Energy Flow Through Ecosystems: Study Guide (Biology 8.2)

Energy in, energy used, energy out

Every organism needs energy to organise its cells, grow, reproduce and maintain homeostasis. Energy enters the living world when autotrophs capture it, then passes to heterotrophs as they eat. At every step, much of it is released as heat by cellular respiration. Heat cannot be used again to build molecules, so energy flows through an ecosystem in one direction.

Matter is different. The carbon, nitrogen, phosphorus and water in your body have been part of other organisms and of the air, soil and sea before. Matter cycles. Keep this one contrast in mind and most of this topic falls into place.

How individual organisms use energy

Endotherms and ectotherms

• Endotherms (birds and mammals) use thermal energy released by their own metabolism to keep body temperature steady. In the cold, they raise their metabolic rate to make more heat. This costs a lot of food.
• Ectotherms (most fish, amphibians, reptiles and invertebrates) do not have efficient internal ways of holding their body temperature steady. Body temperature tends to follow the surroundings. Many still regulate it by behaviour: basking in the sun, moving into shade, or clustering together.

Because an ectotherm does not pay to heat itself, it needs far less food than an endotherm of the same mass. Worked example 2 puts numbers on this.

Energy budgets

Think of an organism’s energy like a bank account:

• Net gain (more energy taken in than used): the extra is stored (for example as fat), used for growth, and allows more reproduction.
• Net loss (more used than taken in): the organism uses its stores and loses mass, reproduces less and, if the loss continues, dies.

Reproductive strategies and energy

Reproduction is expensive, so many organisms time it to energy supply. Many animals and plants breed in the season when food is plentiful. A biennial plant stores energy in its first year and flowers and sets seed only in its second. Some animals pause reproduction when food is short (reproductive diapause).

Some organisms switch between asexual and sexual reproduction in response to energy availability. Water fleas (Daphnia) and aphids reproduce asexually when food is plentiful, producing many offspring quickly. When food runs short or other conditions worsen (crowding in Daphnia; shorter, colder autumn days in aphids), they switch to sexual reproduction. In Daphnia this produces tough resting eggs that can survive until conditions improve, and the offspring are genetically varied.

Levels of organisation

Level	Meaning	Example
Population	all individuals of one species in an area	the field mice in one meadow
Community	all populations of different species in an area	mice, grasses, owls, beetles and fungi in the meadow
Ecosystem	the community plus the abiotic environment	the meadow’s organisms with its soil, water, air and light
Biome	a large region with a characteristic climate and community	temperate grassland, tropical rainforest, tundra

Trophic levels: who eats whom

A trophic level is a feeding position in a food chain or web.

• Producers (autotrophs) make organic molecules from inorganic ones.
• Primary consumers eat producers (herbivores). Secondary consumers eat primary consumers. Tertiary and quaternary consumers feed higher still.
• Decomposers (many bacteria and fungi) break down dead matter and wastes from every level.

Autotrophs: where energy enters

• Photosynthetic organisms (plants, algae, cyanobacteria) capture light energy. The rate at which they store energy in organic matter is primary productivity.
• Chemosynthetic organisms (certain bacteria and archaea) capture energy from small inorganic molecules in their surroundings, such as hydrogen sulfide or ammonia. This can happen without oxygen and without light. Around deep-sea hydrothermal vents, whole communities depend on chemosynthetic microbes.

Heterotrophs: passing energy on

Herbivores, carnivores, omnivores, scavengers and decomposers are all heterotrophs. They take in organic matter that came, directly or indirectly, from autotrophs. They break down carbohydrates, lipids and proteins to release energy (making ATP in respiration), and they build some of the matter into their own tissues.

Why so little energy reaches the top

At each level, most energy does not become new biomass in the next level:

• much is released as heat in respiration;
• some biomass is never eaten (roots, wood, bones);
• some eaten food is not absorbed and leaves as faeces, which goes to decomposers.

On average only about 10% passes on. This is why an energy pyramid always narrows upward, and why food chains rarely have more than four or five levels.

Matter cycles: biogeochemical cycles

A biogeochemical cycle moves an element or compound between abiotic reservoirs (air, water, rock, soil) and biotic reservoirs (living things). Each cycle conserves matter: atoms are moved, never made or destroyed. The cycles also depend on each other. Photosynthesis needs water and carbon dioxide, and producers can only build proteins, DNA and ATP if they have enough nitrogen and phosphorus.

The water (hydrologic) cycle

Reservoirs: oceans, surface water, the atmosphere and living organisms. Processes: evaporation (liquid to vapour), condensation (vapour to droplets, forming clouds), precipitation (rain and snow) and transpiration (water vapour lost from leaves).

The carbon cycle

Carbon moves from atmospheric CO₂ into organisms as carbohydrates and other organic molecules, and back again. At the largest scale, four processes are enough:

Process	Direction	What happens
Photosynthesis	removes CO₂ from the air	producers fix CO₂ into carbohydrates
Cellular respiration	returns CO₂	organisms break down organic molecules for energy
Decomposition	returns CO₂	decomposers respire the carbon in dead matter and wastes
Combustion	returns CO₂	burning wood or fossil fuels releases stored carbon quickly

The nitrogen cycle

The largest nitrogen reservoir is the atmosphere (N₂ gas), but most organisms cannot use N₂ directly. Soil microorganisms carry out the conversions.

1. Nitrogen fixation: bacteria convert N₂ into ammonia (NH₃), which picks up hydrogen ions from the soil solution to become ammonium (NH₄⁺).
2. Assimilation: plants take up ammonium or nitrate and build the nitrogen into amino acids, proteins and nucleic acids. Consumers get nitrogen by eating.
3. Ammonification: decomposers break down dead matter and wastes, releasing ammonium.
4. Nitrification: other bacteria convert ammonium to nitrite (NO₂⁻) and then nitrate (NO₃⁻).
5. Denitrification: bacteria convert nitrate back to N₂, returning it to the atmosphere.

The phosphorus cycle

Phosphorus has no major atmospheric reservoir. Weathering of rocks releases phosphate (PO₄³⁻) into soil and groundwater. Producers take it up and build it into molecules such as nucleic acids, ATP and phospholipids. Consumers get it by eating producers. Phosphate returns to the soil through excretion and the decomposition of dead organisms. Because new phosphate only comes slowly from rock, it often limits producer growth.

Changes in energy availability

Energy supply sets the size of every level above it.

• Population size: when food energy rises, more individuals gain net energy, breed and survive, so populations grow. When food energy falls, populations shrink.
• Sunlight: less light (for example under a long period of cloud or ash) means less photosynthesis, less energy entering, and fewer or smaller trophic levels above.
• Producer biomass: removing producers from an area (clearing, overgrazing, a crop failure) cuts the energy available to every consumer level. The top level is often lost first, because it was already the smallest.

Worked example 1: energy through a food chain

Question. Figure 1 shows fictional net production at each level of a grassland, in kJ m⁻² yr⁻¹. The producers’ gross primary production (all the energy they capture) was 24 000; they used 13 200 in their own respiration.

(a) Show that net primary production is 10 800. (b) Calculate the transfer efficiency between each pair of levels. (c) Predict the tertiary consumers’ production if cloud cover cut producer production by 30%, and state your assumption.

1. (a) Net primary production = gross − respiration = 24 000 − 13 200 = 10 800 kJ m⁻² yr⁻¹. So 55% of captured energy (13 200 ÷ 24 000) was used by the producers themselves.
2. (b) Producers → primary: 1200 ÷ 10 800 × 100 = 11.1%. Primary → secondary: 130 ÷ 1200 × 100 = 10.8%. Secondary → tertiary: 12 ÷ 130 × 100 = 9.2%. Each is close to the 10% rule of thumb.
3. Overall, only 12 ÷ 10 800 × 100 ≈ 0.11% of producer energy reaches the tertiary level.
4. (c) Producers: 10 800 × 0.70 = 7560. Assuming the same transfer efficiencies, every level falls by 30%: tertiary = 12 × 0.70 = 8.4 kJ m⁻² yr⁻¹.

Interpretation. A fifth level would receive roughly 10% of 12, about 1 kJ m⁻² yr⁻¹, too little to support a population. That is why chains are short. The 30% fall in sunlight-driven production may push the tertiary population below a viable size: the top level is the most vulnerable. In reality, consumers might switch prey, so the efficiencies could change.

Worked example 2: the cost of being warm

Question. A fictional 20 g mouse and a 20 g lizard are kept at 20 °C. Oxygen use: mouse 3.6 mL O₂ g⁻¹ h⁻¹; lizard 0.20 mL O₂ g⁻¹ h⁻¹. Take 20 J released per mL of O₂ and 17 kJ per gram of dry food.

(a) Calculate each animal’s daily energy use and daily food need. (b) The lizard eats 0.5 g of dry food a day and the mouse 1.5 g. Predict what happens to each, assuming all food energy is absorbed.

1. Mouse oxygen per day: 3.6 × 20 g × 24 h = 1728 mL. Energy: 1728 × 20 J = 34 560 J = 34.6 kJ per day. Food: 34.56 ÷ 17 = 2.0 g per day.
2. Lizard oxygen per day: 0.20 × 20 × 24 = 96 mL. Energy: 96 × 20 = 1920 J = 1.92 kJ per day. Food: 1.92 ÷ 17 = 0.11 g per day.
3. The mouse uses 3.6 ÷ 0.20 = 18 times as much energy, because it is generating heat to keep its body near 37 °C.
4. (b) Lizard: 0.5 × 17 = 8.5 kJ taken in, 1.92 kJ used: a net gain of about 6.6 kJ per day, available for storage, growth and reproduction.
5. Mouse: 1.5 × 17 = 25.5 kJ taken in, 34.56 kJ used: a net loss of about 9.1 kJ per day. It will use fat stores, lose mass and, if this continues, stop breeding and eventually die.

Check. Units: (mL g⁻¹ h⁻¹) × g × h = mL; mL × J mL⁻¹ = J. Interpretation. At lower air temperatures the mouse’s oxygen use would rise further (more heat needed), while the lizard’s would fall; at warmer temperatures, up to a point, the reverse. That is why ectotherms can thrive where food is scarce, and why endotherms can stay active in the cold.

Common misconceptions

• “Energy is recycled in ecosystems.” Matter is recycled. Energy flows through and leaves as heat.
• “Ninety percent of energy is wasted as heat.” Heat is a large part, but some energy is also in uneaten biomass and faeces, which feed decomposers.
• “Ectotherms cannot control their temperature at all.” They regulate it by behaviour, such as basking or seeking shade.
• “Decomposers are not part of the food web.” They are a trophic role that receives energy from every level and releases nutrients.
• “Plants get nitrogen from the air.” Plants absorb ammonium and nitrate from the soil. Only certain bacteria fix N₂.
• “All producers need sunlight.” Chemosynthetic autotrophs use energy from inorganic molecules, even in darkness.
• “Phosphorus cycles through the atmosphere like carbon.” It has no major gas phase; it moves from rock to soil, organisms and back.

Where this leads

Energy supply limits how fast populations grow and how many individuals an environment can support. That is the next topic, Population Ecology. First, test yourself with the practice questions, then use the revision notes and the checklist.