B4.1 Adaptation to environmentIB Biology HL: Revision notes

A habitat is the place where a community, species, population or organism lives. A description can include the geographical location (e.g. coasts of north-west Europe), the physical location (e.g. mobile sand dunes) and the type of ecosystem.

Marram grass (Ammophila arenaria) on sand dunes: leaves roll in dry weather with stomata in hairy grooves on the inner surface, trapping humid air; deep roots reach water; underground stems grow up through buried sand.

Red mangrove (Rhizophora mangle) in mangrove swamps: prop roots with lenticels supply oxygen to roots in waterlogged, anoxic mud; roots exclude salt; seeds germinate on the tree, producing seedlings that establish quickly in tidal mud.

Abiotic variables affect distribution. Plants: light intensity, soil pH, soil moisture, salinity, temperature, mineral nutrients. Animals: temperature, water availability, oxygen concentration, pH, salinity.

Each species' adaptations give it a range of tolerance for each variable; outside it the species cannot survive or reproduce. A transect records species abundance and an abiotic variable at intervals (sensors can measure temperature, light and soil pH), so the two can be correlated. Semi-natural habitats are influenced by humans but dominated by wild species.

Reef-building corals contain photosynthetic algae (zooxanthellae) and deposit calcium carbonate. Reefs form where water is:

• shallow (usually less than about 50 m), so light reaches the algae;
• clear, with little suspended sediment;
• warm, about 23–29 °C;
• of normal sea salinity;
• slightly alkaline, pH about 8.0–8.4 — lower pH reduces calcium carbonate deposition.

For any combination of temperature and rainfall, one natural ecosystem type is likely to develop. A biome is a group of ecosystems with similar communities, due to similar abiotic conditions and convergent evolution.

• Tropical forest: hot, very wet all year.
• Temperate forest: mild, moderate rainfall, distinct seasons.
• Taiga: cold winters, short summers, moderate precipitation.
• Grassland: moderate temperatures, rainfall too low for forest.
• Tundra: very cold, low precipitation, permafrost.
• Hot desert: hot, very low rainfall.

Hot desert: saguaro cactus (Carnegiea gigantea) stores water in its stem, has spines instead of leaves and opens stomata at night; fennec fox (Vulpes zerda) is nocturnal with large ears to lose heat; kangaroo rats produce concentrated urine.

Tropical rainforest: kapok tree (Ceiba pentandra) emerges above the canopy with buttress roots; drip-tip leaves shed rain; spider monkeys (Ateles) use a prehensile tail in the canopy.