WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE A Q A Geography revision. 00:00:04.125 --> 00:00:06.618 Topic G 5: Ecosystems. 00:00:06.768 --> 00:00:11.621 Use your school’s selected case studies if they differ from these revision examples. 00:00:14.300 --> 00:00:20.645 An ecosystem is a system in which living things interact with each other and with their non-living surroundings. 00:00:20.795 --> 00:00:24.131 Ecosystems range from a pond to a large forest. 00:00:26.800 --> 00:00:31.864 Biotic components are living things, such as plants, animals and microorganisms. 00:00:32.014 --> 00:00:37.357 Abiotic components include sunlight, temperature, water, air and minerals in soil. 00:00:40.033 --> 00:00:45.440 Producers, such as green plants and algae, make food using sunlight through photosynthesis. 00:00:45.590 --> 00:00:51.753 Consumers get energy by eating other organisms; herbivores eat plants and carnivores eat animals. 00:00:54.433 --> 00:00:59.766 Decomposers, including many bacteria and fungi, break down dead material and waste. 00:00:59.916 --> 00:01:04.545 This returns nutrients to soil or water so producers can use them again. 00:01:07.200 --> 00:01:09.749 A food chain shows one feeding pathway. 00:01:09.899 --> 00:01:15.369 Arrows point from the food to the organism eating it, showing the direction of energy transfer. 00:01:15.519 --> 00:01:18.357 A food web joins several linked food chains. 00:01:21.033 --> 00:01:25.834 Energy enters most ecosystems through sunlight and is eventually lost as heat. 00:01:25.984 --> 00:01:32.969 Nutrients are recycled between organisms, dead material, soil and water; energy is not recycled in the same way. 00:01:35.633 --> 00:01:37.681 Components depend on each other. 00:01:37.831 --> 00:01:45.905 Plants need water and minerals; animals need food and shelter; decomposers need dead material and suitable moisture and temperature. 00:01:48.567 --> 00:01:52.993 Ecosystems change naturally through seasons and over longer periods. 00:01:53.143 --> 00:01:58.849 Balance is dynamic: populations fluctuate, rather than remaining exactly the same size. 00:02:01.500 --> 00:02:05.080 A U K freshwater pond is a small-scale ecosystem. 00:02:05.230 --> 00:02:12.253 Light, water temperature, oxygen levels, water depth and nutrient supply affect which organisms can survive. 00:02:14.933 --> 00:02:17.493 Algae and aquatic plants are producers. 00:02:17.643 --> 00:02:24.329 Pond snails and some insect larvae feed on plants or algae; predatory insects and fish eat other animals. 00:02:24.479 --> 00:02:29.086 Bacteria and fungi decompose dead material in the pond and its sediment. 00:02:31.767 --> 00:02:38.890 One possible pond food chain is algae is eaten by water fleas is eaten by small fish is eaten by heron. 00:02:39.040 --> 00:02:44.319 A real pond food web has several pathways; the exact species vary between ponds. 00:02:44.469 --> 00:02:46.588 A possible pond food chain. 00:02:46.738 --> 00:02:50.613 Arrows run from food to consumer, showing energy transfer. 00:02:50.763 --> 00:02:54.999 This is an illustrative U K pond chain, not a fieldwork record. 00:02:59.667 --> 00:03:02.163 Plants provide shelter as well as food. 00:03:02.313 --> 00:03:10.716 Submerged plants release oxygen during photosynthesis, while organisms also use oxygen for respiration, including at night. 00:03:13.367 --> 00:03:16.117 Dead leaves and organisms add nutrients. 00:03:16.267 --> 00:03:19.323 Decomposers release them, and plants take them up. 00:03:19.473 --> 00:03:23.569 Temperature and oxygen affect how quickly decomposition happens. 00:03:26.233 --> 00:03:30.552 Too much fertiliser entering a pond can cause rapid algal growth. 00:03:30.702 --> 00:03:39.053 When algae die, decomposition uses oxygen, so fish and other oxygen-dependent organisms may die: this is eutrophication. 00:03:41.733 --> 00:03:47.823 Removing a predator can increase its prey population and change food available to other organisms. 00:03:47.973 --> 00:03:52.101 Draining a pond removes habitat and can break many links at once. 00:03:54.767 --> 00:03:59.789 A biome is a large region with characteristic climate, vegetation and animals. 00:03:59.939 --> 00:04:07.319 Temperature and rainfall strongly influence its distribution, although soils, altitude and human activity also matter. 00:04:10.000 --> 00:04:16.143 Tropical rainforests occur near the equator, including the Amazon, Congo Basin and Southeast Asia. 00:04:16.293 --> 00:04:22.000 They are hot and wet throughout the year and support dense vegetation and high biodiversity. 00:04:24.667 --> 00:04:30.073 Tropical grasslands (savannas) occur either side of the equatorial rainforest belt. 00:04:30.223 --> 00:04:36.045 Warm temperatures and wet and dry seasons support grasses, scattered trees and grazing animals. 00:04:38.700 --> 00:04:44.853 Hot deserts occur mainly near 30 degrees north and south, including the Sahara and Australian deserts. 00:04:45.003 --> 00:04:50.986 Very low rainfall limits vegetation; water-saving adaptations help organisms survive. 00:04:53.667 --> 00:04:58.935 Temperate deciduous forests occur in mid-latitudes, including parts of western Europe. 00:04:59.085 --> 00:05:02.763 Rainfall supports trees that shed leaves during the colder season. 00:05:05.433 --> 00:05:11.447 Temperate grasslands occur in continental interiors with too little rainfall for extensive forest. 00:05:11.597 --> 00:05:15.357 Grasses dominate; many areas have been converted to farming. 00:05:18.033 --> 00:05:24.110 Coniferous forests (taiga) stretch across high northern latitudes in Canada, Scandinavia and Russia. 00:05:24.260 --> 00:05:28.728 Long cold winters and short summers favour hardy evergreen conifers. 00:05:31.400 --> 00:05:36.549 Tundra occurs beyond the northern tree line and in some high-altitude environments. 00:05:36.699 --> 00:05:43.045 Low temperatures, short growing seasons and often permafrost favour mosses, lichens and low shrubs. 00:05:45.700 --> 00:05:50.509 Polar environments are very cold and usually receive little precipitation. 00:05:50.659 --> 00:05:54.757 Extensive ice and limited plant growth distinguish them from tundra. 00:05:57.433 --> 00:05:59.396 That completes Ecosystems. 00:05:59.546 --> 00:06:02.891 Revisit the notes and test yourself on the revision website.