Comparing Ecosystems

Explain means giving the reasons. The question contains a comparison, so the answer must say what makes water different rather than only describing what happens in water.

5 marks. Compare the availability of oxygen in the two media, give the figures for how temperature affects it, work through a consequence, then state what this means for comparing ecosystems.

Air is about 21 per cent oxygen by volume, an effectively unlimited supply that is well mixed and does not vary appreciably between terrestrial habitats. Oxygen in water is dissolved, and even when fully saturated fresh water holds only a few milligrams per litre, so the supply is orders of magnitude smaller, is replenished slowly by diffusion at the surface and by turbulence, and varies substantially from place to place and hour to hour.

The solubility of oxygen in water falls as temperature rises. Fresh water holds roughly 11 milligrams per litre of dissolved oxygen at 10 degrees Celsius but only about 8 milligrams per litre at 25 degrees Celsius, a fall of about 3 milligrams per litre, which is roughly a 27 per cent reduction, across a 15 degree warming.

This produces a compounding problem. As water warms, less oxygen can dissolve, while the metabolic rate of the organisms living in it rises, so their oxygen demand increases at the same time as the supply falls. A warm, slow, lowland pool is therefore oxygen-poor and supports only species tolerant of low oxygen, while a cold, turbulent mountain stream is oxygen-rich and can support species with high oxygen demands such as many trout and mayfly larvae.

None of this applies to a terrestrial organism, because it is surrounded by air at an essentially constant 21 per cent oxygen whatever the temperature. Warming raises a land animal's oxygen demand too, but the supply does not fall to meet it, so oxygen availability is not a limiting factor on land. Terrestrial organisms are limited instead by temperature itself and by water availability, which is why those are the abiotic factors measured on land.

It follows that the abiotic factors worth measuring are not the same in every ecosystem, and a comparison must be made on factors that are meaningful in both. Dissolved oxygen and salinity are informative in aquatic systems and largely meaningless in terrestrial ones, while soil moisture and wind exposure are informative on land and not applicable in open water. Temperature and pH can be compared across both, though even then only if measured in the equivalent medium, such as soil pH against water pH, rather than treated as the same quantity.

Compare requires similarities and differences on shared criteria. Three ecosystems and two criteria means six statements, so decide the structure before writing: take one criterion and run all three ecosystems through it, then the next.

6 marks. Set out the figures, compare on temperature, compare on salinity, then give the demand each set of conditions makes, then draw out the general principle.

A coral reef runs at about 24 to 29 degrees Celsius, very stable, at a salinity of about 35 parts per thousand, full-strength sea water, and a pH of about 8.1. An estuary runs at about 14 to 26 degrees Celsius with salinity varying from about 5 to 25 parts per thousand as fresh and salt water mix with the tide and with river flow. A desert runs from about 0 to 45 degrees Celsius, an extreme daily range, and salinity is not a meaningful measure in the same way, though soil salinity can be high where evaporation concentrates salts.

The three differ far more in stability than in average. The reef varies by about 5 degrees Celsius, the estuary by about 12, and the desert by about 45 degrees within a single day. The reef is the most stable by a wide margin, and the desert the least stable by a wider one. Water is the reason: its high specific heat capacity means a large body of water changes temperature slowly, so aquatic systems buffer temperature in a way an air-and-soil system cannot.

On salinity the ordering is different. The reef is high and almost perfectly constant at about 35 parts per thousand; the estuary is intermediate and highly variable, swinging across roughly 5 to 25 parts per thousand on a tidal cycle; the desert has no equivalent aquatic salinity, though soil salinity may be high and is generally stable rather than fluctuating. So the reef is stable on both criteria, the desert is extremely variable in temperature but not in salinity, and the estuary is the only one of the three that is substantially variable in both.

Stable conditions permit specialisation. Reef organisms such as corals can operate within narrow tolerance ranges, because conditions rarely leave them, and this is why a rise of only 1 to 2 degrees Celsius above normal can trigger mass bleaching. Extreme but predictable variability demands tolerance or avoidance: desert organisms need heat and drought adaptations, or behaviours such as sheltering and nocturnal activity that avoid the extremes rather than enduring them. Variability on two axes at once demands the most: an estuarine organism must both tolerate a moderate temperature range and osmoregulate across a wide salinity range, so only species that can actively regulate their internal salt and water balance can live there.

The comparison shows that what limits organisms is the variability of conditions at least as much as their average value, and that variability on more than one axis multiplies the demand rather than adding to it. It also shows why abiotic data explain rather than merely describe an ecosystem: the reef's stable warmth and clear water suit corals, the desert's extremes require drought and heat adaptations, and an estuary's swinging salinity admits only species able to osmoregulate across a range. On that reading the hardest of the three to live in is the estuary, because it demands two kinds of tolerance simultaneously, even though the desert has the more extreme single figure.