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B4.1 Adaptation to environment

Adaptation to environment explains how abiotic conditions, tolerance ranges, habitats and biomes shape organism survival, distribution and specialised traits over time.

Syllabus
First assessment 2025
Topic
B4.1
Level
HL

Learning objectives

B4.1.1Habitat definition• Habitat is the place where an organism, population, species, or community lives• Habitat descriptions can include geographic location, physical position, and ecosystem type• Microhabitats are small areas with conditions different from the surrounding habitatB4.1.2Adaptations to abiotic environment• Adaptations improve survival under abiotic conditions such as water availability, salinity, oxygen, and wind• Marram grass has rolled, hairy leaves and thick cuticle to reduce water loss in dunes• Mangroves use aerial roots, pneumatophores, salt exclusion, or salt excretion in saline low-oxygen swampsB4.1.3Abiotic variables affecting distribution• Species distribution depends on abiotic variables such as temperature, salinity, pH, light, rainfall, humidity, and substratum• Limiting factors can restrict survival, growth, reproduction, or activity by deficiency or excess• Plant limiting factors include light, water, nutrients, carbon dioxide, and temperatureB4.1.4Range of tolerance of limiting factor• A range of tolerance lies between critical minimum and maximum limits• The optimum zone supports highest survival, growth, abundance, and reproduction• Zones of stress and intolerance explain reduced abundance or absence along environmental gradientsB4.1.5Coral reef formation conditions• Coral reefs need warm, shallow, clear, sunlit, saline water with suitable pH• Reef-building corals rely on photosynthetic symbiotic algae• Low nutrients, sediment, cold water, or reduced light can limit reef formationB4.1.6Terrestrial biome distribution• Terrestrial biome distribution is mainly determined by temperature, rainfall, and insolation• Productivity depends strongly on water availability and photosynthesis rate• Latitude and seasonal variation help explain rainforest, temperate forest, taiga, grassland, tundra, and desert distributionB4.1.7Biomes• Biomes are groups of ecosystems with similar communities due to similar abiotic conditions• Similar biomes can evolve similar forms through convergent evolution• Examples include tropical rainforest, temperate forest, taiga, grassland, tundra, and hot desertB4.1.8Adaptations to hot deserts and tropical rainforest• Hot desert strategies include expire, evade, or endure under heat and water scarcity• Camels, cacti, scorpions, and kangaroo rats show water-saving, heat-avoidance, or storage adaptations• Tropical rainforest adaptations include pitcher plant nutrient capture, gibbon brachiation, flying lizard gliding, and orchid mantis mimicry

A Habitat Is a Place Plus Its Conditions

A habitat is the physical and biological environment where an organism lives. Describe location together with resources, abiotic conditions and interactions.

Static ecosystem scene showing one broad habitat and two microhabitats within it, such as bark crevices and damp soil beneath leaf litter, with labels for geographic location, physical position, and ecosystem type.

A microhabitat is a smaller local environment with distinct conditions. A niche goes further by describing how a species uses resources and interacts within its habitat.

An Adaptation Solves a Particular Abiotic Challenge

An adaptation is an inherited feature that increases survival or reproduction under particular conditions. Explain it as challenge → feature → physical effect → fitness benefit.

Micrographs show a cross-section of a marram grass blade and a magnified section with a thick cuticle, thick-walled epidermal cells, and a folded, hairy inner epidermis that traps water vapour.

marram: rolled leaf + hairs + sunken stomata → humid trapped air → smaller water-vapour gradient
mangrove: aerial roots and salt exclusion/excretion → aeration and ion balance in saline, anoxic mud

Abiotic Limits Filter Where a Species Can Persist

Temperature, water, light, salinity, pH, oxygen, minerals and substrate can restrict distribution when they fall outside the range for survival, growth or reproduction.

sample abundance at fixed positions along a transect + measure candidate abiotic variables with calibrated sensors + repeat with equal effort → test whether distribution changes with the gradient

A Tolerance Curve Has an Optimum, Stress Zones and Limits

For one environmental factor, a species has a critical minimum and maximum. Performance is highest near an optimum and declines through zones of physiological stress toward either limit.

Static tolerance curve with abundance on the y-axis and one abiotic factor on the x-axis. Label critical minimum, critical maximum, lower stress zone, optimum zone, upper stress zone, and intolerance beyond both limits.

choose an x-value → locate performance on the curve → classify optimum, stress or intolerance → remember another factor may still prevent the species occurring

Coral Reefs Require Several Conditions at Once

Reef-building corals depend on warm, shallow, clear, well-lit seawater with suitable salinity and carbonate chemistry. Photosynthetic symbionts supply much of the energy supporting calcification.

Coral reef system diagram showing shallow sunlit water, coral polyps with zooxanthellae, clear water column, saline seawater, and side arrows indicating limiting factors such as sediment, reduced light, cold upwelling, and lower pH.

greater depth or turbidity → less light
thermal stress → bleaching risk
lower pH → harder CaCO₃ deposition
sediment/nutrient disturbance → damaged or shaded coral

Climate Narrows the Likely Terrestrial Biome

Long-term temperature and rainfall are the main predictors of terrestrial biomes. Seasonality and insolation refine the prediction because they change water availability, growing season and productivity.

Show simplified climograph and named biome examples so the learner can predict A Biome From Climate.

warm + wet year-round → tropical forest
warm + very dry → hot desert
cold + short growing season → tundra
seasonal/moderate rain → grassland or temperate forest, depending on water balance

Similar Climates Can Produce Similar Biomes

A biome is a group of ecosystems with similar abiotic conditions and broadly similar communities. It is defined by climate and dominant vegetation, not by a single species list or a political boundary.

Separated regions can experience the same selection pressures. Natural selection may therefore favour independently evolved features with similar functions—convergent evolution—so their communities can have similar forms even though the species differ.

Climate predicts the broad biome; soil, altitude, fire, grazing and local history can shift the ecosystem found at one site, so biome boundaries are gradual rather than perfectly sharp.

Desert Species Evade Heat or Conserve Water

Hot deserts combine intense daytime heat, large temperature changes and scarce, unpredictable water. Survival depends on evading the worst conditions or enduring them while minimizing water loss.

  • cactus: succulent photosynthetic stem stores water; spines replace broad leaves and reduce leaf area
  • kangaroo rat: nocturnal burrowing avoids daytime heat; concentrated urine and no sweating reduce water loss
  • camel: variable body temperature and water-conserving excretion reduce losses; hump fat supplies energy and metabolic water

The feature must be linked to its physical effect: a camel's hump stores fat, not liquid water, and a cactus spine helps conserve water because it replaces a leaf and deters herbivores.

Rainforest Species Exploit Layers and Scarce Resources

Tropical rainforests are warm and wet, but light, space and mineral nutrients are unevenly distributed through their vertical layers. Adaptations improve access to these resources or reduce exposure to predators.

Several pitcher plants hang among green rainforest leaves; their modified leaves form deep fluid-filled traps.
  • pitcher plant: modified leaves trap and digest animals → mineral ions supplement nutrient-poor rooting sites
  • flying lizard: skin stretched between extended ribs forms a gliding surface → movement between trees without descending
  • gibbon: long forelimbs, hook-like fingers and mobile shoulders → efficient brachiation through the canopy
  • orchid mantis: flower-like form attracts prey and provides camouflage

Pitcher plants still photosynthesize for carbon and energy; captured animals mainly supply scarce mineral nutrients such as nitrogen.

Summary: Predict Distribution from Environment and Adaptation

measure environment → locate tolerance limits and optimum → predict possible distribution → test abundance in the field → explain how adaptations change performance

microhabitat: local moisture, light and temperature
species range: interacting tolerance, adaptation and dispersal limits
biome: long-term climate filters whole communities; similar pressures can produce convergent forms

Habitat definition

1 mark

Define habitat.

Adaptations to abiotic environment

2 marks

Explain one feature of tree roots that help trees to survive in mangrove swamps.

Abiotic variables affecting distribution

4 marks

Discuss how abiotic factors can affect the distribution of species in an ecosystem.

Range of tolerance of limiting factor

2 marks

Using the data in the graphs, discuss whether species in Group 1 or Group 2 are more likely to be adversely affected by increases in soil temperature due to global warming.

Coral reef formation conditions

2 marks

Outline one way in which reef-building corals are affected by increasing atmospheric carbon dioxide.

Terrestrial biome distribution

3 marks

Outline how the type of stable ecosystem that will develop in an area can be predicted based on climate.

Biomes exam focus

2 marks

State what a biome is.

Adaptations to hot deserts and tropical

7 marks

Describe adaptations that are typical of plants growing in hot deserts throughout the world.