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Environmental Science Unit 1: Ecosystems

Ecosystems move energy from the sun through food webs while recycling matter through biogeochemical cycles.

Covers: Biomes, energy flow, trophic levels, food webs, biogeochemical cycles (carbon, nitrogen, phosphorus, water)

Study guide

An ecosystem includes all the living things in an area and their physical environment. Biomes are large regions defined by climate (temperature and precipitation) and their dominant plants: tropical rainforest, desert, grassland, temperate deciduous forest, boreal forest (taiga) and tundra on land, plus freshwater and marine biomes in water.

Energy flows one way through ecosystems. Producers (autotrophs) capture sunlight through photosynthesis; consumers (heterotrophs) eat producers or other consumers; decomposers and detritivores break down dead matter. Gross primary productivity is the total energy producers capture, and net primary productivity is what's left after their own respiration, the energy available to consumers.

Organisms are grouped into trophic levels: producers, primary consumers (herbivores), secondary consumers and tertiary consumers. Only about 10% of the energy at one level passes to the next, the rest being lost as heat. Energy pyramids are therefore wide at the bottom and narrow at the top, and food chains rarely have more than four or five levels. Food webs show the many connected feeding relationships in a real ecosystem.

Matter cycles rather than flowing one way. The carbon cycle moves carbon through photosynthesis, respiration, decomposition, ocean exchange and fossil fuel combustion; sedimentary rock is the largest reservoir. In the nitrogen cycle, bacteria fix N₂ gas into ammonia, nitrification converts it to nitrates plants absorb, and denitrification returns N₂ to the air. The phosphorus cycle has no gas phase; it moves slowly through rocks, soil, water and organisms, so phosphorus often limits growth. The water (hydrologic) cycle includes evaporation, transpiration, condensation, precipitation, runoff and infiltration into groundwater.

Ecological succession is the gradual change in a community after a disturbance. Primary succession starts on bare rock with no soil, beginning with pioneer species like lichens and mosses. Secondary succession, which is faster, happens where soil remains, such as after a fire or on an abandoned farm. Savannas (tropical grasslands) are warm all year, with distinct wet and dry seasons and grasses with scattered trees. Temperate seasonal forests have four distinct seasons, moderate precipitation spread through the year and deciduous trees. Shrublands (chaparral) have hot, dry summers and mild, wet winters and are adapted to fire.

Species interact through competition, predation, parasitism, mutualism (both benefit) and commensalism (one benefits and the other is unaffected). Competing species can coexist through resource partitioning, using the same resource at different times, in different places or in different ways. This reduces direct competition. A keystone species has an outsized effect on its ecosystem compared with its numbers, and it is often a top predator. Removing a predator can cause a trophic cascade: its prey increases and then overconsumes the next level down, such as plants.

Key ideas

• Energy flows one way; matter cycles.
• About 10% of energy passes up each trophic level.
• Carbon cycles through photosynthesis, respiration, combustion and the oceans.
• Nitrogen fixation by bacteria makes nitrogen usable to plants.
• Phosphorus has no gas phase, so it cycles slowly through rock.

Formulas and rules

Common mistakes

• Saying energy is recycled in ecosystems.
• Forgetting phosphorus has no atmospheric phase.

Words to know

Biome

A large region defined by climate and vegetation.

Phosphorus cycle

The only major nutrient cycle with no gas phase.

Nitrogen fixation

Bacteria converting N₂ into ammonia plants can use.

10% rule

About 10% of energy passes to the next trophic level.

Carbon reservoir

Where carbon is stored; sedimentary rock is largest, then the oceans.

Ecosystem

Living things in an area plus their physical environment.

Producer (autotroph)

Makes its own food, usually by photosynthesis.

Consumer (heterotroph)

Gets energy by eating other organisms.

Decomposer

Breaks down dead matter and recycles nutrients.

Gross primary productivity

Total energy captured by producers.

Net primary productivity

GPP minus producers' respiration: energy available to consumers.

Trophic level

A feeding level: producer, primary consumer, secondary consumer…

Energy pyramid

Wide at the bottom, narrow at the top, because ~90% is lost each level.

Carbon cycle

Moves carbon through photosynthesis, respiration, oceans, rocks and combustion.

Nitrification / denitrification

Ammonia → nitrates plants use / nitrates → N₂ gas.

Water cycle

Evaporation, transpiration, condensation, precipitation, runoff, infiltration.

Tundra

Cold, treeless biome with permafrost.

Tropical rainforest

Warm, wet biome with the highest biodiversity.

Sample practice questions

15 more questions like these, from level 1 to 5, are in the practice set.