Abiotic Factors & Distribution

Why does marram grass grow on a bare sand dune where almost nothing else will? Not because it is tougher in some vague way — because it has specific structures that solve a specific problem. This page works through two named examples in the detail examiners expect, then explains the idea that ties the whole topic together: range of tolerance.

What counts as an abiotic factor

Abiotic simply means non-living. These are the physical and chemical conditions of a habitat:

• Light intensity and wavelength
• Temperature
• Turbidity (cloudiness) of water
• Humidity
• Soil or water pH
• Soil or water salinity
• Soil composition
• Oxygen or carbon dioxide concentration

Adapting to dry conditions: marram grass

Marram grass grows on sand dunes. That habitat throws three abiotic problems at a plant:

• Low water availability — water drains straight through sand.
• High salinity — salt spray blows in off the sea.
• Low nutrient levels — there is barely any organic matter in bare sand.

Marram is a xerophyte: a plant adapted to survive dry conditions. To understand its adaptations you have to understand the problem properly, because every one of them is the same solution to the same thing.

The problem, in two steps

• Dry air means a steep concentration gradient between the wet inside of the leaf and the air outside. Water evaporates fast from cell surfaces inside the leaf and is lost to the environment.
• Because soil water is low, whatever is lost cannot easily be replaced. So the plant has to stop the loss rather than make up for it.

The four adaptations, and what each one does

• Leaves rolled up — the surfaces are no longer exposed to the wind, so water vapour stays trapped inside the roll.
• Stomata sunk in pits — the pit holds a pocket of water vapour right where it is released.
• Inner surface covered in tiny hairs — the hairs hold still, humid air against the leaf.
• Thick waxy cuticle on the outside — this one is different: it is a physical barrier that stops evaporation straight through the leaf surface.

Adapting to salt and low oxygen: mangroves

Mangrove swamps are tropical coastal habitats that are regularly submerged in sea water. That gives three abiotic problems:

• High salinity
• Low fresh water availability
• Low oxygen availability, because the soil is waterlogged

The three challenges

• Root cells that are underwater cannot take in oxygen for respiration.
• The surrounding water is salty, so there is very little fresh water to take up.
• Sea water has a higher solute concentration than the root cells, so the tree risks losing water by osmosis out of its own roots.

Solving the oxygen problem: aerial roots

Mangroves grow aerial root systems: parts of the root stick up above the water and take in oxygen for respiration. Two species do it in different ways:

• Red mangroves have prop roots. These arch down from the trunk into the mud, and are partly above the water. They also give stability in unstable soil, and the underwater tangle gives shelter for marine animals.
• Black mangroves have pneumatophores: narrow structures that grow vertically upwards out of the waterlogged soil, like a field of snorkels.

Solving the salt problem: two opposite strategies

• Red mangroves keep salt out. Their root cells do not allow salt to enter the water-transport system, so the tree takes up fresh water only. Those same cells also stop water moving outwards, which prevents water loss by osmosis.
• Black mangroves let salt in, then throw it out. They take salt water into their cells and excrete the excess salt through salt glands in their leaves.

Range of tolerance

Now the idea that links all of this together. Abiotic factors act as limiting factors on where a species can live, and every species has a range of tolerance.

• There is an optimum — the level of the factor at which the species does best.
• Some variation either side of the optimum can be tolerated. That whole tolerable span is the range of tolerance.
• Push the factor beyond the range and the species is not found there at all.

Mangroves are a good example. They grow best at a salinity of roughly 3–27 parts per thousand, but they can survive in fresh water and in salt concentrations up to about 75 ppt. The optimum is narrow; the tolerance is wide.

Species that live in extremes

Species adapted to harsh places often have an especially wide range of tolerance, which lets them live where others cannot. They may also have an optimum that is higher or lower than average — marram grass, for example, has a lower optimum for water availability than most plants.

There is a trade-off, and examiners like it:

• In the extreme habitat, these species do well because competition is low — almost nothing else can survive there.
• In a mild, comfortable habitat they often do worse, because they are outcompeted by species whose optimum sits right in the middle of those conditions.

Worked examples