WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.883 Unit S B 9: Ecosystems and material cycles. 00:00:10.533 --> 00:00:13.549 An individual organism is one living thing. 00:00:13.699 --> 00:00:19.543 A population is all the organisms of one species in a particular area at a particular time. 00:00:22.200 --> 00:00:26.740 A community consists of the populations of different species living in an area. 00:00:26.890 --> 00:00:32.979 An ecosystem includes that community and its non-living environment, including interactions between them. 00:00:35.633 --> 00:00:38.640 A habitat is the place where an organism lives. 00:00:38.790 --> 00:00:42.709 Different species can share a habitat while using different resources. 00:00:45.367 --> 00:00:50.389 Organisms need resources such as food, water, space and suitable conditions. 00:00:50.539 --> 00:00:56.465 Plants also compete for light and mineral ions; animals may compete for food, mates and territory. 00:00:59.133 --> 00:01:05.052 Interdependence means organisms depend on others, for example for food, pollination or shelter. 00:01:05.202 --> 00:01:09.449 Changing one population can affect other species in the community. 00:01:12.100 --> 00:01:16.235 An ecosystem includes both biotic and abiotic components. 00:01:16.385 --> 00:01:21.353 Do not use population, community and ecosystem as interchangeable terms. 00:01:24.033 --> 00:01:32.640 Abiotic factors are non-living influences, including temperature, light intensity, water availability, soil conditions and pollutants. 00:01:35.300 --> 00:01:44.429 Biotic factors involve living organisms and their interactions, including predation, competition, disease and the availability of food organisms. 00:01:47.100 --> 00:01:50.001 Species are adapted to particular conditions. 00:01:50.151 --> 00:01:58.253 A substantial change in temperature, rainfall or other conditions can change survival, reproduction, abundance and distribution. 00:02:00.933 --> 00:02:05.142 Competition occurs when organisms need the same limited resource. 00:02:05.292 --> 00:02:09.138 Competition can occur within a species or between different species. 00:02:11.800 --> 00:02:17.578 In a simplified predator, prey cycle, prey numbers rise first, giving predators more food. 00:02:17.728 --> 00:02:24.938 Predator numbers may rise after a delay, increasing predation and reducing prey numbers; predator numbers may then fall. 00:02:27.600 --> 00:02:34.038 Real populations are also affected by other foods, disease, migration and environmental changes. 00:02:34.188 --> 00:02:39.915 Predator, prey cycles and food-web responses are not guaranteed to follow one fixed pattern. 00:02:42.567 --> 00:02:45.606 Food chains and webs show feeding relationships. 00:02:45.756 --> 00:02:52.549 An arrow goes from the organism eaten to the organism eating it, showing the direction of energy transfer in food. 00:02:55.200 --> 00:02:59.889 Producers, such as green plants, make biomass using photosynthesis. 00:03:00.039 --> 00:03:07.684 Primary consumers eat producers; secondary consumers eat primary consumers, and tertiary consumers feed further along a chain. 00:03:10.367 --> 00:03:15.145 A food web links several chains, so a consumer may have more than one food source. 00:03:15.295 --> 00:03:22.121 If one prey population falls, its predators may fall or switch to other prey, affecting those populations too. 00:03:22.271 --> 00:03:27.219 A simplified food web: a population change can affect several linked species. 00:03:31.900 --> 00:03:39.568 A change can have indirect effects: fewer predators may allow a prey population to rise, increasing pressure on the prey's food. 00:03:39.718 --> 00:03:43.895 Explain the links and qualify predictions using other relevant factors. 00:03:46.567 --> 00:03:51.259 A parasite benefits at its host's expense, often living on or inside it. 00:03:51.409 --> 00:03:57.303 For example, a flea feeds on a mammal's blood; harm to the host does not mean that it must die immediately. 00:03:59.967 --> 00:04:02.484 Mutualism benefits both partners. 00:04:02.634 --> 00:04:07.721 A pollinating bee gains nectar or pollen while helping a flowering plant reproduce. 00:04:10.400 --> 00:04:20.669 Legumes and bacteria in their root nodules can also be mutualistic: bacteria provide fixed nitrogen, while the plant supplies carbohydrates and a suitable habitat. 00:04:23.333 --> 00:04:30.831 Abundance describes how common a species is, measured using counts, density, frequency or percentage cover as appropriate. 00:04:30.981 --> 00:04:34.212 It is not always simply a total population count. 00:04:36.867 --> 00:04:39.767 A quadrat is a frame enclosing a known area. 00:04:39.917 --> 00:04:46.828 It is useful for sampling plants and animals that remain in place, rather than animals that quickly move into or out of the frame. 00:04:49.500 --> 00:04:56.389 For a representative area estimate, mark a study area and use random coordinates to choose quadrat positions. 00:04:56.539 --> 00:05:01.047 Choosing only convenient or visibly crowded patches introduces bias. 00:05:03.700 --> 00:05:10.439 Identify the species consistently, count individuals and use a consistent rule for plants touching the frame. 00:05:10.589 --> 00:05:15.858 Use percentage cover instead when distinct individuals cannot sensibly be counted. 00:05:18.533 --> 00:05:23.259 Repeat with enough non-overlapping samples distributed across the study area. 00:05:23.409 --> 00:05:28.251 Record quadrat size, locations and counts so the method can be evaluated. 00:05:30.933 --> 00:05:36.681 Density equals total count in sampled quadrats divided by total area sampled. 00:05:36.831 --> 00:05:42.015 Estimated population equals density times total suitable habitat area. 00:05:44.667 --> 00:05:49.723 For ten zero point two five square metre quadrats containing forty plants in total, 00:05:49.873 --> 00:05:56.261 sampled area equals two point five square metres and density equals sixteen plants per square metre. 00:05:56.411 --> 00:06:02.905 Across a representative one hundred square metre area, the estimated population is one thousand six hundred. 00:06:03.055 --> 00:06:07.249 Use the total sampled area, not just the area of one quadrat. 00:06:11.900 --> 00:06:18.352 Alternatively, multiply the mean count per quadrat by the number of quadrat-sized areas in the habitat. 00:06:18.502 --> 00:06:21.913 Convert all areas to the same units before calculating. 00:06:24.567 --> 00:06:27.724 An estimate assumes samples represent the area. 00:06:27.874 --> 00:06:37.408 Patchiness, identification errors, too few quadrats or sampling only part of the habitat can reduce reliability; the estimate is not an exact census. 00:06:40.067 --> 00:06:49.313 A belt transect samples a strip across an environmental gradient, such as from shaded woodland to open grassland, using quadrats along a measured line. 00:06:51.967 --> 00:06:58.471 A continuous belt uses adjacent quadrats; an interrupted belt uses quadrats at specified intervals. 00:06:58.621 --> 00:07:04.039 Record distance along the transect and keep quadrat area and counting rules consistent. 00:07:04.189 --> 00:07:09.279 Record distribution across a gradient; this is different from random area sampling. 00:07:13.933 --> 00:07:23.031 Measure the relevant abiotic factor at sampling positions, for example light intensity or soil moisture, as well as species abundance or cover. 00:07:25.700 --> 00:07:28.867 Repeat transects where possible and compare patterns. 00:07:29.017 --> 00:07:34.733 Keep the measurement procedure consistent and consider weather, time of day and identification errors. 00:07:37.400 --> 00:07:47.062 A transect uses regular sampling positions (systematic sampling) to investigate how species distribution changes along an environmental gradient. 00:07:47.212 --> 00:07:49.153 It is not random sampling. 00:07:49.303 --> 00:07:54.805 Only use it to estimate the whole habitat if the sampled area represents that habitat. 00:07:57.467 --> 00:08:02.043 Plot abundance or cover against distance or a measured abiotic factor. 00:08:02.193 --> 00:08:07.898 A correlation can support an explanation but does not prove that one factor alone caused the pattern. 00:08:10.567 --> 00:08:17.413 Frequency ( percent) equals quadrats containing the species divided by total quadrats times 100. 00:08:17.563 --> 00:08:24.409 A species in 6 of 10 quadrats has 60 percent frequency; that does not mean it covers 60 percent of the ground. 00:08:27.067 --> 00:08:33.711 Work safely in the study area, avoid unnecessary disturbance and leave organisms and habitats intact. 00:08:33.861 --> 00:08:38.693 Record the method and environmental conditions so comparisons are meaningful. 00:08:41.367 --> 00:08:45.489 Biodiversity is the variety of living organisms in an area. 00:08:45.639 --> 00:08:50.886 More individuals of one species do not automatically mean greater biodiversity. 00:08:53.567 --> 00:08:58.537 Maintaining biodiversity protects species and interdependent relationships. 00:08:58.687 --> 00:09:05.736 Diverse ecosystems may be more resilient to disturbance, although outcomes depend on the particular species and event. 00:09:08.400 --> 00:09:15.809 Species can provide food, medicines and other useful materials, while ecosystems provide services such as pollination. 00:09:15.959 --> 00:09:20.403 Conservation also values species beyond their direct uses to people. 00:09:23.067 --> 00:09:31.332 Conservation can protect and restore habitats, reduce pollution, regulate harvesting and protect threatened species locally and globally. 00:09:34.000 --> 00:09:40.453 Reforestation restores tree cover and can provide habitat, store carbon and support biodiversity. 00:09:40.603 --> 00:09:47.152 Using suitable native species and varied habitats can be more beneficial than a single-species plantation. 00:09:49.833 --> 00:09:54.761 Captive breeding can increase numbers of threatened species for possible reintroduction. 00:09:54.911 --> 00:10:02.183 It needs genetic diversity, suitable habitat and management of the original threats to support long-term success. 00:10:04.833 --> 00:10:10.773 Controlling an invasive population, sometimes including culling, may protect native species. 00:10:10.923 --> 00:10:19.571 Evaluate effectiveness, animal welfare, unintended effects and alternatives rather than assuming that killing competitors is always beneficial. 00:10:22.233 --> 00:10:26.788 Overfishing can reduce wild fish populations and disrupt food webs. 00:10:26.938 --> 00:10:34.797 Fish farming can provide food and may reduce some harvesting pressure on wild stocks, depending on the farming system and feed sources. 00:10:37.467 --> 00:10:41.531 High stocking density can increase stress and disease transmission. 00:10:41.681 --> 00:10:49.488 Waste and uneaten feed can add nutrients to nearby water; these are possible impacts rather than proof that every farm is unhealthy. 00:10:52.167 --> 00:10:58.481 Escaped farmed fish may compete or breed with wild populations, and parasites or diseases can spread. 00:10:58.631 --> 00:11:03.029 Better siting, waste management and biosecurity can reduce risks. 00:11:05.700 --> 00:11:12.973 Medicines used in some systems can affect nearby organisms or contribute to antimicrobial-resistance concerns. 00:11:13.123 --> 00:11:19.639 Evaluate evidence and management practices rather than assuming every farm uses antibiotics routinely. 00:11:22.300 --> 00:11:26.673 A non-indigenous species has been introduced outside its native range. 00:11:26.823 --> 00:11:35.823 Some become invasive and harm native populations through competition, predation or disease; not every introduced species becomes invasive. 00:11:38.500 --> 00:11:44.739 An introduced predator or competitor can change several populations indirectly through a food web. 00:11:44.889 --> 00:11:49.933 Predict effects using the organism's interactions and the environmental context. 00:11:52.600 --> 00:12:02.329 Eutrophication can occur when excess nutrients, such as nitrates or phosphates from fertiliser runoff or sewage, enter water and stimulate rapid algal growth. 00:12:05.000 --> 00:12:12.547 A dense algal bloom reduces light reaching submerged plants, reducing photosynthesis and potentially causing plant death. 00:12:12.697 --> 00:12:17.133 Algae also eventually die and provide material for decomposition. 00:12:19.800 --> 00:12:25.922 Microorganisms decompose dead material and respire aerobically, using dissolved oxygen. 00:12:26.072 --> 00:12:30.359 Increased decomposition can greatly reduce oxygen concentration. 00:12:33.033 --> 00:12:38.653 Fish and other organisms requiring oxygen may die or leave when oxygen is too low. 00:12:38.803 --> 00:12:43.987 The main immediate problem is oxygen depletion, rather than carbon dioxide alone. 00:12:44.137 --> 00:12:48.723 Microbial respiration links decomposition to dissolved-oxygen loss. 00:12:53.400 --> 00:13:02.540 Some organisms can survive very low oxygen levels, so eutrophication changes the community rather than necessarily killing everything in the water. 00:13:05.200 --> 00:13:11.289 Reducing nutrient inputs through appropriate fertiliser use and sewage treatment helps prevention. 00:13:11.439 --> 00:13:18.743 Explain the sequence from nutrient addition to oxygen loss, rather than saying fertiliser directly poisons all fish. 00:13:21.400 --> 00:13:26.967 Carbon is present in carbohydrates, lipids, proteins and other components of biomass. 00:13:27.117 --> 00:13:35.586 Materials cycle between organisms and the non-living environment; energy flows through ecosystems and is not recycled in the same way. 00:13:38.267 --> 00:13:42.145 Photosynthesis takes carbon dioxide from air or water. 00:13:42.295 --> 00:13:46.813 Producers use its carbon atoms to make organic molecules such as glucose. 00:13:49.467 --> 00:13:55.449 Feeding transfers carbon-containing material from producers to consumers and between consumers. 00:13:55.599 --> 00:13:59.925 Carbon atoms are rearranged rather than created anew at each transfer. 00:14:02.600 --> 00:14:08.463 Plants, animals and microorganisms respire, returning carbon dioxide to the environment. 00:14:08.613 --> 00:14:11.428 Plants respire as well as photosynthesise. 00:14:14.100 --> 00:14:20.509 Death, fallen leaves and waste supply organic material to decomposers, including bacteria and fungi. 00:14:20.659 --> 00:14:27.175 Decomposition makes materials available again, while decomposer respiration releases carbon dioxide. 00:14:29.833 --> 00:14:37.887 Some organic carbon can be stored for long periods, for example in fossil fuels formed under particular conditions over geological time. 00:14:38.037 --> 00:14:40.639 This does not happen to all dead material. 00:14:40.789 --> 00:14:45.303 Main transfer pathways; dead material does not all become fossil fuel. 00:14:49.967 --> 00:14:54.417 Combustion of fossil fuels or biomass releases carbon dioxide. 00:14:54.567 --> 00:14:59.879 Deforestation can reduce carbon uptake and storage; reforestation can increase them. 00:15:02.533 --> 00:15:08.345 Water is needed for cell reactions and is a major component of cytoplasm and blood plasma. 00:15:08.495 --> 00:15:13.381 It moves between oceans, freshwater, the atmosphere, land and living organisms. 00:15:16.033 --> 00:15:19.875 Energy from the Sun drives evaporation from water surfaces. 00:15:20.025 --> 00:15:24.614 Transpiration from plants also transfers water vapour into the atmosphere. 00:15:27.267 --> 00:15:32.504 Water vapour cools and condenses into droplets or ice crystals that form clouds. 00:15:32.654 --> 00:15:37.731 Condensation changes gas to liquid; it is not the same process as rainfall. 00:15:40.400 --> 00:15:45.077 Precipitation returns water to the surface as rain, snow or other forms. 00:15:45.227 --> 00:15:48.754 Water may collect in lakes and rivers or return to the ocean. 00:15:51.433 --> 00:15:56.840 Some water soaks into soil and rock (infiltration) and moves underground as groundwater. 00:15:56.990 --> 00:15:59.667 Some flows over the land as surface runoff. 00:15:59.817 --> 00:16:02.822 Rivers and groundwater can return water to the sea. 00:16:02.972 --> 00:16:08.923 Water moves between stores through several processes; cloud formation and rainfall are different steps. 00:16:13.600 --> 00:16:20.521 Organisms take up water, and return it through processes including transpiration, respiration and excretion. 00:16:20.671 --> 00:16:26.217 Water is continuously redistributed rather than permanently used up by one organism. 00:16:28.900 --> 00:16:32.893 Nitrogen is needed for amino acids, proteins and D N A. 00:16:33.043 --> 00:16:42.459 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. 00:16:45.133 --> 00:16:51.169 Nitrogen-fixing bacteria convert nitrogen gas into ammonia or ammonium compounds. 00:16:51.319 --> 00:16:56.310 Fixation does not directly turn nitrogen gas into nitrate in a single step. 00:16:58.967 --> 00:17:04.682 Some nitrogen-fixing bacteria live freely in soil; others live in legume root nodules. 00:17:04.832 --> 00:17:09.015 The bacteria and the legume can both benefit from their association. 00:17:11.667 --> 00:17:17.393 Decomposers break down dead organisms and waste, releasing ammonium compounds into soil. 00:17:17.543 --> 00:17:23.142 This returns nitrogen from organic material to a form that can undergo further processing. 00:17:25.800 --> 00:17:33.051 Nitrifying bacteria convert ammonium compounds to nitrites and then nitrates under oxygenated conditions. 00:17:33.201 --> 00:17:37.841 Nitrates can be absorbed by plant roots and used to build organic molecules. 00:17:40.500 --> 00:17:47.433 Animals obtain nitrogen by eating plants or other animals, not by absorbing atmospheric nitrogen directly. 00:17:47.583 --> 00:17:51.821 Feeding transfers nitrogen-containing compounds through the community. 00:17:54.500 --> 00:18:03.171 Denitrifying bacteria can convert nitrates to nitrogen gas, returning nitrogen to the atmosphere, especially in oxygen-poor soil. 00:18:03.321 --> 00:18:06.508 Distinguish this from fixation and nitrification. 00:18:06.658 --> 00:18:15.659 Fixation, nitrification and denitrification are distinct processes; decomposition returns organic nitrogen to ammonium compounds. 00:18:20.333 --> 00:18:28.044 Fertilisers supply mineral nutrients, including nitrogen compounds that can increase plant growth when these nutrients are limiting. 00:18:28.194 --> 00:18:32.464 They do not supply the carbon in glucose produced by photosynthesis. 00:18:35.133 --> 00:18:39.629 Crop rotation changes the crop grown on a field in successive seasons. 00:18:39.779 --> 00:18:44.319 Including legumes can introduce biologically fixed nitrogen into the system. 00:18:47.000 --> 00:18:54.092 Nitrogen in legume residues can become available to later crops after decomposition and bacterial processing. 00:18:54.242 --> 00:18:59.670 Nitrogen is not instantly transferred from atmospheric gas to every neighbouring root. 00:19:02.333 --> 00:19:07.707 Returning suitable organic matter to soil can provide nutrients as decomposers act. 00:19:07.857 --> 00:19:12.443 Excess nutrient applications can still cause runoff and eutrophication. 00:19:15.100 --> 00:19:22.000 Evaluate farming practices using crop yield, soil conditions, fertiliser demand and environmental impacts. 00:19:22.150 --> 00:19:26.918 More fertiliser does not always produce more growth if another factor is limiting. 00:19:29.600 --> 00:19:33.736 Potable water is safe to drink and need not be chemically pure water. 00:19:33.886 --> 00:19:39.527 It must contain acceptably low levels of harmful dissolved substances and microorganisms. 00:19:42.200 --> 00:19:51.585 Freshwater treatment commonly removes solids by settling and filtration, then uses disinfection, such as chlorine, to reduce harmful microorganisms. 00:19:51.735 --> 00:19:55.187 Ordinary filtration does not remove all dissolved salts. 00:19:57.867 --> 00:20:02.882 Where freshwater is scarce, desalination removes dissolved salts from seawater. 00:20:03.032 --> 00:20:08.951 Distillation evaporates water and condenses the vapour, leaving most dissolved salts behind. 00:20:11.633 --> 00:20:20.069 Reverse osmosis is another desalination method: pressure pushes water through a suitable membrane while rejecting much of the dissolved salt. 00:20:22.733 --> 00:20:29.815 Desalination provides an additional water supply but requires energy and suitable management of concentrated brine. 00:20:29.965 --> 00:20:33.865 Compare these costs and impacts with local water availability. 00:20:36.533 --> 00:20:45.685 Different treatment steps have different purposes: filtration removes particles, disinfection reduces microorganisms and desalination removes salts. 00:20:45.835 --> 00:20:49.053 One step does not automatically replace all the others. 00:20:51.733 --> 00:20:55.272 Energy enters most ecosystems through photosynthesis. 00:20:55.422 --> 00:21:03.005 At each feeding level (trophic level), some energy transfers through respiration and movement and eventually heats the surroundings. 00:21:03.155 --> 00:21:07.702 Uneaten parts and waste do not become part of the next consumer’s biomass. 00:21:07.852 --> 00:21:12.519 Respiration, waste and uneaten material reduce transfer between each level. 00:21:17.200 --> 00:21:23.865 Pyramids of biomass usually become narrower at higher trophic levels because less biomass can be supported. 00:21:24.015 --> 00:21:31.395 Energy losses limit food-chain length; a pyramid of numbers can have a different shape if organisms differ greatly in size. 00:21:34.067 --> 00:21:44.763 Transfer efficiency ( percent) equals energy or biomass transferred to the next level divided by energy or biomass available at the previous level times 100. 00:21:44.913 --> 00:21:52.100 Compare matching measurements and time periods; biomass values often use dry mass to exclude variable water content. 00:21:54.767 --> 00:21:58.793 Food security means reliable access to enough suitable food. 00:21:58.943 --> 00:22:09.843 It can be reduced by population growth, greater meat and fish consumption, pests, pathogens, environmental changes and the costs of farming supplies such as fertiliser and feed. 00:22:12.500 --> 00:22:17.343 Growing crops for biofuels can compete with food production for land and water. 00:22:17.493 --> 00:22:27.380 Animal farming uses feed that could sometimes feed people directly; however, some grazing land cannot grow human food crops, so evaluate the particular system. 00:22:30.033 --> 00:22:38.041 Sustainable responses include reducing waste, maintaining soils and water supplies, managing pests and protecting fish stocks. 00:22:38.191 --> 00:22:42.554 A higher short-term yield alone does not show a system is sustainable. 00:22:45.233 --> 00:22:50.693 (Higher tier) Water low in oxygen can support pollution-tolerant bloodworms and sludgeworms, 00:22:50.843 --> 00:22:55.355 while freshwater shrimps and stonefly larvae are generally associated with cleaner, 00:22:55.505 --> 00:22:57.467 well-oxygenated water. 00:22:57.617 --> 00:23:03.418 Use a community survey and supporting measurements rather than treating one animal as conclusive proof. 00:23:06.100 --> 00:23:12.733 (Higher tier) Lichen communities can indicate air quality because species differ in sensitivity to pollutants. 00:23:12.883 --> 00:23:16.872 Blackspot fungus on roses is sensitive to sulfur dioxide, 00:23:17.022 --> 00:23:21.801 so its occurrence can support evidence of relatively low sulfur-dioxide pollution; 00:23:21.951 --> 00:23:24.989 weather and habitat also affect observations. 00:23:27.667 --> 00:23:32.083 Decomposers need suitable temperatures, water and often oxygen. 00:23:32.233 --> 00:23:36.713 Increasing temperature towards an optimum accelerates enzyme reactions, 00:23:36.863 --> 00:23:39.786 but very high temperatures can denature enzymes; 00:23:39.936 --> 00:23:44.724 freezing slows microbial activity rather than sterilising all food. 00:23:47.400 --> 00:23:52.508 Refrigeration, drying and limiting oxygen can slow decomposition in food. 00:23:52.658 --> 00:23:58.577 Some microbes can grow without oxygen, so removing oxygen alone is not a guarantee of safety. 00:24:01.233 --> 00:24:04.387 Composting benefits from moisture, warmth and air. 00:24:04.537 --> 00:24:11.330 Turning a heap improves oxygen supply; waterlogging fills air spaces and reduces aerobic decomposition. 00:24:11.480 --> 00:24:14.167 Microbial respiration can warm the heap. 00:24:16.833 --> 00:24:22.273 Calculate decomposition rate as mass lost divided by time, using matching units. 00:24:22.423 --> 00:24:30.591 Compare the same starting material and conditions, include repeats and consider water loss as a possible reason for apparent mass reduction. 00:24:33.267 --> 00:24:36.263 That completes Ecosystems and material cycles. 00:24:36.413 --> 00:24:39.759 Revisit the notes and test yourself on the revision website.