Energy and biomass in ecosystems practice questions.

Food chains rarely contain more than four or five trophic levels mainly because

• predators at the top are physically too large to be supported
• nutrients are lost from the ecosystem at every transfer
• biomass is destroyed rather than transferred at each level
• only about 10 % of the energy is passed on at each transfer

Answer: only about 10 % of the energy is passed on at each transfer

With roughly a tenth of the energy crossing each transfer, the flow left after four or five levels is too small to support a viable population. Nutrients, unlike energy, are recycled and are not the limiting factor here.

Which ecological pyramid can never be inverted?

• A pyramid of numbers for a woodland with one oak tree
• A pyramid of biomass for open ocean plankton in spring
• A pyramid of productivity for any ecosystem
• A pyramid of numbers for a herd of grazing herbivores

Answer: A pyramid of productivity for any ecosystem

Productivity is a rate of energy flow per unit area per year, and energy is always lost between levels, so each bar must be smaller than the one below it. Pyramids of numbers and of biomass are snapshots and can invert when producers are few and large, or small and fast-reproducing.

A caterpillar eats a leaf but cannot digest the cellulose, which leaves its body in the faeces. In an energy-flow diagram this energy is

• transferred into the net secondary productivity of the caterpillar
• egested, and passes to the decomposer food chain
• lost from the ecosystem as heat from respiration
• excreted, having first been absorbed and metabolised

Answer: egested, and passes to the decomposer food chain

Egested material was never absorbed across the gut wall, so it never entered the caterpillar's assimilated energy; excretion, by contrast, removes the waste products of substances that were absorbed. The egested energy stays in the ecosystem and flows to detritivores and decomposers.

Biomass in an ecological pyramid is normally expressed as dry mass per unit area because

• water content varies and carries no usable energy
• drying the sample destroys the organic matter present
• wet mass cannot be measured accurately in the field
• dry mass includes the mineral content of the soil

Answer: water content varies and carries no usable energy

Water makes up a variable share of fresh mass and contains no chemical energy, so comparing wet masses would compare hydration rather than energy. The trade-off is ethical and practical: obtaining dry mass usually means killing the organisms sampled.

Of the energy assimilated by a secondary consumer, the largest share is typically

• stored as new body tissue for growth
• released as heat through cellular respiration
• passed on intact to the next trophic level
• returned to decomposers in faeces

Answer: released as heat through cellular respiration

Movement, maintenance and body warmth all run on respiration, which dissipates most assimilated energy as heat. Faecal energy was never assimilated in the first place — putting it inside the assimilated budget is the classic energy-flow-diagram error.

Which food chain would produce an inverted pyramid of numbers?

• grass grazed by rabbits that are hunted by foxes
• phytoplankton eaten by zooplankton eaten by herring
• a single oak tree supporting thousands of leaf-eating insects
• maize plants eaten by locusts that are eaten by lizards

Answer: a single oak tree supporting thousands of leaf-eating insects

A pyramid of numbers counts individuals, so one enormous producer carrying thousands of tiny consumers flips the base. The plankton example inverts a pyramid of biomass (through rapid turnover), not a pyramid of numbers — separating those two inversions is the point of this question.