Adaptation to environment
🎯What you need to be able to do
- Define a habitat and describe the habitat of a species.
- Explain the adaptations of a sand dune grass and a mangrove tree to the abiotic conditions of their habitats.
- Give examples of abiotic variables that affect the distribution of plants and animals, and explain range of tolerance.
- Use transect data to correlate the distribution of a species with an abiotic variable.
- State the conditions needed for coral reef formation.
- Explain how temperature and rainfall determine the distribution of terrestrial biomes, using a biome graph.
- Describe the climate of tropical forest, temperate forest, taiga, grassland, tundra and hot desert.
- Explain adaptations of named plants and animals to hot deserts and tropical rainforest.
📚The biology
Habitat
A habitat is the place in which a community, species, population or individual organism lives. A description of a habitat can include:
- its geographical location — for example, the coasts of the Indian and Pacific Oceans;
- its physical location within that area — for example, the intertidal mud of sheltered estuaries;
- the type of ecosystem — for example, mangrove swamp.
Adaptations to the abiotic environment
Abiotic factors are the non-living, physical and chemical features of an environment: temperature, water availability, light, salinity, soil type, wind and so on. Two contrasting examples show how species are adapted to harsh abiotic conditions.
Marram grass (Ammophila arenaria) colonizes coastal sand dunes. Sand dunes are dry, because water drains away rapidly through sand; they are windy and exposed; the sand is salty and low in nutrients; and sand is constantly blown over the plants.
In dry conditions the leaves roll inwards, with the stomata on the inner surface, trapping humid air and reducing water loss.
Stomata sit in pits and are surrounded by hairs, which hold still, moist air near them and further reduce transpiration.
On the outer surface of the rolled leaf, reducing evaporation.
Roots reach down to water deep in the sand, and underground stems spread widely, binding the sand.
Growth is stimulated by being buried, so the plant keeps pace with accumulating sand.
Grey mangrove (Avicennia marina) grows in tidal swamps on tropical coasts, including those of Bali. Its habitat is waterlogged, salty mud that contains almost no oxygen, and is flooded twice a day by the tide.
Vertical “breathing roots” grow up out of the mud. They are exposed at low tide and have pores through which oxygen diffuses to the roots buried in anoxic mud.
Salt glands on the leaves actively secrete excess salt, which can be seen as crystals on the leaf surface; the roots also exclude much of the salt when taking up water.
A wide network of cable roots anchors the tree in soft, unstable mud against waves and tides.
The embryo starts to grow while the fruit is still attached, so it can root quickly after dropping into mud before being washed away.
Reduce water loss, which matters because obtaining fresh water from seawater is costly.
Abiotic variables and species distribution
The distribution of a species — where it is found — is affected by abiotic variables:
temperature, light intensity, water availability, soil pH, soil mineral nutrients, salinity, wind exposure.
temperature, water availability, oxygen concentration (especially in water), salinity, pH of water, humidity, availability of shelter.
A species’ adaptations give it a range of tolerance for each abiotic variable. Within an optimum range, the species survives, grows and reproduces well. Towards either end of its range, it is under stress: it may survive but grow poorly or fail to reproduce. Beyond the limits of tolerance, it cannot survive. Where one abiotic variable is outside a species’ range, that variable becomes a limiting factor for its distribution, however suitable everything else is.
Using a transect
A transect is a line across a habitat along which samples are taken at regular intervals. It is used where conditions change gradually across an area — from the shore inland across a dune system, from the edge of a wood into its centre, or down a rocky shore. At each sampling point:
- record the abundance of the species, for example as percentage cover in a quadrat or the number of individuals;
- measure one or more abiotic variables, ideally with sensors: temperature, light intensity, soil moisture, soil pH, wind speed, salinity.
Collect data yourself from a natural or semi-natural habitat. A semi-natural habitat has been influenced by humans but is dominated by wild, not cultivated, species — for example a grazed meadow or a managed woodland. Plot the abundance against the abiotic variable as a scatter graph and calculate a correlation coefficient to assess how strongly they are related.
Conditions for coral reef formation
Coral reefs are built by hard corals that live in partnership with photosynthetic algae called zooxanthellae (C4.1). The algae need light and the corals need conditions that allow them to deposit calcium carbonate skeletons. Reefs therefore form only where:
Shallow — most reef-building corals grow at depths of less than about 50 m, where enough light penetrates for photosynthesis.
Clear water, with little suspended sediment, lets light through and does not smother the corals.
Warm, roughly 20–30 °C. Too cold and corals grow very slowly; too warm and they bleach.
Fully marine salinity; corals do not grow near river mouths where fresh water dilutes the sea.
Slightly alkaline seawater (about pH 8.0–8.3). Lower pH makes it harder to deposit calcium carbonate (D4.3).
Biomes and the climate graph
A biome is a group of ecosystems with similar communities, found in different parts of the world where the abiotic conditions are similar. On land, the two dominant variables are temperature and rainfall. For any given combination of the two, one type of natural ecosystem is likely to develop.
This can be shown on a graph with mean annual temperature on one axis and mean annual rainfall on the other, with regions of the graph labelled by biome. Hot and very wet gives tropical rainforest; hot and very dry gives desert; cold and dry gives tundra; and so on.
Biomes in different continents contain unrelated species that look and behave alike, because similar abiotic conditions have produced similar selection pressures. Cacti in the Americas and euphorbias in Africa, for example, are unrelated but both have water-storing stems and spines instead of leaves. This is convergent evolution (A4.1).
Hot all year (about 25–27 °C), very high rainfall (over 2000 mm per year), little seasonal variation.
Warm summers and cool or cold winters, moderate rainfall spread through the year; distinct seasons.
Long, very cold winters and short, mild summers; low to moderate precipitation, much as snow.
Moderate temperatures (hot in tropical savanna), rainfall too low or too seasonal for forest; often a long dry season and fires.
Very cold, with a short growing season and low precipitation; subsoil permanently frozen (permafrost).
Very low rainfall (under about 250 mm per year), very hot days, often cold nights; high evaporation.
Adaptations to hot deserts
Leaves reduced to spines, which reduce water loss and deter animals.
A thick, pleated stem stores water and can expand like an accordion after rain.
Shallow, widespread roots absorb rain quickly before it evaporates.
A thick waxy cuticle, and stomata that open at night when it is cooler (CAM photosynthesis).
Very large ears with many blood vessels radiate body heat.
Nocturnal: it hunts in the cool of night and shelters in burrows by day.
Kidneys that produce concentrated urine, conserving water; it gets most of its water from food.
Furry soles protect the feet from hot sand.
Adaptations to tropical rainforest
Grows very tall, emerging above the canopy to reach light.
Buttress roots spread out from the base of the trunk, supporting the huge tree in shallow, nutrient-poor soil.
Shallow roots absorb nutrients quickly from rapidly decomposing leaf litter.
Many rainforest leaves also have drip tips, pointed tips that shed rainwater quickly so fungi and algae cannot grow on the leaf.
Very long arms and hook-shaped hands and feet for climbing and swinging through the canopy.
Arboreal: rarely descends to the ground, where food is scarce and predators are found.
A diet of fruit, which is available year-round in the rainforest, supplemented by leaves and bark.
Excellent memory of where and when trees fruit across a large area.
✏️Worked example
9.5, 8.0, 6.1, 4.2, 2.8, 2.0
40, 42, 22, 15, 3, 2
(b) Using a calculator, the correlation coefficient is \( r = 0.975 \) and the coefficient of determination is \( R^{2} = 0.95 \). Interpret these values.
(c) A classmate concludes that wind causes marram grass to grow. Evaluate this conclusion.
(a) There is a positive correlation: as wind speed increases, marram grass cover increases. Cover is highest (40–42%) at the most exposed points nearest the sea and falls to 2–3% where wind speed is lowest, furthest inland.
(b) An \( r \) of 0.975 is close to +1, so the positive correlation is very strong. An \( R^{2} \) of 0.95 means that about 95% of the variation in marram cover is associated with variation in wind speed, so a straight line fits the data very well.
(c) The conclusion is not justified. Correlation does not show causation. Several other abiotic variables change along the same transect: nearer the sea there is more blowing sand, which stimulates marram growth, and further inland the sand becomes more stable, with more organic matter and moisture, so other plants grow and out-compete marram grass. Wind speed may simply vary in step with these factors. Only six points were sampled along one transect, so the result may not be representative. A controlled experiment, or measuring several variables along several transects, would be needed to identify the cause.
📝Practise
Work through these on paper, then reveal the answer.
1. Explain two adaptations of marram grass that reduce water loss.
2. Explain how a mangrove tree obtains oxygen for its roots.
3. Explain what is meant by the range of tolerance of a species for an abiotic variable.
4. State four abiotic conditions required for coral reefs to form, and explain why one of them is needed.
5. Explain why tropical rainforests in South America, Africa and Southeast Asia have similar vegetation even though their species are mostly unrelated.
6. For a named desert animal, explain two adaptations that help it survive.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- Field Studies Council — practical guides to transects, quadrats and measuring abiotic factors on sand dunes and rocky shores.
- NOAA Ocean Service — clear explanations of the conditions coral reefs need and why.
- Khan Academy — Biomes, including the temperature–precipitation (Whittaker) graph.