Ecosystems run on two things: matter that cycles (carbon, nitrogen, phosphorus, water) and energy that flows one way, from the sun through producers to consumers, losing most of itself as heat at every step. This unit is about how species interact, where the biomes are, and how to read and calculate those cycles and flows.
Species interact as predator and prey, through symbiosis (mutualism, commensalism, parasitism), and through competition for limited resources. Resource partitioning, using a resource in different ways, places or times, reduces competition.
A biome is a set of plant and animal communities adapted to a particular climate. The major terrestrial biomes are taiga, temperate rainforest, temperate seasonal forest, tropical rainforest, shrubland, temperate grassland, savanna, desert and tundra. Climate, latitude, altitude, soil and nutrients shape where they are, and their distribution shifts as the climate changes.
Freshwater biomes (streams, rivers, ponds, lakes) supply drinking water. Marine biomes (oceans, coral reefs, marshland, estuaries) include algae that produce much of Earth's oxygen and absorb CO₂. Salinity, depth, turbidity, nutrients and temperature decide which organisms live where.
Carbon moves between sources and sinks. Some reservoirs (sedimentary rock, fossil fuels, deep ocean) hold it for millions of years; others (atmosphere, living things) for short times. Photosynthesis and respiration cycle it through life, and burning fossil fuels moves long-stored carbon quickly into the atmosphere as CO₂.
The atmosphere is the largest nitrogen reservoir, but most organisms can't use N₂. Soil bacteria fix it into ammonia, which becomes ammonium; then come assimilation, ammonification, nitrification and denitrification. Fixation is slow, so nitrogen often limits plant growth.
Phosphorus is stored mainly in rock and ocean sediments and has no significant atmospheric component. Rocks weather slowly, so phosphorus is scarce and often limits producers, especially in freshwater.
Powered by the sun, water moves as ice, liquid and vapor between sources and sinks. The oceans are by far the main surface reservoir; ice caps and groundwater are much smaller.
Primary productivity is the rate at which producers turn sunlight into organic compounds. Gross primary productivity is the total; net primary productivity is what's left after the producers' own respiration. It's measured in energy per area per time, like kcal/m²/yr. In water, light quality with depth limits photosynthesis.
Ecosystems need a continuous inflow of high-quality energy, almost always from the sun. Energy flows one way, from producers up to higher trophic levels, while matter cycles through biogeochemical cycles and is conserved.
Only about 10% of the energy in one trophic level is passed to the next. The rest is used for life processes and lost as heat, as the laws of thermodynamics predict. So each level up has about a tenth of the energy below it.
A food chain shows energy and matter moving from producers to herbivores to carnivores and omnivores. Decomposers and detritivores return nutrients to the soil. A food web links many food chains, and adding or removing one species affects the rest, through positive and negative feedback.