archaea include ○ Hydrogen gas ○ Ammonia ○ Methane ○ Hydrogen sulphide Example: Sulfolobus acidocaldarius: Found in hot, acidic environments such as hot springs and volcanic vents, this archaeon oxidises sulphur compounds (like hydrogen sulphide, H₂S) to sulfuric acid (H₂SO₄) to generate energy. Page 11 of 64  Oxidation of carbon compounds Heterotrophic archaea get their carbon compounds (such as sugars) from other organisms, and then use these carbon compounds to generate ATP Example: Methanosarcina acetivorans commonly found in marine and soil environments. Break down organic compounds particularly acetate, a simple carbon compound derived from the decomposition of plant material to produce methane. Page 12 of 64 Relationship between dentition and diet in omnivorous and herbivorous Hominidae B4.2.8—Relationship between dentition and the diet of omnivorous and herbivorous representative members of the family Hominidae. Humans are part of the Hominidae family, along with chimps, gorillas, orangutans, and gibbons Studying the skulls of existing hominid species shows that the jaw and dentition, or teeth, of each species are specialised for their particular diet. Species will often have different combinations of teeth types and sizes to enable them to better chew then digest their diet. Incisor teeth are chisel shaped for cutting and biting Canine teeth are pointed for holding and tearing Premolars and molars are flat and ridged for grinding Page 13 of 64  Hominidae Classification Diet Dentition Other Feature Chimps Omnivore Fruit Small incisor teeth Small jaw muscles Insects Long canines to which are only Small mammals bite and tear strong enough to meat) chew softer fruit Pre-molars and animal tissue Molars for grinding vegetation Gorillas Herbivore Leafy vegetation Incisors Wider face for Insects Long canines strong jaw Large molar and muscles to bite premolars for and grind tough grinding vegetation vegetation Thick tooth enamel for protection against tough plant material Humans Omnivore Fruit Incisors Moderately strong Vegetables Small canines jaw muscles Grain Pre-molars Flexibility in jaw Meat Molars movements suitable for a varied diet Teeth are highly mineralised (calcium phosphate) and durable which makes them less susceptible to decomposition. This allows teeth to persist long after other parts of the body have decayed. By looking at the relationship between diet and the dentition in currently existing hominid species, it is possible to apply this principle to extinct hominid species. Page 14 of 64 Task: Use the pictures of the skulls of extinct hominids below to identify their diet based on their detention and skull structure. Use the extract from Kognity to help you. https://app.kognity.com/study/app/ibdp-biology-slhl-2025-pc/sid-422-cid-242422/book/dentition-id-46626/ Paranthropus robustus existed about 1.8 to 1.2 million years ago in South Africa. Homo floresiensis existed between 100 000 to 50 000 years ago in Indonesia. Page 15 of 64 Adaptations of herbivores and plants for feeding and resisting herbivory B4.2.9—Adaptations of herbivores for feeding on plants and of plants for resisting herbivory. For herbivore adaptations, include piercing and chewing mouthparts of leaf-eating insects. Plants resist herbivory using thorns and other physical structures. Plants also produce toxic secondary compounds in seeds and leaves. Some animals have metabolic adaptations for detoxifying these toxins Herbivore adaptation of leaf eating insects Aphids (Aphidoidea), have specialised piercing mouthparts called stylets. These stylets allow them to pierce through plant tissues and access the phloem sap directly, which they then suck up as a nutrient source. Caterpillars (larvae of butterflies and moths) have mouthparts are adapted to grind and break down plant leaves, allowing the caterpillar to consume large amounts of leaf material efficiently Plant Adaptations for Resisting Herbivory Thorns and Spines: Plants like roses and acacia trees have developed thorns and spines to deter herbivores from feeding on their leaves and stems. Page 16 of 64  Tough Leaves: Some plants, like holly (Ilex aquifolium), have tough, waxy leaves that are difficult for herbivores to chew. Plant toxins Nicotine in tobacco plants acts as a potent neurotoxin against herbivores. Tannins in oak trees reduce the digestibility of the leaves, making them less appealing to herbivores. Metabolic adaptations of animals Page 17 of 64  Koalas primarily feed on eucalyptus leaves, which contain toxic compounds like terpenes and phenolic glycosides. Koalas have specialised liver enzymes that detoxify these chemicals, allowing them to safely consume leaves that would be harmful to most other animals. Monarch caterpillars feed exclusively on milkweed plants, which contain toxic cardiac glycosides (cardenolides). These caterpillars have evolved to not only tolerate these toxins but use them in their bodies, making the adult butterflies poisonous to predators. Page 18 of 64 Adaptations of predators for finding, catching, and killing prey and of prey for resisting predation B4.2.10—Adaptations of predators for finding, catching and killing prey and of prey animals for resisting predation. Students should be aware of chemical, physical and behavioural adaptations in predators and prey. Adaptations of Predators Chemical adaptation: The King Cobra possesses venom that contains neurotoxins, which paralyse the nervous system of its prey. This chemical adaptation allows the snake to immobilise its prey quickly, making it easier to capture and consume. Physical adaptation: Cheetahs in the African grasslands have long, slender bodies and powerful leg muscles to run at high speeds to catch swift prey like gazelles. They also have sharp claws and teeth for capturing and killing their prey. Behavioural adaptation: Wolves exhibit pack hunting behaviour, which allows them to work together to catch larger prey like deer. This cooperative strategy increases their hunting success and enables them to take down animals too for an individual wolf. Page 19 of 64 Adaptations of prey Chemical adaptations: Poison dart frogs have skin that secretes toxic alkaloids, which are highly poisonous to predators. This chemical defence deters predators from attempting to eat them, as the toxins can cause harm or death. Physical adaptations: Chameleons can change their skin colour to blend in with their surroundings, making them less visible to predators. This helps them avoid detection and reduces the likelihood of being caught by predators. Behavioural adaptations: Opossums exhibit a behaviour known as "playing dead" or thanatosis. When threatened, they will collapse and appear lifeless, which can confuse predators or make them lose interest, avoiding capture. Page 20 of 64 Adaptations of plants for harvesting light in forest ecosystems B4.2.11—Adaptations of plant form for harvesting light. Include examples from forest ecosystems to illustrate how plants in forests use different strategies to reach light sources, including trees that reach the canopy, lianas, epiphytes growing on branches of trees, strangler epiphytes, shade-tolerant shrubs and herbs growing on the forest floor. (Emergent Trees) Kapok Tree - Ceiba pentandra These trees grow more rapidly than other vegetation to reach above the canopy. (Up to 60m) Their tall trunks and expansive crowns enable them to capture sunlight unavailable to lower layers of the forest. Monstera deliciosa (Lianas) Lianas are woody vines that use taller trees to climb up towards the light. Instead of investing in a thick trunk, lianas save energy by relying on trees for support, which allows them to reach the canopy and access sunlight more efficiently.. Bromeliads (Epiphytes): Epiphytes grow on the branches or trunks of trees rather than in the soil. They have specialised root systems that anchor them to their host trees. They absorb water and nutrients directly from the air and rain. This elevated position allows them to access light in the canopy or sub-canopy without having to compete with ground-based plants. Page 21 of 64  Strangler Fig (Strangler epiphytes). Strangler figs start life as epiphytes, growing on the branches of a host tree. Over time, they send down aerial roots that envelop the host tree's trunk, eventually outcompeting it for light, water and nutrients, finally killing it and taking its place. Bird's Nest Fern (shade tolerant plants and shrubs) Shade-tolerant plants have adapted to survive under the low-light conditions of the forest floor. These plants have large, broad leaves that maximise light capture, even in the dappled sunlight filtering through the canopy. Fundamental and realised niches B4.2.12—Fundamental and realised niches. Appreciate that fundamental niche is the potential of a species based on adaptations and tolerance limits and that realised niche is the actual extent of a species niche when in competition with other species. The ecological niche of a species can be divided into two categories: the fundamental niche and the realised niche. The fundamental niche refers to the potential range of an organism based on its adaptations and tolerance limits. It represents the full range of conditions and resources that a species can utilise in the absence of competition or other constraints. The realised niche, is the actual extent of an organism's niche when in competition with other species. It may be smaller than the fundamental niche due to competition for resources, predation, or other limiting factors. Page 22 of 64 Competitive exclusion and uniqueness of ecological niches B4.2.13—Competitive exclusion and the uniqueness of ecological niches. Include elimination of one of the competing species or the restriction of both to a part of their fundamental niche as possible outcomes of competition between two species. Competitive exclusion: Where two species that have the same requirements cannot occupy the same niche for an extended period of time. One species will eventually outcompete the other, resulting in elimination of the weaker species or the restriction of both species to a part of their fundamental niche. Example: Barnacles Competition: Balanus is more competitive in the lower and middle intertidal zones, where it outgrows and outcompetes Chthamalus, limiting the latter's realised niche to the upper zone. Adaptation: Chthamalus, although capable of living in the entire intertidal zone, is restricted to the upper zone because it cannot compete effectively with Balanus in the lower zones. The uniqueness of ecological niches: Arises from competition and the need to avoid direct competition with other species. Species have evolved adaptations and behaviours that allow them to exploit different resources or occupy different niches within an ecosystem, reducing competition and promoting coexistence. Example: coral reef ecosystem Different species of fish may occupy different niches based on their feeding habits and habitat preferences. Some fish may feed on algae near the reef, while others may feed on plankton in the open water. This niche differentiation allows for the coexistence of multiple species within the same ecosystem. Page 23 of 64  Learning Objectives C4.1 Populations and Communities (SL/HL: 5 hours) Guiding questions How do interactions between organisms regulate sizes of populations in a community? What interactions within a community make its populations interdependent? C4.1.1—Populations as interacting groups of organisms of the same species living in an area Should understand that members of a population normally breed and that reproductive isolation is used to distinguish one population of a species from another. C4.1.2—Estimation of population size by random sampling. Should understand reasons for estimating population size, rather than counting every individual, and the need for randomness in sampling procedures. NOS: Students should be aware that random sampling, instead of measuring an entire population, inevitably results in sampling error. In this case the difference between the estimate of population size and the true size of the whole population is the sampling error. C4.1.3—Random quadrat sampling to estimate population size for sessile organisms. Both sessile animals and plants, where the numbers of individuals can be counted, are suitable. Application of skills: Should understand what is indicated by the standard deviation of a mean. You do not need to memorise the formula used to calculate this. In this example, the standard deviation of the mean number of individuals per quadrat could be determined using a calculator to give a measure of the variation and how evenly the population is spread. C4.1.4—Capture–mark–release–recapture and the Lincoln index to estimate population size for motile organisms. Application of skills: Students should use the Lincoln index to estimate population size. Population size estimate = M × N/R , where M is the number of individuals caught and marked initially, N is the total number of individuals recaptured and R is the number of marked individuals recaptured. Should understand the assumptions made when using this method. C4.1.5—Carrying capacity and competition for limited resources. A simple definition of carrying capacity is sufficient, with some examples of resources that may limit carrying capacity. C4.1.6—Negative feedback control of population size by density-dependent factors. Numbers of individuals in a population may fluctuate due to density-independent factors, but density- dependent factors tend to push the population back towards the carrying capacity. In addition to competition for limited resources, include the increased risk of predation and the transfer of pathogens or pests in dense populations. C4.1.7—Population growth curves. Should study at least one case study in an ecosystem. Students should understand reasons for exponential growth in the initial phases. A lag phase is not expected as a part of sigmoid population growth. NOS: The curve represents an idealised graphical model. Students should recognize that models are often Page 24 of 64 simplifications of complex systems. Application of skills: Students should test the growth of a population against the model of exponential growth using a graph with a logarithmic scale for size of population on the vertical axis and a non- logarithmic scale for time on the horizontal axis. C4.1.8—Modelling of the sigmoid population growth curve. Application of skills: Should collect data regarding population growth. Yeast and duckweed are recommended but other organisms that proliferate under experimental conditions could be used. C4.1.9—Competition versus cooperation in intraspecific relationships. Include reasons for intraspecific competition within a population. Also include a range of real examples of competition and cooperation. C4.1.10—A community as all of the interacting organisms in an ecosystem. Communities comprise all the populations in an area including plants, animals, fungi and bacteria. C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationships within communities. Include each type of ecological interaction using at least one example. C4.1.12—Mutualism as an interspecific relationship that benefits both species. Include these examples: root nodules in Fabaceae (legume family), mycorrhizae in Orchidaceae (orchid family) and zooxanthellae in hard corals. In each case include the benefits to both organisms. Note: When referring to organisms in an examination, either the common name or the scientific name is acceptable. C4.1.13—Resource competition between endemic and invasive species. Choose one local example to illustrate competitive advantage over endemic species in resource acquisition as the basis for an introduced species becoming invasive. C4.1.14—Tests for interspecific competition. Interspecific competition is indicated but not proven if one species is more successful in the absence of another. Should appreciate the range of possible approaches to research: laboratory experiments, field observations by random sampling and field manipulation by removal of one species. NOS: Students should recognize that hypotheses can be tested by both experiments and observations and should understand the difference between them. C4.1.15—Use of the chi-squared test for association between two species. Application of skills: Should be able to apply chi-squared tests on the presence/absence of two species in several sampling sites, exploring the differences or similarities in distribution. This may provide evidence for interspecific competition. C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations. Include a real case study. C4.1.17—Top-down and bottom-up control of populations in communities. Should understand that both of these types of control are possible, but one or the other is likely to be dominant in a community. C4.1.18—Allelopathy and secretion of antibiotics. These two processes are similar in that a chemical substance is released into the environment to deter potential competitors. Include one specific example of each—where possible, choose a local example. GLOSSARY Page 25 of 64 Population: A group of organisms of the same species living and interacting in a particular area. Reproductive Isolation: A situation where different populations of a species do not interbreed, leading to the distinction of separate populations. Random Sampling: A method used to estimate population size by selecting random samples from a population to avoid bias. Sampling Error: The difference between the estimated population size and the actual population size due to the randomness of sampling. Quadrat Sampling: A method used to estimate the population size of sessile organisms by counting individuals within a defined square area. Sessile Organisms: Organisms that are fixed in one place and do not move, such as plants and some animals like barnacles. Standard Deviation: A measure of the amount of variation or dispersion in a set of values, indicating how spread out the numbers are from the mean. Capture-Mark-Release-Recapture: A method used to estimate the population size of motile organisms by capturing, marking, releasing, and then recapturing individuals. Lincoln Index: A formula used to estimate population size using capture-mark-release-recapture data: Population size estimate = (M × N) / R. Carrying Capacity: The maximum number of individuals in a population that an environment can sustainably support. Density-Dependent Factors: Factors that influence population size based on the population density, such as competition, predation, and disease. Exponential Growth: A phase of population growth where the population size increases rapidly without being limited by resources. Sigmoid Growth Curve: A population growth curve that shows an initial exponential growth phase, followed by a plateau as the population reaches carrying capacity. Intraspecific Competition: Competition between individuals of the same species for limited resources. Community: All the interacting populations of different species living in a particular area. Interspecific Competition: Competition between individuals of different species for the same resources. Mutualism: A type of interspecific relationship where both species benefit from the interaction. Invasive Species: A species that is not native to a particular area and tends to spread, often outcompeting native species. Chi-Squared Test: A statistical test used to determine whether there is a significant association between two categorical variables. Allelopathy: A process by which plants release chemicals into the environment to inhibit the growth of competing plants. Interaction of Populations C4.1.1—Populations as interacting groups of organisms of the same species living in an area Page 26 of 64 Should understand that members of a population normally breed and that reproductive isolation is used to distinguish one population of a species from another. C4.1.10—A community as all of the interacting organisms in an ecosystem. Communities comprise all the populations in an area including plants, animals, fungi and bacteria. Prior knowledge: Definitions Species: A species is defined as a group of organisms that are capable of interbreeding and producing fertile offspring under natural conditions. Community: A community is a group of different species that live in the same area and interact with each other. Population: A population is a group of organisms of the same species that live in the same area and interact with each other. Ecosystem: An ecosystem is a community of living organisms (BIOTIC plants, animals, and microbes) interacting with each other and their physical environment (ABIOTIC air, water, and soil) in a specific area. Images of some of the inhabitants in the Serengeti National Park Page 27 of 64 Use the pictures and the objectives to provide examples of the following terms. Species:…………………………………………………………………………………………………………………… …………………………………… Community……………………………………………………………………………………………………………… …………………………………… Population……………………………………………………………………………………………………………… …………………………………… Ecosystem………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… Reproductive Isolation Page 28 of 64  Temporal Temporal isolation occurs when species reproduce at different times, preventing them from interbreeding even if they live in the same area Example: The tree frog, bullfrog, pickerel frog, and wood frog breed at different times of the year. This difference in breeding seasons keeps their gene pools separate and maintains distinct species. They have evolved to breed at a time that maximises its reproductive success, avoids competition for resources to improve offspring's survival. Ecological Ecological isolation occurs when species occupy different habitats or niches. Example: Viola arvensis typically grows in open, disturbed soils like fields, while Viola tricolor prefers less disturbed habitats like meadows. Different ecological preferences cause them to flower and breed at different times, reducing the chances of cross-pollination and maintaining their separation as distinct species. Behavioural Occurs with differences in behaviour, such as mating rituals, calls or displays. Example: The blue-footed booby exhibits an elaborate courtship dance that involves showing off its bright blue feet and unique calls recognised only by other blue-footed boobies. This behaviour ensures that blue-footed boobies only mate with their own kind, preventing interbreeding with closely related species. Page 29 of 64  Mechanical This occurs when differences in the reproductive structures of species prevent successful mating. Example: In frogs, this can happen if the male's reproductive organs are not physically compatible with the female's reproductive structures preventing the transfer of sperm. Differences in body size or the shape of their mating structures can make it impossible for them to align properly during mating, preventing fertilisation. This helps maintain species boundaries by preventing interbreeding between species with incompatible reproductive anatomy. Quadrats C4.1.2—Estimation of population size by random sampling. Should understand reasons for estimating population size, rather than counting every individual, and the need for randomness in sampling procedures.NOS: Students should be aware that random sampling, instead of measuring an entire population, inevitably results in sampling error. In this case the difference between the estimate of population size and the true size of the whole population is the sampling error. Random sampling of a large forested area Reasons for using a quadrat Importance of estimation: Estimating population size is more practical than counting every individual. Example: Counting all trees in a forest is impractical, so sampling is used. Random sampling necessity: Ensures that the sample is representative of the whole population. Example: Sampling fish in a lake by randomly selecting different locations ensures diverse coverage. Sampling error: The difference between the estimated and true population size due to randomness. Page 30 of 64  Example: Estimating the number of daisies in a field might vary depending on the sample location. How to use a quadrat. Summary from Kognity. There is also a really useful video to help with understanding. https://app.kognity.com/study/app/ibdp-biology-slhl-2025-pc/sid-422-cid-242422/book/estimating-population- sizes-id-46388/ Randomly Place Quadrats: Use a random sampling method (like a random number generator) to determine where to place the quadrats within the study area. Ensure that quadrats are placed without bias to get a representative sample. Count Organisms: Within each quadrat, count the number of individuals for the species being studied. For organisms partially inside the quadrat: ○ Count it if more than half of its body lies within the quadrat. ○ Establish this counting rule consistently for all quadrats. Record Data: Count the number of organisms found in each quadrat. Repeat Sampling: Page 31 of 64  Aim to sample at least 10 quadrats to obtain reliable data. Calculate Mean Population Density: Add up the counts from all quadrats. Divide the total count by the number of quadrats sampled to find the mean number of organisms per quadrat. Estimate Population Size: Multiply the mean number of organisms per quadrat by the total number of quadrats that could fit into the entire study area to estimate the total population size. Analyse Spatial Distribution: Use the collected data to analyse how the population is distributed across the study area, looking for patterns or trends. C4.1.3—Random quadrat sampling to estimate population size for sessile organisms. Both sessile animals and plants, where the numbers of individuals can be counted, are suitable. Quadrat sampling: A method to estimate population size for non-moving organisms (sessile). Page 32 of 64  o Example: Counting the number of barnacles on a rock using a quadrat. Applicability: Suitable for counting both sessile animals and plants. o Example: Estimating the number of clover plants in a grassland. Standard deviation: Indicates the variation and evenness of the population within quadrats. o Example: High standard deviation in a quadrat study of seaweed suggests uneven distribution. Application of skills: Should understand what is indicated by the standard deviation of a mean. You do not need to memorise the formula used to calculate this. In this example, the standard deviation of the mean number of individuals per quadrat could be determined using a calculator to give a measure of the variation and how evenly the population is spread. Page 33 of 64 Page 34 of 64 The Lincoln Index C4.1.4—Capture–mark–release–recapture and the Lincoln index to estimate population size for motile organisms. Application of skills: Students should use the Lincoln index to estimate population size. Population size estimate = M × N/R , where M is the number of individuals caught and marked initially, N is the total number of individuals recaptured and R is the number of marked individuals recaptured. Should understand the assumptions made when using this method. Page 35 of 64 Purpose of the Lincoln Index: Scientists use the Lincoln Index to estimate the population size of motile (moving) organisms in a specific area. Directly counting every individual in a population is often impractical or impossible, especially for animals that move or hide. Capture-Mark-Release-Recapture Method: A sample of organisms is captured, marked in a harmless way, and then released back into the environment. After allowing time for the marked individuals to mix back into the population, another sample is captured. Scientists count how many of the recaptured organisms are marked. Page 36 of 64 Why It’s Used: Efficiency: It provides a practical and efficient way to estimate population size without needing to observe every individual. Applicability: Suitable for a wide range of motile organisms, including birds, fish, insects, and mammals. Non-Invasive: The method is minimally invasive, as it involves capturing and releasing organisms with no lasting harm. Population Insights: The index can give insights into population density, survival rates, and movement patterns within the population. Assumptions: The marked individuals have the same chance of being recaptured as unmarked ones. There is no significant change in the population size between the two sampling events (e.g., due to births, deaths, or migration). Marking does not affect the organism's survival or behaviour. Example: For this exercise you will need several boxes of matches and a pen. Work in a group of 2-3 students to ‘sample’ the population of matches in the full box by using the mark and recapture method. Each match will represent one animal. Instructions: 1. Take out 10 matches from the box and mark them on 4 sides with a pen so that you will be able to recognise them from the other unmarked matches later. Page 37 of 64 2. Return the marked matches to the box and shake the box to mix the matches. 3. Take a sample of 20 matches from the same box and record the number of marked matches and unmarked matches. 4. Determine the total population size by using the equation table. Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 R = number of marked individuals recaptured Average (R) Estimate of 𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 𝑀𝑥 𝑁 𝑅 Population M = number of individuals marked initially = 10 N = total number of individuals recaptured = 20 Class average Estimate of total population = ________ Carrying Capacity and Competition for Limited Resources C4.1.5—Carrying capacity and competition for limited resources. A simple definition of carrying capacity is sufficient, with some examples of resources that may limit carrying capacity. Page 38 of 64 Carrying capacity: is the maximum number of individuals of a species that an environment can sustainably support over time without degrading the environment. Examples of Resources that Limit Carrying Capacity: Food: Limits the number of organisms an environment can support. Eg. a forest can only support a certain number of deer based on the amount of vegetation available for grazing. Water: Access to water is crucial for survival. In a desert, the carrying capacity for animals like camels is limited by the availability of water sources. Space: The amount of space available can limit populations, particularly for species that require specific territories for breeding, such as birds or large predators like tigers. Shelter: Adequate shelter is necessary for protection from predators and harsh weather. For example, the number of nesting sites available can limit the population of birds in a forest. Nutrients: In aquatic environments, the availability of nutrients like nitrogen and phosphorus can limit the growth of algae and other aquatic organisms. Predation C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations. Include a real case study. Page 39 of 64  As prey increase, predators increase as there is more food available Prey numbers will then fall as the are being eaten by the predators As prey numbers fall, the predators will then fall due to lack of food C4.1.6—Negative feedback control of population size by density-dependent factors. Numbers of individuals in a population may fluctuate due to density-independent factors, but density- dependent factors tend to push the population back towards the carrying capacity. In addition to competition for limited resources, include the increased risk of predation and the transfer of pathogens or pests in dense populations. Page 40 of 64 Examples of density-dependent and density-independent factors Density-dependent Density-independent Density-dependent factors affect the population Density-independent factors affect population size size in relation to the population's density. These regardless of the population's density. These factors are usually biotic (living). factors are usually abiotic (non-living) and can impact a population suddenly and drastically. Competition for Resources: As the population grows, individuals compete more for limited Weather Events: Severe weather conditions such resources like food, water, and shelter. as hurricanes, droughts, or frost can impact populations regardless of their density. Predation: In dense populations, predators may find it easier to catch prey, which can reduce the Natural Disasters: Events like wildfires, prey population. earthquakes, or volcanic eruptions can reduce population size irrespective of how many Disease and Parasitism: High population density individuals are present. can facilitate the spread of diseases and parasites. Human Activities: Pollution, habitat destruction (deforestation) and climate change can affect populations without regard to their density. Both density-dependent and density-independent interact to regulate population sizes in ecosystems. Density-dependent factors help stabilise populations by preventing them from exceeding the environment’s carrying capacity. Density-independent factors can cause sudden changes in population size, often unpredictably. Negative feedback control of population size by density-dependent factors. Page 41 of 64  Example: Negative Feedback control of population size by density-dependent factors As the rabbit population increases there is more food for foxes, leading to an increase in the fox population. With more foxes hunting, the rabbit population starts to decrease because more rabbits are being preyed upon. As the rabbit population declines, there is less food available for the foxes, causing the fox population to decrease as well. With fewer foxes, the pressure on the rabbit population lessens, allowing it to recover. Population Growth Curves C4.1.7—Population growth curves. Should study at least one case study in an ecosystem. Should understand reasons for exponential growth in the initial phases. A lag phase is not expected as a part of sigmoid population growth. NOS: The curve represents an idealised graphical model. Students should recognize that models are often simplifications of complex systems. Application of skills: Students should test the growth of a population against the model of exponential growth using a graph with a logarithmic scale for size of population on the vertical axis and a non- logarithmic scale for time on the horizontal axis. Page 42 of 64  Log phase: Bacterial growth curve characterised by rapid and constant cell division (binary fission) Growth is at its maximum rate. Number of bacterial cells doubles at regular intervals. Optimal conditions (nutrient availability, pH, temperature). Stationary phase: The rate of bacterial growth slows down and eventually stabilises. The number of new cells being produced equals the number of cells dying. Limiting Factors: when essential nutrients in the growth medium become depleted, or waste products accumulate to levels that inhibit further growth. Change in pH or temperature Death phase: Stage in a bacterial growth curve where the number of dying cells exceeds the number of new cells being produced. Resources in the environment become depleted, and waste products accumulate to toxic levels. Bacterial cells begin to die at an exponential rate. C4.1.8—Modelling of the sigmoid population growth curve. Application of skills: Should collect data regarding population growth. Yeast and duckweed are recommended but other organisms that proliferate under experimental conditions could be used. Page 43 of 64 C4.1.9—Competition versus cooperation in intraspecific relationships. Include reasons for intraspecific competition within a population. Also include a range of real examples of competition and cooperation. Intraspecific competition: Occurs when individuals of the same species compete for resources. Page 44 of 64 Limited Resources: In any given environment, resources such as food, water, shelter, and mates are finite. As the population size of a species grows, these resources become more limited, leading to competition among individuals of the same species. Similar Needs: Individuals of the same species have similar needs because they share the same ecological niche. For example, they might require the same type of food or habitat, making them direct competitors for these resources. Carrying Capacity: When the population of a species approaches the carrying capacity of the environment, the available resources become insufficient to support all individuals, leading to increased competition. Territoriality: Some species establish and defend territories to secure resources like food, shelter, or mating opportunities. Competition arises when multiple individuals or groups try to claim the same territory. Mating Opportunities: Intraspecific competition often occurs during the breeding season when individuals compete for mates. This can involve direct competition, such as fighting, or indirect competition, like displaying traits or behaviours to attract mates. Survival of the Fittest: Intraspecific competition can be a driving force in natural selection, where individuals with advantageous traits are more likely to survive, reproduce, and pass on their genes, while those less adapted may not. Population Density: As the density of a population increases, individuals are more likely to encounter each other, leading to more frequent competition for resources. Intraspecific COMPETITION Intraspecific COOPERATION Page 45 of 64 Deer competing for territory and mates. Wolves hunting in packs to take down larger prey. Male peacocks compete for the attention of females Female elephants cooperate in raising their young, by displaying their elaborate tail feathers. with older, experienced females (matriarchs). They also work together to find water and food resources. During spawning season, male salmon compete for Honeybees are highly cooperative within their prime nesting sites in streams. They often engage hives. They work together to build the hive, forage in physical confrontations to secure a spot where for food, defend against predators, and care for the they can fertilise the eggs laid by females. queen and larvae. Page 46 of 64  Dolphins often hunt in coordinated groups, working In dense forests, oak trees compete for sunlight, together to herd fish into tight balls, making them water, and nutrients. Taller trees may outcompete easier to catch. They also assist each other in shorter ones by shading them out and absorbing caring for sick or injured individuals. more nutrients from the soil. Page 47 of 64 C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationships within communities. Include each type of ecological interaction using at least one example. Carry out some research to find your own example of different relationships within communities. Herbivory: One species feeds on plants. Example: Cows grazing on grass. Your example: Predation: One species hunts another. Example: Lions preying on zebras. Your example: Interspecific competition: Different species compete for the same resources. Example: Lions and hyenas competing for the same prey. Your example: Mutualism: Both species benefit. Example: Bees pollinating flowers, gaining nectar while helping plants reproduce. Your example: Page 48 of 64  Parasitism: One species benefits at the expense of another. Example: Fleas feeding on a dog's blood. Your example: Pathogenicity: One species causes disease in another. Example: Bacteria causing tuberculosis in humans. Your example: Page 49 of 64 C4.1.12—Mutualism as an interspecific relationship that benefits both species. Include these examples: root nodules in Fabaceae (legume family), mycorrhizae in Orchidaceae (orchid family) and zooxanthellae in hard corals. In each case include the benefits to both organisms. Note: When referring to organisms in an examination, either the common name or the scientific name is acceptable. Symbiotic Bacteria: Root nodules in Fabaceae (legume family) contain nitrogen-fixing bacteria (Rhizobium) that convert atmospheric nitrogen into a form the plant can use (ammonia). Nutrient Exchange: The plant provides the bacteria with carbohydrates and a protected environment within the root nodules. Mutual Benefit: This mutualistic relationship enhances soil fertility by increasing nitrogen availability, benefiting both the legume and surrounding plants. Nutrient Exchange: Orchids form a symbiotic relationship with mycorrhizal fungi, where the fungi colonize the orchid's roots, assisting in the absorption of water and essential nutrients, particularly phosphorus, from the soil. Seed Germination Support: Orchid seeds are tiny and lack sufficient nutrients for germination. The mycorrhizal fungi provide the necessary nutrients, allowing the seeds to germinate and develop into seedlings. Mutual Benefit: While the orchid benefits from enhanced nutrient uptake and successful seed germination, the fungi receive carbohydrates and other organic compounds produced by the orchid through photosynthesis. Page 50 of 64  Symbiotic Partnership: Zooxanthellae are photosynthetic algae that live within the tissues of hard corals, providing the coral with nutrients (like glucose) produced through photosynthesis. Energy and Growth: The coral relies on these nutrients for energy, growth, and the building of calcium carbonate skeletons, which form coral reefs. Protection and Resources: In return, zooxanthellae benefit from a protected environment and access to the coral’s waste products (like carbon dioxide) needed for photosynthesis Page 51 of 64 C4.1.13—Resource competition between endemic and invasive species. Choose one local example to illustrate competitive advantage over endemic species in resource acquisition as the basis for an introduced species becoming invasive. Endemic Species Definition: An endemic species is a species that is native to and found only within a specific geographic area, such as an island, region, country, or other defined location and are not naturally found anywhere else in the world. Example: Endemic Species Invasive species with competitive advantage Asir Magpie - Saudi Arabia Prosopis juliflora (commonly known as mesquite) Introduced into arid regions (including KSA) As livestock fodder To prevent soil erosion Reforestation Fuel and timber Competitive advantage: Reported to spread aggressively, altering habitats where native species like the Asir magpie live due to: High seed production-produces a large number of seeds that can remain viable in the soil for many years, allowing it to spread rapidly. Drought resistant-can thrive in poor, dry soils where other plants struggle Allelopathy-releases chemicals into the soil that inhibit the growth of other plants Page 52 of 64 C4.1.14—Tests for interspecific competition. Interspecific competition is indicated but not proven if one species is more successful in the absence of another. Should appreciate the range of possible approaches to research: laboratory experiments, field observations by random sampling and field manipulation by removal of one species. NOS: Should recognize that hypotheses can be tested by both experiments and observations and should understand the difference between them. Page 53 of 64 C4.1.15—Use of the chi-squared test for association between two species. Application of skills: Should be able to apply chi-squared tests on the presence/absence of two species in several sampling sites, exploring the differences or similarities in distribution. This may provide evidence for interspecific competition. The chi-squared test (SavemyExams) A statistical test called the chi-squared test determines whether or not there is a significant difference between the observed and expected results in an experiment Its purpose is to assess whether any difference in these results is due to chance, or due to an association between the variables being tested A chi-squared test can be used to analyse data from quadrat sampling to determine whether or not there is a statistically significant association between the distributions of two species To the eye there may appear to be an association between the two species, but if it is not statistically significant then researchers can conclude that species distributions are independent of each other, and any appearance of association is due to chance If an association is statistically significant then it must be due to an important factor, such as a symbiotic relationship A chi-squared test enables scientists to test hypotheses A hypothesis is a testable statement about the expected outcome of an experiment There are two types of hypothesis: ➔ A null hypothesis states that there is no significant difference, or association, between data sets e.g. that there is no association between the distributions of two species ➔ An alternative hypothesis states that there is a significant difference, or association, between data sets e.g. that there is an association (either positive or negative) between the distributions of two species The result of a chi-squared test enables scientists to either accept or reject a null hypothesis Using the chi-squared test to test for association Step 1: Construct a contingency table for your results This allows the number of quadrats that contain one, both, or neither species to be recorded Step 2: Calculate the row, column, and overall totals for your contingency table Step 3: Calculate the expected values (E) for your table The results recorded in the contingency table are the observed values (O); to calculate the chi-squared value we need to calculate the expected values for each data point. The expected values are what we would expect to see if the null hypothesis were correct Note that this is the first step towards calculating the chi-squared value, the equation for which is: Page 54 of 64  Σ = sum of O = observed value E = expected value Step 4: Calculate the difference between the observed and expected values Subtract the expected values from the observed values (O - E); some of the resulting values will be negative Step 5: Square each difference This eliminates negative values Step 6: Divide each squared difference by the expected value Step 7: Add all of the results from step 6 together This gives the chi-squared value Step 8: Calculate the degrees of freedom Step 9: Establish a probability level or p-value As biologists, we work with a probability level of 0.05, or 5 % This means that we can be 95 % certain that any significant difference or association is not due to chance Some studies require a higher level of certainty than this, e.g. medical researchers may use a smaller p-value Step 10: Use a critical values table and the results of steps 8-9 to find the critical value In order to understand what the chi-squared value says about the data, a table relating chi-squared values to probability is needed; this critical values table displays the probabilities that the differences between expected and observed values are due to chance Step 11: Compare the chi-squared value with the critical value to assess the significance Page 55 of 64 Worked example A researcher decided to test for an association between the distribution of two types of mollusc on a rocky shore; limpets and dog whelks. Their null hypothesis was that there was no association between the distributions of limpets and dog whelks. They carried out 50 randomly placed quadrat samples on the rocky shore, recording either the presence or the absence of both limpets and dog whelks in each quadrat. They obtained the following results: Quadrats containing limpets only: 14 Quadrats containing dog whelks only: 21 Quadrats containing both limpets and dog whelks: 7 Quadrats containing neither limpets nor dog whelks: 8 Use the chi-squared test to determine whether or not there is a statistically significant association between the distributions of limpets and dog whelks. Answer: Step 1: Construct a contingency table Contingency table Limpets present Limpets absent Dog whelks present 7 21 Dog whelks absent 14 8 Step 2: Calculate the row, column, and overall totals for your contingency table Limpets present Limpets absent Row Total Dog whelks present 7 21 28 Dog whelks absent 14 8 22 Column Total 21 29 50 Page 56 of 64  O E O-E (O-E)2 (O-E)2/E Limpets only 14 9.24 4.76 22.66 2.45 Dog whelks only 21 16.24 4.76 22.66 1.4 Both dog whelks and 7 11.76 -4.76 22.66 1.93 limpets Neither dog whelks 8 12.76 -4.76 22.66 1.78 nor limpets Page 57 of 64 Step 7: Add all of the results from step 6 together to obtain the chi-squared value 2.45 + 1.4 + 1.93 + 1.78 = 7.56 (this is the chi-squared value) Step 8: Calculate the degrees of freedom Degrees of freedom can be calculated using the following equation: Degrees of freedom = (number of columns - 1) x (number of rows - 1) Columns and rows refer to the original contingency table In this example, there are 2 columns and 2 rows in the contingency table Degrees of freedom = (2 - 1) x (2 - 1) =1x1 =1 Step 9: Determine the probability level As biologists, we work at a probability of 0.05, or 5% Step 10: Use a critical values table and the results of steps 8-9 to find the critical value Probability that the difference between O and E is due to chance Degrees of freedom 0.1 0.05 0.01 0.001 1 2.71 3.84 6.64 10.83 2 4.6 5.99 9.21 13.82 3 6.25 7.82 11.34 16.27 4 7.78 9.49 13.28 18.46 With degrees of freedom as 1, and a probability level of 0.05, the critical value can be read from the table as 3.84 Page 58 of 64 Step 11: Compare the chi-squared value with the critical value to assess significance The chi-squared value of 7.56 is larger than the critical value of 3.84 This means that there is a significant association between the two species (see section below on statistical significance) Step 7: Add all of the results from step 6 together to obtain the chi-squared value 2.45 + 1.4 + 1.93 + 1.78 = 7.56 (this is the chi-squared value) Step 8: Calculate the degrees of freedom Degrees of freedom can be calculated using the following equation: Degrees of freedom = (number of columns - 1) x (number of rows - 1) Columns and rows refer to the original contingency table In this example, there are 2 columns and 2 rows in the contingency table Degrees of freedom = (2 - 1) x (2 - 1) =1x1 =1 Step 9: Determine the probability level As biologists, we work at a probability of 0.05, or 5% Step 10: Use a critical values table and the results of steps 8-9 to find the critical value Probability that the difference between O and E is due to chance Degrees of freedom 0.1 0.05 0.01 0.001 1 2.71 3.84 6.64 10.83 2 4.6 5.99 9.21 13.82 3 6.25 7.82 11.34 16.27 Page 59 of 64  4 7.78 9.49 13.28 18.46 With degrees of freedom as 1, and a probability level of 0.05, the critical value can be read from the table as 3.84 Step 11: Compare the chi-squared value with the critical value to assess significance The chi-squared value of 7.56 is larger than the critical value of 3.84 This means that there is a significant association between the two species (see section below on statistical significance) Statistical significance The chi-squared value, once calculated, can be compared to a critical value; this allows statistical significance to be assessed If the chi-squared value is larger than the critical value, there is a statistically significant difference between observed and expected values, or a statistically significant association between two sets of results ○ In this case, the null hypothesis can be rejected If the chi-squared value is equal to or smaller than the critical value, there is no statistically significant difference between observed and expected values, or no statistically significant association between two sets of results ○ In this case, the null hypothesis can be accepted To determine the critical value biologists generally use a probability level, or p-value, of 0.05, or 5 % ○ This means that if a difference or association is shown to be statistically significant at this level, there is only a 5 % probability (i.e. probability = 0.05) that this result might be due to chance Page 60 of 64 Top-down and bottom-up control C4.1.17—Top-down and bottom-up control of populations in communities. Should understand that both of these types of control are possible, but one or the other is likely to be dominant in a community. Top-down Control Bottom-up Control Top-down control: Predators or higher trophic levels regulate the population. o Example: Sharks controlling fish populations in coral reefs. Bottom-up control: Resource availability at lower trophic levels determines population sizes. o Example: The availability of plankton affects fish populations in oceans. Dominance: One type of control often dominates in a given community. o Example: In some ecosystems, predators (top-down) are the primary regulators, while in others, resources (bottom-up) dominate. Page 61 of 64 Allelopathy C4.1.18—Allelopathy and secretion of antibiotics. These two processes are similar in that a chemical substance is released into the environment to deter potential competitors. Include one specific example of each—where possible, choose a local example. Allelopathy occurs in various organisms, including plants, bacteria, and fungi. It involves the secretion of certain chemicals or compounds that can have detrimental effects on other organisms in their environment. Allelopathy in plants Release of chemicals by some plants through root, leaf litter, or volatiles (chemicals that can diffuse in the air) to inhibit competitors. Example: Black walnut trees release juglone, a chemical compound found in its leaves, roots, bark, inhibiting root growth of nearby plants. Allelopathy in fungi (antibiotics) Some fungi secrete antibiotics to deter competitors. Example: Penicillium mould secretes penicillin, which inhibits bacterial growth by interfering with their cell wall synthesis. Allelopathy in bacteria (antibiotics) Some bacteria outcompete other microorganisms by secreting antibiotics, which can inhibit the growth or survival of other bacteria. Example: Streptomyces commonly found in soil, produce antibiotics such as streptomycin, tetracycline, and erythromycin Page 62 of 64 ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… …………………?