Feeding relationships and energy flow

The Sun: where the energy in a marine food chain starts

The Sun is the principal source of energy input to biological systems. Light energy arriving at the surface of the sea is captured by producers - the seagrasses, seaweeds, diatoms, dinoflagellates and cyanobacteria that photosynthesise - and converted into the chemical energy stored in carbohydrates. Every feeding relationship in this topic is a way of passing that captured energy on.

Because the energy arrives as light, production only happens where light reaches. Water absorbs light quickly, so the photosynthesising organisms are concentrated in the upper, sunlit part of the water column. Below that depth there is too little light for photosynthesis, and the food that arrives is material that has sunk or been carried down from above, which is why even a deep-water community depends on the surface.

Energy flows through a food chain; it does not cycle. At every transfer most of it is lost, mainly as heat, and heat cannot be used again by a living organism. Nutrients such as nitrate and phosphate are recycled through the ecosystem, but energy is not. That is why a food chain needs a continuous supply of sunlight and why it can only ever be a few links long.

• Sun -> producer -> consumer: light energy is converted to chemical energy in carbohydrates, then passed on as food.
• Producers are the entry point of energy into every marine food chain, so a chain that starts anywhere else is wrong.
• Energy flows one way and is lost as heat; nutrients cycle. Never write that energy is recycled.
• Only a small fraction of the energy at one trophic level reaches the next, which is the single idea behind the rest of this topic.

Food chains: energy moving from one organism to the next

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. Each organism feeds on the one before it, and the arrow points from the organism being eaten to the organism that eats it, so the arrow shows the direction in which the energy travels.

Get the arrow the right way round. `Diatom -> copepod` means energy passes from the diatom to the copepod. It does not mean that the copepod is eaten by the diatom. A chain drawn with the arrows reversed, or labelled 'eaten by' along the top, is the most common single error in this topic and it costs marks even when every organism is correct.

A four-level marine chain is diatom -> copepod -> sandeel -> puffin. The diatom is a producer, the copepod a primary consumer, the sandeel a secondary consumer and the puffin a tertiary consumer. A shorter one from a seagrass bed is seagrass -> green turtle -> tiger shark, which shows how quickly a chain can reach its top predator.

Decomposers are normally left off the end of a chain. They do not wait for the top predator to die: they break down dead bodies, shed shells, faeces and dissolved waste from every level at once, so they are better drawn as a side branch receiving material from all of the trophic levels rather than as the last link in a single line.

Food webs: many chains joined into a network

A food web is a network of interconnected food chains. It is a more honest picture of a real community than a single chain, because in the sea most consumers eat more than one kind of food and are eaten by more than one predator. A seal does not eat only one species of fish, and a fish does not have only one predator.

To construct a web, write each organism once, then draw one arrow for every feeding link you are given, with the arrow pointing from the food to the feeder. Every chain in the web should still begin at a producer, and no arrow should point downwards. Check your finished web by tracing two or three chains through it from a producer to a top predator.

To interpret a web, follow the arrows forwards to answer 'what eats this?' and backwards to answer 'what does this depend on?' Removing one organism has effects in both directions. Take a kelp forest: the sea otter eats sea urchins, the urchins graze kelp, and the kelp shelters and feeds many other species. Remove the otters through hunting and the urchin population grows, the kelp is grazed down to bare rock, and the whole community that depended on the kelp changes with it.

The same organism can sit at different trophic levels in different chains of one web. A crab that grazes seaweed is a primary consumer in that chain, and a crab that eats a dead fish is a detritivore, and a crab that eats a small mussel is a secondary consumer. This is exactly why the syllabus says consumers can feed at different trophic levels in a food web, and why a web - not a chain - is the realistic model.

• A web is chains joined together; it shows every feeding link that has been recorded, not just one route to the top.
• One arrow per link, always from the organism being eaten to the organism that eats it.
• Follow arrows forwards for predators and backwards for food; a change in one species travels along both sets of arrows.
• In a web, name the chain before you name the trophic level, because one organism may occupy several levels.

Trophic levels and the classes of consumer

A trophic level is the position of an organism in a food chain, food web or ecological pyramid. The producer is always the first trophic level. A primary consumer is the second, a secondary consumer the third and a tertiary consumer the fourth. Count from the producer, never from the top predator, and never start at zero.

A consumer is an organism that gets its energy by feeding on other organisms; unlike a producer it cannot make its own food. Consumers are classed as primary, secondary or tertiary according to their position in a food chain, and because an organism can appear in more than one chain in a web, one consumer can feed at more than one trophic level.

The three types of consumer the syllabus names are defined by what they eat. A herbivore gets its energy by eating producers, a carnivore gets its energy by eating other animals, and an omnivore gets its energy by eating both plants and other animals. A detritivore is a fourth kind: an animal that gets its energy from dead or waste organic material, which is not the same as a decomposer.

The labels answer different questions, and examiners expect you to keep them apart. 'Primary consumer' tells you the position in the chain; 'herbivore' tells you what the animal eats; 'consumer' tells you that it cannot photosynthesise. A parrotfish is a herbivore and a primary consumer when it scrapes algae from rock, and may be a carnivore and a secondary consumer in another chain where it eats small invertebrates.

Decomposers, detritivores, predators and prey

A decomposer is an organism that gets its energy by breaking down dead or waste organic material. The decomposers in the sea are mainly bacteria and fungi. They digest the material outside their bodies using enzymes, then absorb the soluble products, which is why they do not need to swallow anything. In doing so they release the nutrients locked in dead tissue back into the water and the sediment, where producers can take them up again.

A detritivore is an animal that gets its energy from eating dead or waste organic material. Lugworms swallow mud and digest the organic fragments in it, sea cucumbers work over the sediment, and amphipods shred dead kelp. The difference from a decomposer is the method: a detritivore ingests the material and digests it inside its body, whereas a decomposer breaks the material down externally and absorbs the products. Both get their energy from dead matter, and both are essential, but they are not interchangeable in an answer.

A predator is an animal that captures, kills and eats another animal, which is its prey. The prey is the animal that is caught, so in the pair 'sandeel and puffin' the sandeel is the prey and the puffin is the predator. Predators are consumers and are usually carnivores. A herbivore cropping seagrass is not a predator in this sense, because no animal is captured and killed.

The decomposer pathway is not a dead end. In the open sea a large share of the organic matter produced by phytoplankton is never eaten by a grazer at all: the cells die, clump together and sink, and are broken down on the way down or on the sea bed. That material still carries energy and nutrients, and the decomposers that process it are the reason a food web can keep running in the same patch of water year after year.

• Decomposer - bacteria or fungi; breaks dead or waste material down externally and absorbs the products.
• Detritivore - an animal such as a lugworm or sea cucumber; eats dead or waste material and digests it internally.
• Predator - captures, kills and eats another animal; the animal it catches is its prey.
• Both decomposers and detritivores take material from every trophic level, not just from the top of the chain.

Biomass, and the four ways energy leaves a food chain

Biomass is the mass of living matter. It is normally measured as dry mass per unit area, in grams per square metre, because marine organisms are mostly water and wet mass varies enormously with what the organism has been doing. Biomass can be quoted for a single species or for a whole trophic level, and it is what a pyramid of biomass is built from.

Energy is lost from a food chain in four ways, and the syllabus limits the answer to these. Respiration is the largest: every cell breaks down glucose to release the energy it needs, and the energy that is not used for life processes is released as heat. Movement uses energy in muscle contraction - swimming, burrowing, pumping water through gills, catching food - and that energy also ends up as heat. Excretion removes waste products such as ammonia and urea, which still contain energy, in solution into the surrounding water. Removal or harvesting takes organisms out of the chain altogether, for example when a fishery catches them, when they are washed ashore, or when they are killed and not eaten, so the energy in their bodies never reaches the next trophic level.

Put numbers on it. Suppose the producers in a patch of sea fix 100 000 kJ per square metre per year. If about 10 per cent of the energy at each level is passed on, the primary consumers receive 10 000, the secondary consumers 1 000, the tertiary consumers 100 and a fourth-level consumer only 10. Roughly 90 per cent has gone at every step, most of it as heat from respiration, with smaller amounts lost in movement, in excretion and through removal.

Two consequences follow, and both are worth a mark. Food chains are short, because after four or five transfers there is too little energy left to support another population. And the organisms at the top are few and widely spread, because a top predator needs a very large area of ocean to collect enough energy to live. That is also why a fishery that catches low in the chain, such as plankton-feeding fish, can support more human food than one that catches the predators.

Pyramids of numbers, biomass and energy

An ecological pyramid shows the trophic levels of a food chain or web as stacked bars, with the producers at the bottom and each bar's width proportional to the quantity being measured. Each bar represents a whole trophic level, not one species, so a pyramid summarises a web rather than a single chain. Every pyramid needs a title, labelled levels and a stated unit, or it cannot be interpreted.

A pyramid of numbers shows how many organisms there are at each level. It is the easiest to collect and the least reliable to interpret, because it ignores size: one large producer can support a very large number of small consumers. That is why a pyramid of numbers can be inverted, with a narrow bar at the bottom and a wide one above it - for example one large kelp plant carrying many small grazers, or a single host fish carrying many parasites.

A pyramid of biomass shows the mass of living matter at each level, in grams per square metre. It corrects for size and is usually upright, but in open water it can appear inverted: the phytoplankton that support a zooplankton community can have a smaller standing biomass than the zooplankton themselves, because the phytoplankton divide and are eaten so quickly that the crop present at any one moment is small, even though the amount produced over a year is very large. The pyramid is a snapshot, and a fast turnover hides behind a small bar.

A pyramid of energy shows the energy passing through each level per unit area per unit time, in kilojoules per square metre per year. It is the only one that can never be inverted, because the energy reaching a level is always less than the energy that entered the level below it, whatever the size, number or turnover of the organisms. That is why a pyramid of energy is the best evidence for energy loss, and why examiners will not accept 'the energy pyramid is inverted'.

Reading pyramids and doing the arithmetic

The efficiency of a transfer is the energy at one level divided by the energy at the level below, multiplied by 100. If producers hold 250 000 kJ m-2 yr-1 and primary consumers hold 25 000, the transfer is 25 000 / 250 000 x 100 = 10 per cent. If the secondary consumers hold 2 500, the energy lost between the primary and secondary levels is 25 000 - 2 500 = 22 500 kJ m-2 yr-1. Always show the division, give the unit and check that your loss is the difference between the two levels, not the amount passed on.

Read a pyramid in the same order as the energy flows. Start at the bottom bar, note the value and the unit, and work upwards, calculating the transfer at each step. The pattern to describe is that each bar is roughly a tenth of the one below. If one bar breaks the pattern, say so and explain it - a wide bar of zooplankton under a narrow one of phytoplankton is fast turnover, not a mistake in the data.

This is also the arithmetic behind management decisions. A fishery that catches a plankton-feeding fish such as the Peruvian anchoveta is catching at the third trophic level, close to the large amount of energy fixed by phytoplankton; a fishery for tuna is catching at the fourth or fifth, where only a hundredth or a thousandth of that energy is left. The same catch weight therefore costs the ecosystem far more when it is taken from the top.

For a prediction question, follow the arrows and the energy. If a fishery removes much of the primary consumer, the secondary consumers have less food, so their numbers fall, and the producer may briefly increase because it is being grazed less. If a top predator is removed, the level below is released from predation and increases, and the level below that is grazed down. Say which direction each change goes and why, in terms of energy reaching the level.

• Efficiency = (energy at this level / energy at the level below) x 100, in per cent.
• Energy lost = energy at the level below minus energy at this level; give it in kJ m-2 yr-1.
• Prediction: work along the arrows. Less food below means fewer organisms above; fewer predators above means more organisms below.
• Never calculate a loss by subtracting the efficiency from 100 and calling it energy - the loss must be in the same unit as the data.