Components of ecosystems, investigating them, and the open ocean

The six words ecology is built on

Every question in this topic is marked against precise definitions, so learn these six words as six sentences rather than as a general idea of 'nature'. A species is a group of organisms that can reproduce to produce fertile offspring. Horses and donkeys can breed, but the mule they produce is sterile, so they are two species and not one.

A population is a group of organisms of one species, living in the same area, at the same time - all the limpets on one rocky shore, or all the sandeels in one bay. A community is all of the populations of different species in an area, so the community of that shore includes the limpets, the barnacles, the seaweeds, the crabs and the fish. One species is a population; every species together is a community.

A habitat is the area where an organism lives and interacts with its environment and other organisms - a rock pool, a burrow in mud, the underside of a kelp frond. The environment is the external surroundings of an organism or population, and it is made of biotic (living) components, such as predators and competitors, and abiotic (non-living) components, such as temperature, light, salinity and dissolved oxygen.

An ecosystem is a unit containing the community of organisms and their environment, interacting together. A rocky shore is an ecosystem, and so is a coral reef, a mangrove forest or the open ocean. The word matters because it includes the non-living part: the reef ecosystem is the corals, fish and algae together with the water, light, temperature and rock, and a change in any of those changes the whole unit.

What controls the rate of population growth

A population grows when the number of individuals added - by birth and by immigration - is greater than the number lost by death and emigration. The syllabus limits the factors you must explain to four: food supply, disease, competition and predation. All four are biotic, because all four involve other living things.

Food supply sets the ceiling. With plenty of food, individuals have the energy to grow and reproduce, more offspring are produced and more of them survive, so the population grows rapidly. As the population increases, the food per individual falls, growth slows, and the population levels off at the number the habitat can just support. Disease acts most strongly where density is high: infections pass more easily between crowded individuals, so an outbreak can kill many at once and make the population crash. A dense population is a disease's best opportunity.

Competition happens when two or more individuals need the same limited resource - food, space, light, dissolved oxygen, a place to settle. It can be between members of the same species (intraspecific) or between different species (interspecific), and it reduces the growth, reproduction and survival of the losers. Predation removes individuals directly: more predators means more prey eaten, so the prey population falls, and then the predators starve and their numbers fall too, which lets the prey recover. That lag is why predator and prey numbers often rise and fall in cycles.

Put the four together for a named organism and you have an explanation rather than a list. Take the limpet population on a rocky shore: growth is fastest in spring when the film of algae the limpets graze is thickest, it is slowed when limpets crowd together and compete for space on the rock, it is cut when oystercatchers and crabs take them, and it can be reduced sharply when a disease spreads through a dense band of them.

Two types of marine ecosystem: open ocean and coast

The syllabus divides marine ecosystems into two groups. The open-ocean ecosystem is the water beyond the edge of the continental shelf: deep, far from land, with no seabed within reach of sunlight over most of its area and no tidal exposure at all. Everything in it lives either in the water column or on a seabed hundreds or thousands of metres down.

Coastal ecosystems are found along the continental shelf or continental slope and are affected by tides. The continental shelf is the shallow, gently sloping submerged edge of a continent, and the slope is the steeper drop beyond it. Because coastal ecosystems are shallow and next to land, sunlight reaches the seabed, run-off from rivers arrives directly, and the tide rises and falls over them twice a day.

Tides matter in three ways. They expose intertidal organisms to air and to drying, they drive currents that mix the water and carry nutrients and larvae, and they shift the position of the water's edge so that the same patch of shore is submerged, then exposed, then submerged again. The syllabus names eight coastal ecosystems: wetlands, coral reefs, sandy shores, muddy shores, rocky shores, kelp forests, seagrass beds and mangrove forests. Learn them as a list, because a question can simply ask you to name them.

Why the coast is more productive and more vulnerable

Coastal ecosystems generally have a higher productivity than the open-ocean ecosystem, partly because run-off from the land carries nutrients into the coastal water. Nitrates and phosphates washed out of soils, rivers and estuaries fertilise the shallow water, so algae and seagrasses grow fast and support dense populations of grazers and predators. Two other things help: the water is shallow, so light reaches the seabed and rooted plants and attached seaweeds can photosynthesise, and the seabed is close to the sunlit water, so nutrients released by decay in the sediment are recycled straight back into the growing zone.

The open ocean is the opposite. Its surface water is nutrient-poor, because there is no land to wash nutrients into it, and because dead organisms and waste sink out of the sunlit layer before they can be broken down and recycled there. Production is therefore concentrated in the top 200 m and is limited by nutrients rather than by light, which is why the clearest blue water is often the least productive.

Coastal ecosystems are more susceptible than the open-ocean ecosystem to human activity, and the reasons are all about proximity and scale. They sit exactly where people live, work and holiday, so they receive the run-off, sewage, fertiliser, plastics and oil that come off the land. The volume of water is small, so a pollutant is diluted far less than it would be in the open ocean, and semi-enclosed bays and estuaries trap it. Tides concentrate material at the strandline instead of dispersing it.

The organisms make it worse. Seaweeds, corals, barnacles and seagrasses are attached or rooted, so they cannot move away from a pollutant, and many have larvae that settle in the same narrow coastal band. The seabed is shallow enough to be trawled, dredged, anchored on and built over, and coastal ecosystems are the nursery grounds where the juveniles of open-ocean fish spend their first weeks. Lose the mangrove forest or the seagrass bed and you lose the nursery, which is why coastal damage is felt far out at sea as well.

• Higher coastal productivity: run-off carries nutrients (nitrates and phosphates) into shallow water; light reaches the seabed; nutrients are recycled quickly between sediment and water.
• Low open-ocean productivity: no land run-off, and dead material sinks out of the sunlit layer before its nutrients can be recycled there.
• Proximity: coastal ecosystems are where people live, farm, build, ship and holiday, so they receive the pollution and the physical damage directly.
• Dilution: a small volume of coastal water dilutes a pollutant far less than the open ocean, and enclosed bays and estuaries trap it.
• Immobility: attached and rooted organisms cannot escape, and the shallow seabed can be trawled, dredged and built on.
• Nurseries: coastal beds and forests are the nursery grounds for many open-ocean species, so damage there spreads out to sea.

Random sampling: quadrats, population size and species richness

You cannot count every organism on a shore, so you count a sample and multiply up. A quadrat is a square frame of known area - commonly 0.5 m x 0.5 m, which is 0.25 m2 - often divided by string into 100 small squares so that percentage cover can be estimated by eye. Random sampling is used because it avoids bias: if you choose where to put the quadrat, you will put it where the interesting organisms are, and your estimate will be too high.

The method. Stretch two tape measures at right angles along the edges of the area to make a grid. Use a random number generator or random number table to pick a pair of coordinates, walk to that point and lay the quadrat down. Repeat until you have at least ten quadrats. In each quadrat record the number of individuals of each species (for animals you can count), the percentage cover of each seaweed or sessile species, or the frequency - the number of the 100 small squares in which the species occurs. Species richness is simply the number of different species you have found, so it is read off your list rather than calculated.

The calculation. Population size = mean number per quadrat x (total area / area of one quadrat). If the mean is 8 limpets per 0.25 m2 quadrat and the shore is 400 m2, then the shore holds 400 / 0.25 = 1600 quadrats' worth, so 8 x 1600 = 12 800 limpets - an estimate, not a count. Increasing the number of quadrats narrows the uncertainty, and you should keep sampling until the running mean stops changing.

Limitations to quote. Animals hidden under weed, in crevices or buried in sand are missed, so the estimate is too low; the edge of the frame cuts through organisms; mobile animals may move away as you approach; percentage cover is judged by eye and differs between recorders; and a quadrat sample says nothing about how the organisms are arranged, which is why distribution needs a transect instead.

Systematic sampling and measuring the physical shore

Random sampling tells you how much there is; systematic sampling tells you where it is. Samples are taken at regular intervals along a line, which is the right method whenever the shore changes in a gradient - up the beach, away from the water's edge, or across a reef flat. A line transect is a tape laid along the shore on which you record the organisms actually touching the line, or the species present at each marked interval. A belt transect is a strip of fixed width laid along the same line - for example 1 m wide - with a quadrat placed at every interval, which gives you abundance as well as position.

Measuring the profile of a shore or slope. Run a tape measure from the low water mark straight up the shore, at right angles to the waterline, and mark it at intervals of, say, 1 m or 2 m. At each mark, measure the vertical height of the shore above a fixed datum: hold a metre rule upright, stretch a string line horizontally from the previous station, check it is level with a spirit level, and read off the height difference. A surveying level and a levelling staff do the same job more accurately over a long shore. Record distance along the tape and height at each station, then plot height against distance to give the profile; a clinometer can be used instead to give the slope angle between stations.

Measuring the particle profile of sand or sediment. Collect a sample of the surface sediment to a measured depth, for example the top 2 cm, and dry it. Weigh the dry sample, then pass it through a stack of sieves with meshes of decreasing size, shaking it so that each fraction falls through. Weigh the sediment left in each sieve and express it as a percentage of the total mass, then plot the results as a bar chart or a cumulative curve. Coarse sand and gravel mean a high-energy shore where fine material has been washed away; fine silt and mud mean a sheltered shore where it settles.

Measuring moisture content. Take a known mass of fresh sediment, weigh it immediately in a sealed container so it does not dry on the way back, then dry it in an oven at about 105 °C until the mass stops changing. Weigh it again. Moisture content = (wet mass - dry mass) / wet mass x 100, given as a percentage. Take several replicates at each station and average them, because a single sample can sit in a puddle or on a ridge. Higher up the shore, moisture content falls, and the animals living there are the ones that can survive drying.

The open ocean: pelagic and benthic, and three zones of light

The open ocean is first divided by where the organisms are. The pelagic zone includes the whole column of open water, from the surface down to the seabed, and everything that swims or drifts in it lives there. The benthic zone is the seabed, from the shore edge out to the deepest trench, and everything that lives on or in it lives there. A fish swimming halfway down is pelagic; a flatfish lying on the bottom is benthic.

It is then divided by light into three zones. The sunlight zone is the region between 0 m and 200 m depth, where there is enough light for photosynthesis. The twilight zone is between 200 m and 1000 m, where there is very little light and not enough for photosynthesis. The midnight zone is everything below 1000 m, where there is no light at all except what living things make themselves.

Four conditions change with depth, and you should be able to link each one to its cause. Light is absorbed and scattered by the water itself, so it fades quickly with depth, and only about the top 200 m has enough for photosynthesis. Dissolved oxygen is high near the surface because phytoplankton produce it there and waves and wind mix it in from the atmosphere; below that it is used up by respiration and by the decay of sinking material, and there is no way to replace it, so it falls with depth. Temperature in the sunlight zone is warmed by the Sun and varies with season and latitude, while deep water stays cold and very stable. Pressure rises by about one atmosphere for every 10 m of depth, so an organism at 4000 m is under roughly 400 times the pressure at the surface.

Plankton, and why the sunlight zone holds so much life

Plankton is a collection of organisms that drift in water currents. The word describes a way of life, not a group of species: planktonic organisms may swim weakly, but they cannot make headway against a current, so where the water goes, they go. That is why plankton is the first community in the open ocean to be affected by a change in the current, and why it can be carried far from where it grew.

Phytoplankton are microscopic producers, and the syllabus names three groups: diatoms, which have silica shells; dinoflagellates, which have two flagella and can cause red tides; and cyanobacteria, the tiny blue-green bacteria that are among the oldest photosynthetic organisms on Earth. All three photosynthesise, so all three need light and dissolved nutrients, and all three sit at the base of every open-ocean food chain.

Zooplankton are consumers. They include the larvae of fish and invertebrates, which can be microscopic, and also larger animals such as jellyfish. Some graze phytoplankton, some hunt other zooplankton, and all of them are food for fish, seabirds and whales. Producer and consumer are both drifting in the same current, which is why the two are found together in the surface water.

The sunlight zone contains a large mass of living organisms because that is the only place where the food is made. Photosynthesis needs light, so the phytoplankton - and therefore the zooplankton that graze them, and the fish that eat the zooplankton - are concentrated in the top 200 m. The same layer has the most dissolved oxygen, because the phytoplankton produce it and the surface mixes in more, and it is the warmest water, which speeds up growth and reproduction. Below 200 m there is no production at all, only the slow fall of dead material, so biomass drops sharply with depth. The whole ocean's living mass is therefore stacked into a layer about 200 m deep over water that is often 4000 m deep.

• Plankton: organisms that drift in water currents; weak swimmers that cannot move against a current.
• Phytoplankton: microscopic producers - diatoms, dinoflagellates and cyanobacteria; they photosynthesise and need light and nutrients.
• Zooplankton: consumers - larvae of fish and invertebrates, which can be microscopic, and larger animals such as jellyfish.
• Why the biomass is high: light for photosynthesis -> phytoplankton -> food for zooplankton and fish, all in the same sunlit layer.
• Why it falls with depth: no light below 200 m means no production, only the slow fall of dead material.

Adaptations in the twilight, midnight and benthic zones

Sunlight and twilight zone adaptations. The syllabus names four. Many species are migratory, for food or a breeding ground: the daily vertical migration, in which animals rise to the surface at night to feed and sink into the dark by day, is the largest movement of biomass on the planet. Species are able to use a wide range of food sources, because what drifts past changes from day to day. They are fast swimmers, because there is no shelter in open water - no weed to hide in and no rock to hide behind - so speed and a streamlined body are the only defences. And they are countershaded for predator avoidance: dark on the upper surface and pale below, so a predator looking down sees dark water and a predator looking up sees the bright surface, and the fish disappears against both.

Midnight zone conditions and deep-sea adaptations. Below 1000 m there is no light, little dissolved oxygen, a very stable low temperature and very high pressure. Five adaptations follow. Bioluminescence - light made by the organism itself - is used to attract mates or prey, or to illuminate predators, since it is the only light available. Deep-sea species are slow moving with a long lifespan, because low oxygen and low temperature slow metabolism and there is little food. They are dark brown or black, so they are not easily visible in the light produced by other organisms' bioluminescence. They have gelatinous bodies, because a jelly-like body is supported by the high pressure outside it and needs little oxygen. And many fish have large backward-facing teeth, so that once prey is caught it cannot escape.

Benthic adaptations. Life on the seabed has different problems, and the syllabus gives four solutions. Benthic animals can camouflage on, or burrow into, the seabed to avoid predation. Some invertebrates move using legs or tube feet on their ventral (lower) surface, and have a hard spiny covering on the dorsal (upper) surface for protection from predators - the pattern of a sea urchin or a starfish. Flatfish lie flat on one side, and during development one eye moves around the head so that both eyes end up on the same side and protrude, which lets the fish bury itself in the sediment and still look out for predators and prey. Skates and rays are dorsally flattened, with elongated pectoral and pelvic fins around the edge of the body, which work as a single continuous wing for swimming just above the bottom.