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Unit 1
6–8% of examThis unit is about how ecosystems work: how living things interact, why different biomes are found where they are, and how energy and matter move through nature. Energy flows one way from the sun through food chains, while elements like carbon, nitrogen and phosphorus get recycled over and over. These ideas are the foundation for almost everything else in APES.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Living things in an ecosystem interact in a few main ways: predators eat prey, species compete for limited resources, and symbiosis is a close, long-term relationship between two species. Species can avoid competing head-on through resource partitioning, which means sharing a resource by using it at different times, in different places or in different ways.
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A biome is a large region with a typical climate and the plants and animals adapted to it. Average temperature and rainfall largely decide which land biome you find, such as tropical rainforest, desert, temperate grassland, taiga or tundra, and where biomes sit can shift as the climate changes.
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Water biomes are sorted by things like salinity (saltiness), depth, temperature, flow and how much sunlight gets in. Freshwater biomes such as rivers and lakes supply drinking water, while marine biomes such as oceans, coral reefs, marshes and estuaries are very productive, and algae in the water make a large share of Earth's oxygen.
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Carbon moves between reservoirs such as the air, the ocean, living things, rocks and fossil fuels. Photosynthesis pulls carbon dioxide out of the air and respiration puts it back, while burning fossil fuels quickly releases carbon that was locked underground for millions of years.
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The atmosphere is the biggest reservoir of nitrogen, as N₂ gas, which plants and animals can't use directly. Soil bacteria do most of the converting: in nitrogen fixation they turn N₂ into ammonia (NH₃), which soon becomes ammonium (NH₄⁺) that plants can absorb, and the other steps (assimilation, ammonification, nitrification and denitrification) move nitrogen through living things and back to the air. Because fixation is slow, usable nitrogen often limits how much plants can grow.
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Phosphorus is stored mostly in rocks and ocean sediments and has no major gas stage in the air, so it cycles slowly as rocks weather. Because so little is available at any time, phosphorus is often the nutrient that limits plant and algae growth, especially in fresh water.
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Powered by the sun, water moves between the oceans, the air, ice, groundwater and living things by evaporation, transpiration, condensation, precipitation, infiltration and runoff. The ocean is by far the biggest reservoir, and only a small fraction of Earth's water is fresh water that people can easily use.
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Primary productivity is how fast producers turn sunlight into stored chemical energy through photosynthesis. Gross primary productivity (GPP) is the total, and net primary productivity (NPP) is what's left after producers use some for their own respiration (NPP = GPP − respiration), which is the energy available to the rest of the food web.
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A trophic level is a feeding position in an ecosystem: producers first, then primary, secondary and tertiary consumers. Every ecosystem needs a steady input of energy, usually sunlight, because some energy is lost as heat at every level, while decomposers return matter to the soil to be reused.
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Only about 10% of the energy at one trophic level ends up stored at the next; most of the rest is lost as heat. That's why energy pyramids are wide at the bottom and narrow at the top, and why food chains rarely have more than four or five levels.
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A food chain is one path that energy and matter take from producers up through consumers, and a food web shows how many chains link together. Adding or removing one species can ripple through the whole web, and feedback loops can either dampen or amplify those changes.
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