Lesson 11 of 11 · 13 min

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Must-know facts

20 facts

1An ecosystem is a functional unit of nature: organisms interacting with each other and with their physical environment.
2Terrestrial: forest, grassland, desert. Aquatic: pond, lake, wetland, river, estuary. Man-made: crop field, aquarium.
3Structural features: species composition and stratification (trees on top, then shrubs, then herbs and grasses).
4Four functional aspects: productivity, decomposition, energy flow, nutrient cycling.
5Pond: phytoplankton and plants produce; zooplankton consume; fungi, bacteria and flagellates decompose.
6Productivity units: g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
7GPP − R = NPP; NPP is what heterotrophs can use. Secondary productivity is the rate of new organic matter formed by consumers.
8Biosphere NPP about 170 billion tons dry weight a year; oceans, about 70 per cent of the surface, give only 55 billion tons.
9Decomposition steps: fragmentation, leaching, catabolism, humification, mineralisation.
10Lignin and chitin slow decomposition; nitrogen and sugars speed it; warm and moist favour it, cold and anaerobiosis inhibit it.
11The sun powers all ecosystems except deep-sea hydrothermal ones.
12PAR is under 50 per cent of incident sunlight; plants capture 2-10 per cent of PAR.
13Energy flow is unidirectional: sun → producers → consumers.
14Grazing food chain: grass → goat → man. Detritus food chain starts with dead organic matter.
15Aquatic: GFC is the main energy channel. Terrestrial: more energy flows through the DFC.
1610 per cent law: only 10 per cent of energy passes to the next trophic level.
17Standing crop: living biomass or number per unit area at a trophic level; dry weight is more accurate.
18Pyramid of number inverted for a single tree; pyramid of biomass inverted in the sea; pyramid of energy always upright.
19A sparrow is a primary consumer eating seeds and a secondary consumer eating insects.
20Pyramids ignore food webs, species at two levels, and saprophytes.

Common traps

Where marks are lost

Saying the pyramid of energy can be inverted.

Energy is lost as heat at every transfer, so the pyramid of energy is always upright.

Writing NPP = GPP + R.

Respiration is a loss: NPP = GPP − R.

Saying the grazing food chain carries most energy on land.

On land a much larger share flows through the detritus food chain; the GFC dominates in water.

Saying leaching makes nutrients more available.

Leaching carries soluble nutrients down the soil and precipitates them as unavailable salts.

Thinking humus decomposes quickly.

Humus resists microbial action and decomposes extremely slowly; it is a nutrient reservoir.

Saying lignin-rich detritus decomposes fast.

Lignin and chitin slow decomposition; nitrogen and sugars speed it up.

Giving PAR as 2-10 per cent of sunlight.

PAR is under 50 per cent of sunlight; plants capture 2-10 per cent of that PAR.

Assigning each species one fixed trophic level.

A trophic level is functional: the sparrow is a primary consumer on seeds and a secondary consumer on insects.

Saying the sea's pyramid of biomass is upright.

It is generally inverted: a small standing crop of phytoplankton supports a larger biomass of consumers.

Key terms

18 terms

Ecosystem: A functional unit of nature where organisms interact with each other and their physical surroundings.
Stratification: Vertical layering of species at different levels, such as trees, shrubs and herbs in a forest.
Species composition: The identity and number of plant and animal species in an ecosystem.
Primary production: Biomass made per unit area over a period by plants in photosynthesis.
Gross primary productivity (GPP): Rate of production of organic matter in photosynthesis.
Net primary productivity (NPP): GPP minus respiration losses; the biomass available to heterotrophs.
Secondary productivity: Rate of formation of new organic matter by consumers.
Detritus: Dead plant and animal remains, including fecal matter; the raw material of decomposition.
Detritivore: An organism such as the earthworm that breaks detritus into smaller pieces.
Humus: Dark, amorphous, colloidal matter that resists decay and stores nutrients.
Mineralisation: Release of inorganic nutrients as microbes break down humus.
PAR: Photosynthetically active radiation, under 50 per cent of incident sunlight.
Saprotroph: A decomposer that feeds by breaking down dead organic matter.
Food web: Interconnected food chains, including links between grazing and detritus chains.
Trophic level: An organism's place in a food chain based on its source of food.
Standing crop: Mass or number of living organisms at a trophic level at a given time.
10 per cent law: Only about 10 per cent of energy passes from one trophic level to the next.
Ecological pyramid: Bars showing number, biomass or energy at successive trophic levels.

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