IB Biology Topic B4.1.2: Adaptations of Organisms to the Abiotic Environment of Their Habitat Notes

Abiotic Stress Drives the Adaptations That Let Species Survive Where Others Cannot

1. The abiotic conditions of a habitat, such as water, salinity, temperature and oxygen, set the challenges an organism must overcome to survive and reproduce.
2. Over many generations, natural selection favours individuals whose inherited traits fit these conditions, so the whole population becomes adapted.
3. An adaptation is a feature of a population produced by this process, not a change an individual chooses to make in response to its surroundings.
4. Adaptations fall into three categories.
   1. Structural adaptations are physical features of the body, such as specialised roots or leaves.
   2. Physiological adaptations are internal chemical or cellular processes, such as excreting excess salt or storing solutes to hold water.
   3. Behavioural adaptations are inherited patterns of activity, such as feeding at night to avoid daytime heat.

Lyme Grass: Structural Traits That Beat Drought on Shifting Sand

1. Lyme grass lives on coastal sand dunes, where the sand is dry, salty, nutrient-poor and constantly shifting.
2. It is a xerophyte, meaning its features are adapted to reduce water loss in dry conditions.
3. Its key adaptations reduce water loss and keep it anchored.
   1. A thick waxy cuticle cuts water loss through the leaf surface by transpiration.
   2. Sunken stomata sit in pits that trap humid air, which lowers the rate of evaporation.
   3. Rolled leaves reduce the surface area exposed to dry air during drought.
   4. Tough sclerenchyma supports the leaf and stops it wilting when water is scarce.
   5. Rhizomes grow upward as sand builds up, keeping the plant anchored and reaching deeper water.
   6. Stored fructans lower the cell water potential, which helps the plant pull water from salty soil.

A Common Mistake About Dune Roots

Mangroves: Traits for Salt, Low Oxygen and Soft Mud

1. Mangrove trees grow in tropical and subtropical coastal swamps, where the mud is waterlogged, very salty and low in oxygen.
2. Their adaptations solve three problems: too much salt, too little oxygen, and unstable ground.
3. Managing salt
   1. Salt glands excrete excess salt from the leaves.
   2. Suberin-coated roots block much of the salt from entering the plant in the first place.
4. Getting oxygen
   1. Pneumatophores are aerial roots that project above the water and absorb oxygen from the air.
   2. Cable roots stay near the surface, where oxygen is more available.
5. Staying anchored
   1. Stilt roots prop the tree up in soft mud.
   2. Buttress roots flare out at the trunk base to give a broad, stable footing.
6. Accumulating mannitol lowers the cell water potential so the roots can absorb water from salty soil.
7. Buoyant seeds float on the tide to reach and colonise new areas of mud.

These Traits Are Evolved Solutions to Abiotic Stress, Not Perfect Designs

1. Each trait exists because past individuals that had it left more offspring in that habitat, so natural selection made it common in the population.
2. The same traits often come at a cost, such as the energy spent excreting salt or building deep roots.
3. This trade-off is why a species adapted to one set of abiotic conditions usually cannot thrive in a very different one.