Adaptations to hot deserts and tropical rainforestSpec B4.1.8
In short
Organisms in hot deserts are adapted to conserve water and cope with heat, while rainforest organisms are adapted to high rainfall, competition for light and life in trees. The saguaro cactus stores water and uses CAM photosynthesis, the dromedary camel tolerates rising body temperature, and the kapok tree has buttress roots to support it in shallow soil.
Hot deserts
| Species | Adaptation | How it helps |
|---|---|---|
| Saguaro cactus (Carnegiea gigantea) | Leaves reduced to spines; photosynthesis in the green stem | Small surface area for water loss; spines deter herbivores and shade the stem |
| Saguaro cactus | Pleated stem that swells to store water | Stores water taken up after rare rain |
| Saguaro cactus | Shallow, widespread roots | Absorb rainwater quickly before it evaporates |
| Saguaro cactus | CAM photosynthesis: stomata open at night | CO₂ is taken in when it is cool, so less water is lost |
| Dromedary camel (Camelus dromedarius) | Body temperature can rise by several °C during the day | Less water is used for sweating; heat is lost in the cool night |
| Dromedary camel | Very concentrated urine and dry faeces | Reduces water loss in excretion and egestion |
| Dromedary camel | Fat stored in the hump, not spread under the skin; thick fur on the back | Heat is lost easily from the rest of the body; fur insulates against the sun |
Tropical rainforest
| Species | Adaptation | How it helps |
|---|---|---|
| Kapok tree (Ceiba pentandra) | Very tall trunk that emerges above the canopy | Reaches full sunlight above competing trees |
| Kapok tree | Buttress roots | Support a tall tree in shallow, nutrient-poor soil |
| Many rainforest trees | Leaves with drip tips and a waxy surface | Rain runs off quickly, so fewer fungi, algae and mosses grow on the leaf |
| Spider monkey (Ateles geoffroyi) | Long limbs and a prehensile tail | Swings and grips branches to move and feed in the canopy |
| Strawberry poison frog (Oophaga pumilio) | Toxic alkaloids in the skin with bright warning colours | Predators learn to avoid it, allowing it to be active by day |
The guide asks for named species. Either the common or the scientific name is accepted, but the adaptation must be explained, not just listed.
Quick check
Define habitat.
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The place in which a community, species, population or organism lives.
Why does marram grass roll its leaves?
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To trap humid air around the stomata on the inner surface, reducing transpiration.
Name two factors needed for coral reef formation.
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Any two: shallow water, clear water, warm temperature, normal seawater salinity, slightly alkaline pH.
Which two abiotic factors mainly determine terrestrial biome distribution?
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Temperature and rainfall.
Why do mangrove stilt roots have lenticels?
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The waterlogged mud is anoxic, so oxygen diffuses into the roots from the air through lenticels.
Written and checked against the IB Biology HL specification · Updated October 2026