1How organisms gain and use energy

Organisms use energy to keep themselves organised, to grow, to reproduce and to maintain homeostasis.

Endotherms (birds and mammals) hold a steady body temperature with heat from their own metabolism, which is costly: much of a mouse's food goes on keeping warm. Ectotherms (most reptiles, amphibians, fish and invertebrates) lack efficient internal ways of holding their temperature, so they regulate it by behaviour: basking, moving into shade, huddling. Not burning food for warmth, an ectotherm needs far less food than an endotherm of the same mass.

The balance of energy in and out decides what happens to an organism:

• Net gain of energy: the surplus is stored (as fat or starch), the organism grows, and it can produce more offspring.
• Net loss: the organism uses its stores and loses mass, produces fewer offspring, and, if the loss goes on, dies.

Worked example (invented numbers). A caterpillar eats leaf containing 200 J. It passes 110 J out in its faeces, so it assimilates $200 - 110 = 90$ J. Of that, 62 J is used in cellular respiration and leaves as heat, leaving $90 - 62 = 28$ J stored as new tissue. Only $\frac{28}{200} \times 100 = 14\%$ of what it ate becomes caterpillar, and only that 14% is available to the bird that eats it.

Reproduction follows energy. Many organisms breed in the season when food is plentiful, and some change how they reproduce. Aphids reproduce asexually all summer while sap is rich, making many offspring fast, then switch to sexual reproduction in autumn and lay eggs that survive the winter. A biennial plant stores energy in a root in its first year and flowers in its second; some insects enter diapause, a pause in development, until conditions improve.

2Levels of ecological organisation

• A population is all the individuals of one species in one area.
• A community is all the populations of different species living and interacting in an area.
• An ecosystem is a community together with its non-living (abiotic) surroundings: water, soil, air, light, temperature.
• A biome is a large region of the Earth with a characteristic climate and community, such as tropical rainforest, desert or tundra.

3Autotrophs and heterotrophs

Autotrophs capture energy from the physical or chemical environment.

• Photosynthetic organisms (plants, algae, cyanobacteria) capture light energy. The rate at which they store energy in organic matter is the ecosystem's primary productivity.
• Chemosynthetic organisms (certain bacteria and archaea) capture energy from small inorganic molecules such as hydrogen sulfide, ammonia or iron ions. This can happen with no oxygen and no light: at deep-sea hydrothermal vents whole communities run on chemosynthesis.

Heterotrophs take in organic matter made by autotrophs, break down its carbohydrates, lipids and proteins for energy, and build some of the matter into their own tissues. They include herbivores (eat producers), carnivores (eat animals), omnivores (both), scavengers (eat dead animals they did not kill) and decomposers (bacteria and fungi that digest dead matter and waste and release its nutrients).

4Trophic levels and the flow of energy

A trophic level is a feeding position. Producers are the first level; then primary consumers (herbivores), secondary, tertiary and occasionally quaternary consumers. Decomposers feed on dead matter from every level.

A food chain shows one sequence of feeding; a food web shows all the linked chains in a community. The arrows always point from the organism eaten to the organism that eats it, because they show the direction energy flows. "Grass → rabbit → fox" means energy goes from grass into rabbit into fox.

Why so little energy reaches the next level. Much is never eaten (roots, bones), some is not digested and leaves in faeces, and most of what is absorbed is used in respiration and lost as heat. Heat cannot be captured by any organism, so energy does not cycle. Typically about 10% of the energy at one level ends up in the next, though the real figure varies.

Worked example. Figure 1 is an energy pyramid for an invented grassland, in kJ m⁻² yr⁻¹. The percentage passed from producers to primary consumers is

$$\frac{1{,}620}{18{,}000} \times 100 = 9.0\%$$

From secondary to tertiary consumers it is $\frac{11}{146} \times 100 = 7.5\%$. The loss compounds: tertiary consumers receive only $\frac{11}{18{,}000}$, about 0.06% of the producers' energy. That is why food chains rarely have more than four or five levels and why top predators are few: there is not enough energy left to support many of them.

5Matter cycles

Energy flows through and out; matter cycles. Atoms are not destroyed, so every atom in a living thing came from the environment and will go back to it: each cycle shows the conservation of matter. Each biogeochemical cycle has reservoirs, places where the element is stored, which can be abiotic (air, ocean, rock, soil) or biotic (living organisms), and processes that move it between them. The cycles are interdependent: photosynthesis needs water as well as carbon dioxide, and building proteins needs nitrogen as well as carbon.

The water (hydrologic) cycle. Reservoirs: oceans (by far the largest), surface water such as lakes and rivers, the atmosphere, and living organisms. Processes: evaporation from water surfaces, transpiration from plant leaves, condensation into clouds, and precipitation as rain and snow.

The carbon cycle. Carbon is held in the air as CO₂, in organisms as carbohydrates and other organic molecules, and in fossil fuels, rocks and the ocean. At the simplest level, four processes move it:

The nitrogen cycle. Organisms need nitrogen for proteins and nucleic acids. The largest reservoir is the atmosphere, about 78% N₂, but almost no organism can use N₂ directly. Soil microorganisms carry out every step:

1. Nitrogen fixation: bacteria (some free in the soil, some in the root nodules of legumes) convert N₂ to ammonia, NH₃, which picks up hydrogen ions from the soil solution to become ammonium, NH₄⁺.
2. Ammonification: decomposers turn the nitrogen in dead organisms and wastes into ammonium.
3. Nitrification: nitrifying bacteria convert ammonium to nitrite and then to nitrate, NO₃⁻.
4. Assimilation: plants absorb ammonium and nitrate and build the nitrogen into amino acids and nucleotides; animals get it by eating them.
5. Denitrification: denitrifying bacteria, mostly in waterlogged, low-oxygen soil, convert nitrate back to N₂, which returns to the air.

The phosphorus cycle. Phosphorus has no significant gas, so it cycles slowly through rock, soil and water. Weathering of rocks releases phosphate, PO₄³⁻, into soil and groundwater. Producers absorb it and build it into DNA, RNA, ATP and phospholipids; consumers get it by eating producers. It returns to the soil through decomposition of dead matter and through excretion.

Energy flows through an ecosystem one way and leaves as heat; matter cycles and is never used up.

6When the energy supply changes

Because every level depends on the one below, a change in energy at the bottom spreads upwards.

• A change in energy resources such as sunlight changes the number and size of trophic levels. A lake that gets less light (a cloudy year, or water clouded by sediment) has less primary productivity, so it supports fewer consumers and may lose its top predator entirely.
• A change in the biomass or number of producers in an area changes the other levels. If a disease kills half the grass on a plain, herbivore numbers fall, then predator numbers fall.

Predict it the AP way. Claim: a 40% fall in phytoplankton in a lake will reduce the number of perch (secondary consumers). Reasoning: phytoplankton are the producers; less producer biomass means less energy for the zooplankton that eat them; with only about 10% passed on at each step, less energy reaches the perch, so fewer perch can be supported.