WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.945 Unit C B 9: Ecosystems and material cycles. 00:00:10.600 --> 00:00:13.615 An individual organism is one living thing. 00:00:13.765 --> 00:00:19.610 A population is all the organisms of one species in a particular area at a particular time. 00:00:22.267 --> 00:00:26.807 A community consists of the populations of different species living in an area. 00:00:26.957 --> 00:00:33.046 An ecosystem includes that community and its non-living environment, including interactions between them. 00:00:35.700 --> 00:00:38.707 A habitat is the place where an organism lives. 00:00:38.857 --> 00:00:42.775 Different species can share a habitat while using different resources. 00:00:45.433 --> 00:00:50.456 Organisms need resources such as food, water, space and suitable conditions. 00:00:50.606 --> 00:00:56.532 Plants also compete for light and mineral ions; animals may compete for food, mates and territory. 00:00:59.200 --> 00:01:05.119 Interdependence means organisms depend on others, for example for food, pollination or shelter. 00:01:05.269 --> 00:01:09.516 Changing one population can affect other species in the community. 00:01:12.167 --> 00:01:16.301 An ecosystem includes both biotic and abiotic components. 00:01:16.451 --> 00:01:21.420 Do not use population, community and ecosystem as interchangeable terms. 00:01:24.100 --> 00:01:32.707 Abiotic factors are non-living influences, including temperature, light intensity, water availability, soil conditions and pollutants. 00:01:35.367 --> 00:01:44.495 Biotic factors involve living organisms and their interactions, including predation, competition, disease and the availability of food organisms. 00:01:47.167 --> 00:01:50.068 Species are adapted to particular conditions. 00:01:50.218 --> 00:01:58.320 A substantial change in temperature, rainfall or other conditions can change survival, reproduction, abundance and distribution. 00:02:01.000 --> 00:02:05.209 Competition occurs when organisms need the same limited resource. 00:02:05.359 --> 00:02:09.205 Competition can occur within a species or between different species. 00:02:11.867 --> 00:02:17.645 In a simplified predator, prey cycle, prey numbers rise first, giving predators more food. 00:02:17.795 --> 00:02:25.005 Predator numbers may rise after a delay, increasing predation and reducing prey numbers; predator numbers may then fall. 00:02:27.667 --> 00:02:34.105 Real populations are also affected by other foods, disease, migration and environmental changes. 00:02:34.255 --> 00:02:39.981 Predator, prey cycles and food-web responses are not guaranteed to follow one fixed pattern. 00:02:42.633 --> 00:02:45.673 Food chains and webs show feeding relationships. 00:02:45.823 --> 00:02:52.616 An arrow goes from the organism eaten to the organism eating it, showing the direction of energy transfer in food. 00:02:55.267 --> 00:02:59.955 Producers, such as green plants, make biomass using photosynthesis. 00:03:00.105 --> 00:03:07.751 Primary consumers eat producers; secondary consumers eat primary consumers, and tertiary consumers feed further along a chain. 00:03:10.433 --> 00:03:15.211 A food web links several chains, so a consumer may have more than one food source. 00:03:15.361 --> 00:03:22.187 If one prey population falls, its predators may fall or switch to other prey, affecting those populations too. 00:03:22.337 --> 00:03:27.286 A simplified food web: a population change can affect several linked species. 00:03:31.967 --> 00:03:39.635 A change can have indirect effects: fewer predators may allow a prey population to rise, increasing pressure on the prey's food. 00:03:39.785 --> 00:03:43.962 Explain the links and qualify predictions using other relevant factors. 00:03:46.633 --> 00:03:51.326 A parasite benefits at its host's expense, often living on or inside it. 00:03:51.476 --> 00:03:57.369 For example, a flea feeds on a mammal's blood; harm to the host does not mean that it must die immediately. 00:04:00.033 --> 00:04:02.551 Mutualism benefits both partners. 00:04:02.701 --> 00:04:07.787 A pollinating bee gains nectar or pollen while helping a flowering plant reproduce. 00:04:10.467 --> 00:04:20.736 Legumes and bacteria in their root nodules can also be mutualistic: bacteria provide fixed nitrogen, while the plant supplies carbohydrates and a suitable habitat. 00:04:23.400 --> 00:04:30.897 Abundance describes how common a species is, measured using counts, density, frequency or percentage cover as appropriate. 00:04:31.047 --> 00:04:34.279 It is not always simply a total population count. 00:04:36.933 --> 00:04:39.833 A quadrat is a frame enclosing a known area. 00:04:39.983 --> 00:04:46.895 It is useful for sampling plants and animals that remain in place, rather than animals that quickly move into or out of the frame. 00:04:49.567 --> 00:04:56.455 For a representative area estimate, mark a study area and use random coordinates to choose quadrat positions. 00:04:56.605 --> 00:05:01.114 Choosing only convenient or visibly crowded patches introduces bias. 00:05:03.767 --> 00:05:10.506 Identify the species consistently, count individuals and use a consistent rule for plants touching the frame. 00:05:10.656 --> 00:05:15.925 Use percentage cover instead when distinct individuals cannot sensibly be counted. 00:05:18.600 --> 00:05:23.325 Repeat with enough non-overlapping samples distributed across the study area. 00:05:23.475 --> 00:05:28.317 Record quadrat size, locations and counts so the method can be evaluated. 00:05:31.000 --> 00:05:36.748 Density equals total count in sampled quadrats divided by total area sampled. 00:05:36.898 --> 00:05:42.081 Estimated population equals density times total suitable habitat area. 00:05:44.733 --> 00:05:49.789 For ten zero point two five square metre quadrats containing forty plants in total, 00:05:49.939 --> 00:05:56.327 sampled area equals two point five square metres and density equals sixteen plants per square metre. 00:05:56.477 --> 00:06:02.972 Across a representative one hundred square metre area, the estimated population is one thousand six hundred. 00:06:03.122 --> 00:06:07.315 Use the total sampled area, not just the area of one quadrat. 00:06:11.967 --> 00:06:18.419 Alternatively, multiply the mean count per quadrat by the number of quadrat-sized areas in the habitat. 00:06:18.569 --> 00:06:21.979 Convert all areas to the same units before calculating. 00:06:24.633 --> 00:06:27.791 An estimate assumes samples represent the area. 00:06:27.941 --> 00:06:37.475 Patchiness, identification errors, too few quadrats or sampling only part of the habitat can reduce reliability; the estimate is not an exact census. 00:06:40.133 --> 00:06:49.380 A belt transect samples a strip across an environmental gradient, such as from shaded woodland to open grassland, using quadrats along a measured line. 00:06:52.033 --> 00:06:58.538 A continuous belt uses adjacent quadrats; an interrupted belt uses quadrats at specified intervals. 00:06:58.688 --> 00:07:04.106 Record distance along the transect and keep quadrat area and counting rules consistent. 00:07:04.256 --> 00:07:09.345 Record distribution across a gradient; this is different from random area sampling. 00:07:14.000 --> 00:07:23.097 Measure the relevant abiotic factor at sampling positions, for example light intensity or soil moisture, as well as species abundance or cover. 00:07:25.767 --> 00:07:28.934 Repeat transects where possible and compare patterns. 00:07:29.084 --> 00:07:34.799 Keep the measurement procedure consistent and consider weather, time of day and identification errors. 00:07:37.467 --> 00:07:47.129 A transect uses regular sampling positions (systematic sampling) to investigate how species distribution changes along an environmental gradient. 00:07:47.279 --> 00:07:49.220 It is not random sampling. 00:07:49.370 --> 00:07:54.872 Only use it to estimate the whole habitat if the sampled area represents that habitat. 00:07:57.533 --> 00:08:02.109 Plot abundance or cover against distance or a measured abiotic factor. 00:08:02.259 --> 00:08:07.965 A correlation can support an explanation but does not prove that one factor alone caused the pattern. 00:08:10.633 --> 00:08:17.479 Frequency ( percent) equals quadrats containing the species divided by total quadrats times 100. 00:08:17.629 --> 00:08:24.475 A species in 6 of 10 quadrats has 60 percent frequency; that does not mean it covers 60 percent of the ground. 00:08:27.133 --> 00:08:33.777 Work safely in the study area, avoid unnecessary disturbance and leave organisms and habitats intact. 00:08:33.927 --> 00:08:38.759 Record the method and environmental conditions so comparisons are meaningful. 00:08:41.433 --> 00:08:45.555 Biodiversity is the variety of living organisms in an area. 00:08:45.705 --> 00:08:50.953 More individuals of one species do not automatically mean greater biodiversity. 00:08:53.633 --> 00:08:58.603 Maintaining biodiversity protects species and interdependent relationships. 00:08:58.753 --> 00:09:05.803 Diverse ecosystems may be more resilient to disturbance, although outcomes depend on the particular species and event. 00:09:08.467 --> 00:09:15.876 Species can provide food, medicines and other useful materials, while ecosystems provide services such as pollination. 00:09:16.026 --> 00:09:20.469 Conservation also values species beyond their direct uses to people. 00:09:23.133 --> 00:09:31.399 Conservation can protect and restore habitats, reduce pollution, regulate harvesting and protect threatened species locally and globally. 00:09:34.067 --> 00:09:40.520 Reforestation restores tree cover and can provide habitat, store carbon and support biodiversity. 00:09:40.670 --> 00:09:47.219 Using suitable native species and varied habitats can be more beneficial than a single-species plantation. 00:09:49.900 --> 00:09:54.827 Captive breeding can increase numbers of threatened species for possible reintroduction. 00:09:54.977 --> 00:10:02.250 It needs genetic diversity, suitable habitat and management of the original threats to support long-term success. 00:10:04.900 --> 00:10:10.840 Controlling an invasive population, sometimes including culling, may protect native species. 00:10:10.990 --> 00:10:19.638 Evaluate effectiveness, animal welfare, unintended effects and alternatives rather than assuming that killing competitors is always beneficial. 00:10:22.300 --> 00:10:26.855 Overfishing can reduce wild fish populations and disrupt food webs. 00:10:27.005 --> 00:10:34.863 Fish farming can provide food and may reduce some harvesting pressure on wild stocks, depending on the farming system and feed sources. 00:10:37.533 --> 00:10:41.597 High stocking density can increase stress and disease transmission. 00:10:41.747 --> 00:10:49.555 Waste and uneaten feed can add nutrients to nearby water; these are possible impacts rather than proof that every farm is unhealthy. 00:10:52.233 --> 00:10:58.547 Escaped farmed fish may compete or breed with wild populations, and parasites or diseases can spread. 00:10:58.697 --> 00:11:03.095 Better siting, waste management and biosecurity can reduce risks. 00:11:05.767 --> 00:11:13.039 Medicines used in some systems can affect nearby organisms or contribute to antimicrobial-resistance concerns. 00:11:13.189 --> 00:11:19.705 Evaluate evidence and management practices rather than assuming every farm uses antibiotics routinely. 00:11:22.367 --> 00:11:26.739 A non-indigenous species has been introduced outside its native range. 00:11:26.889 --> 00:11:35.890 Some become invasive and harm native populations through competition, predation or disease; not every introduced species becomes invasive. 00:11:38.567 --> 00:11:44.805 An introduced predator or competitor can change several populations indirectly through a food web. 00:11:44.955 --> 00:11:49.999 Predict effects using the organism's interactions and the environmental context. 00:11:52.667 --> 00:12:02.395 Eutrophication can occur when excess nutrients, such as nitrates or phosphates from fertiliser runoff or sewage, enter water and stimulate rapid algal growth. 00:12:05.067 --> 00:12:12.613 A dense algal bloom reduces light reaching submerged plants, reducing photosynthesis and potentially causing plant death. 00:12:12.763 --> 00:12:17.200 Algae also eventually die and provide material for decomposition. 00:12:19.867 --> 00:12:25.989 Microorganisms decompose dead material and respire aerobically, using dissolved oxygen. 00:12:26.139 --> 00:12:30.426 Increased decomposition can greatly reduce oxygen concentration. 00:12:33.100 --> 00:12:38.720 Fish and other organisms requiring oxygen may die or leave when oxygen is too low. 00:12:38.870 --> 00:12:44.053 The main immediate problem is oxygen depletion, rather than carbon dioxide alone. 00:12:44.203 --> 00:12:48.789 Microbial respiration links decomposition to dissolved-oxygen loss. 00:12:53.467 --> 00:13:02.607 Some organisms can survive very low oxygen levels, so eutrophication changes the community rather than necessarily killing everything in the water. 00:13:05.267 --> 00:13:11.356 Reducing nutrient inputs through appropriate fertiliser use and sewage treatment helps prevention. 00:13:11.506 --> 00:13:18.810 Explain the sequence from nutrient addition to oxygen loss, rather than saying fertiliser directly poisons all fish. 00:13:21.467 --> 00:13:27.034 Carbon is present in carbohydrates, lipids, proteins and other components of biomass. 00:13:27.184 --> 00:13:35.653 Materials cycle between organisms and the non-living environment; energy flows through ecosystems and is not recycled in the same way. 00:13:38.333 --> 00:13:42.212 Photosynthesis takes carbon dioxide from air or water. 00:13:42.362 --> 00:13:46.879 Producers use its carbon atoms to make organic molecules such as glucose. 00:13:49.533 --> 00:13:55.516 Feeding transfers carbon-containing material from producers to consumers and between consumers. 00:13:55.666 --> 00:13:59.991 Carbon atoms are rearranged rather than created anew at each transfer. 00:14:02.667 --> 00:14:08.530 Plants, animals and microorganisms respire, returning carbon dioxide to the environment. 00:14:08.680 --> 00:14:11.495 Plants respire as well as photosynthesise. 00:14:14.167 --> 00:14:20.576 Death, fallen leaves and waste supply organic material to decomposers, including bacteria and fungi. 00:14:20.726 --> 00:14:27.242 Decomposition makes materials available again, while decomposer respiration releases carbon dioxide. 00:14:29.900 --> 00:14:37.953 Some organic carbon can be stored for long periods, for example in fossil fuels formed under particular conditions over geological time. 00:14:38.103 --> 00:14:40.706 This does not happen to all dead material. 00:14:40.856 --> 00:14:45.369 Main transfer pathways; dead material does not all become fossil fuel. 00:14:50.033 --> 00:14:54.484 Combustion of fossil fuels or biomass releases carbon dioxide. 00:14:54.634 --> 00:14:59.945 Deforestation can reduce carbon uptake and storage; reforestation can increase them. 00:15:02.600 --> 00:15:08.412 Water is needed for cell reactions and is a major component of cytoplasm and blood plasma. 00:15:08.562 --> 00:15:13.448 It moves between oceans, freshwater, the atmosphere, land and living organisms. 00:15:16.100 --> 00:15:19.942 Energy from the Sun drives evaporation from water surfaces. 00:15:20.092 --> 00:15:24.681 Transpiration from plants also transfers water vapour into the atmosphere. 00:15:27.333 --> 00:15:32.571 Water vapour cools and condenses into droplets or ice crystals that form clouds. 00:15:32.721 --> 00:15:37.797 Condensation changes gas to liquid; it is not the same process as rainfall. 00:15:40.467 --> 00:15:45.143 Precipitation returns water to the surface as rain, snow or other forms. 00:15:45.293 --> 00:15:48.821 Water may collect in lakes and rivers or return to the ocean. 00:15:51.500 --> 00:15:56.907 Some water soaks into soil and rock (infiltration) and moves underground as groundwater. 00:15:57.057 --> 00:15:59.733 Some flows over the land as surface runoff. 00:15:59.883 --> 00:16:02.889 Rivers and groundwater can return water to the sea. 00:16:03.039 --> 00:16:08.990 Water moves between stores through several processes; cloud formation and rainfall are different steps. 00:16:13.667 --> 00:16:20.588 Organisms take up water, and return it through processes including transpiration, respiration and excretion. 00:16:20.738 --> 00:16:26.284 Water is continuously redistributed rather than permanently used up by one organism. 00:16:28.967 --> 00:16:32.959 Nitrogen is needed for amino acids, proteins and D N A. 00:16:33.109 --> 00:16:42.525 Although nitrogen gas is abundant in air, most plants cannot use it directly and take up nitrogen compounds such as nitrate ions through their roots. 00:16:45.200 --> 00:16:51.235 Nitrogen-fixing bacteria convert nitrogen gas into ammonia or ammonium compounds. 00:16:51.385 --> 00:16:56.377 Fixation does not directly turn nitrogen gas into nitrate in a single step. 00:16:59.033 --> 00:17:04.749 Some nitrogen-fixing bacteria live freely in soil; others live in legume root nodules. 00:17:04.899 --> 00:17:09.082 The bacteria and the legume can both benefit from their association. 00:17:11.733 --> 00:17:17.460 Decomposers break down dead organisms and waste, releasing ammonium compounds into soil. 00:17:17.610 --> 00:17:23.209 This returns nitrogen from organic material to a form that can undergo further processing. 00:17:25.867 --> 00:17:33.118 Nitrifying bacteria convert ammonium compounds to nitrites and then nitrates under oxygenated conditions. 00:17:33.268 --> 00:17:37.908 Nitrates can be absorbed by plant roots and used to build organic molecules. 00:17:40.567 --> 00:17:47.499 Animals obtain nitrogen by eating plants or other animals, not by absorbing atmospheric nitrogen directly. 00:17:47.649 --> 00:17:51.888 Feeding transfers nitrogen-containing compounds through the community. 00:17:54.567 --> 00:18:03.237 Denitrifying bacteria can convert nitrates to nitrogen gas, returning nitrogen to the atmosphere, especially in oxygen-poor soil. 00:18:03.387 --> 00:18:06.575 Distinguish this from fixation and nitrification. 00:18:06.725 --> 00:18:15.725 Fixation, nitrification and denitrification are distinct processes; decomposition returns organic nitrogen to ammonium compounds. 00:18:20.400 --> 00:18:28.111 Fertilisers supply mineral nutrients, including nitrogen compounds that can increase plant growth when these nutrients are limiting. 00:18:28.261 --> 00:18:32.531 They do not supply the carbon in glucose produced by photosynthesis. 00:18:35.200 --> 00:18:39.696 Crop rotation changes the crop grown on a field in successive seasons. 00:18:39.846 --> 00:18:44.386 Including legumes can introduce biologically fixed nitrogen into the system. 00:18:47.067 --> 00:18:54.159 Nitrogen in legume residues can become available to later crops after decomposition and bacterial processing. 00:18:54.309 --> 00:18:59.737 Nitrogen is not instantly transferred from atmospheric gas to every neighbouring root. 00:19:02.400 --> 00:19:07.774 Returning suitable organic matter to soil can provide nutrients as decomposers act. 00:19:07.924 --> 00:19:12.510 Excess nutrient applications can still cause runoff and eutrophication. 00:19:15.167 --> 00:19:22.067 Evaluate farming practices using crop yield, soil conditions, fertiliser demand and environmental impacts. 00:19:22.217 --> 00:19:26.985 More fertiliser does not always produce more growth if another factor is limiting. 00:19:29.667 --> 00:19:33.803 Potable water is safe to drink and need not be chemically pure water. 00:19:33.953 --> 00:19:39.594 It must contain acceptably low levels of harmful dissolved substances and microorganisms. 00:19:42.267 --> 00:19:51.651 Freshwater treatment commonly removes solids by settling and filtration, then uses disinfection, such as chlorine, to reduce harmful microorganisms. 00:19:51.801 --> 00:19:55.253 Ordinary filtration does not remove all dissolved salts. 00:19:57.933 --> 00:20:02.949 Where freshwater is scarce, desalination removes dissolved salts from seawater. 00:20:03.099 --> 00:20:09.017 Distillation evaporates water and condenses the vapour, leaving most dissolved salts behind. 00:20:11.700 --> 00:20:20.136 Reverse osmosis is another desalination method: pressure pushes water through a suitable membrane while rejecting much of the dissolved salt. 00:20:22.800 --> 00:20:29.882 Desalination provides an additional water supply but requires energy and suitable management of concentrated brine. 00:20:30.032 --> 00:20:33.932 Compare these costs and impacts with local water availability. 00:20:36.600 --> 00:20:45.751 Different treatment steps have different purposes: filtration removes particles, disinfection reduces microorganisms and desalination removes salts. 00:20:45.901 --> 00:20:49.120 One step does not automatically replace all the others. 00:20:51.800 --> 00:20:54.797 That completes Ecosystems and material cycles. 00:20:54.947 --> 00:20:58.292 Revisit the notes and test yourself on the revision website.