IB Biology SLTopic 3 — Adapting to the EnvironmentPaper 1 & 2Core idea~14 min read
Biomes
Tell me the average temperature and the average rainfall of a place and I can tell you roughly what grows there, without ever having been. That is not a trick — it is what a biome is.
📚 What you need to know
A biome is a large community of plants and animals that has developed as a result of environmental factors.
Biomes cover huge geographical areas and are named after their dominant vegetation type.
Temperature and rainfall are the two factors that matter most in deciding which biome develops.
A climograph plots mean annual rainfall against mean annual temperature and shows which biome results.
Regions with similar abiotic factors contain the same biome, even on different continents.
Equivalent biomes hold similar communities because of convergent evolution, not shared ancestry.
What a biome actually is
Start with the definition, because it is short and examiners want it exactly: a large community of plants and animals that has developed as a result of environmental factors.
Two things in that sentence do the work. Large — a biome spans continents, not fields. And as a result of environmental factors — a biome is not a place, it is an outcome. Conditions produce it.
The main terrestrial biomes you should be able to name:
Tropical rainforest and temperate rainforest
Temperate deciduous forest
Boreal or coniferous forest, also called taiga
Grassland, which includes temperate grassland and savanna
Tundra
Desert, either hot desert or cold desert
Biomes are named after their plants, never their animals. Nobody talks about a “zebra biome”. Vegetation is what you see from a satellite, and vegetation is what responds most directly to temperature and rainfall.
The climograph
Plot mean annual rainfall on one axis and mean annual temperature on the other, and every combination lands in a region belonging to a particular biome. That graph is a climograph.
Rainfall is in centimetres here. Data tables often use millimetres instead, so check the unit before you read anything off.
🧩 Reading a climograph without falling into the trap
Read both axis labels first. Neither rainfall nor temperature is the independent variable, so either one can be put on either axis. Different textbooks swap them.
Check the direction of the temperature scale. Some climographs run cold at the top and hot at the bottom, which is the opposite of what you expect.
Check the units. Rainfall in cm and rainfall in mm differ by a factor of ten.
Find your rainfall value, find your temperature value, and see which region they meet in.
Do not treat the boundaries as exact. Near an edge, say which two biomes it could be and explain why.
Rainfall and temperature are not the only factors. Soil type, grazing animals and fire all shift where one biome ends and the next begins. On many climographs the uncertain zones are drawn with dotted lines for exactly this reason.
Conditions of the major biomes
Biome
Climate
What the community looks like
Tropical forest
Rainfall 2000 to 10 000 mm; temperature steady at 20 to 25 °C; poor soil because there is no seasonal leaf fall
Layered vegetation with canopy and undercanopy; very high productivity and very high biodiversity
Temperate forest
Rainfall 750 to 1500 mm; seasonal but no extremes; fertile soil built up by leaf fall each autumn
Deciduous trees dominate; productive for part of the year; high biodiversity
Taiga (coniferous forest)
Rainfall 300 to 900 mm plus heavy snow; very short growing season; roughly −40 to 20 °C
Evergreen conifers dominate; low productivity; a small number of well-adapted species
Grassland
Rainfall 500 to 950 mm with distinct wet and dry seasons; roughly −20 to 30 °C depending on region
Grasses dominate; too dry for much tree growth; grazing animals and few top predators
Tundra
Rainfall 150 to 250 mm; dark winters and frozen soil; roughly −50 to 18 °C
Too little water, light and warmth for trees; low productivity; animals hibernate or migrate
Hot desert
Rainfall under 250 mm; days up to about 49 °C and nights down to about 0 °C
Very low productivity because of the lack of water; a small number of well-adapted species
🧠 Spotting the pattern in that table
Read the last column downwards. High biodiversity goes with warm and wet. Low biodiversity goes with extreme. That is not a coincidence — harsh conditions mean fewer species can tolerate them, so only a few well-adapted specialists live there.
Why the same biome looks the same everywhere
Deserts occur in Africa, in North and South America and in Australia. The plants in them look remarkably alike: thick fleshy stems, spines instead of leaves, shallow spreading roots. But the American ones are cacti and the African ones are euphorbias, and the two groups are not close relatives at all.
The word to use is morphology — they have similar body form. Genetically they sit on quite different branches of the plant family tree.
🤔 How convergent evolution works, in four steps
Species in equivalent biomes face the same selection pressures: intense heat, scarce water, hungry grazers. In each population there is natural variation, and individuals that happen to store water better or deter grazers better survive and reproduce more.
Those advantageous features become more common in the population generation after generation through natural selection. Because the pressures were the same in Africa and in America, the same kinds of feature were favoured in both — and two unrelated lineages ended up looking alike.
Adaptations to two very different biomes
Hot desert
The problem is water, plus temperatures that swing wildly between day and night.
Saguaro cactus. A thick waxy cuticle on the stem cuts evaporation. It has spines instead of leaves, which reduces the surface area losing water by transpiration and also deters grazers. Cells in the stem expand to store water after rain. A deep tap root reaches water far underground, while shallow surface roots grab any rainfall quickly before it evaporates.
Kangaroo rat. It spends the day in an underground burrow — a behavioural adaptation that avoids the worst heat. It extracts enough water from its food to survive and produces highly concentrated urine, so it can go a long time without drinking.
Tropical rainforest
Here water and warmth are plentiful, so the problems are different: competition for light, and predation.
Kapok tree.Rapid growth lets it shoot up past its neighbours and reach the upper canopy, where it can absorb enough sunlight in a crowded forest. Wide buttress roots spread out at the base to keep a tall, fast-grown trunk stable.
Orchid mantis. The adult female closely resembles an orchid flower. This mimicry attracts insect pollinators, which then become her prey. The much smaller, plainer male instead uses camouflage among stems and branches to avoid being seen.
The orchid mantis is a nice reminder that adaptations are not only about the physical environment. Competition, predation and mimicry are biotic pressures, and in a rainforest they matter far more than the weather does.
Worked examples
WORKED EXAMPLE
Use the climograph to predict the biome at a mean annual rainfall of 320 cm and a mean annual temperature of 25 °C. [1 mark]
Step 1: find the rainfall on the horizontal axis320 cm sits to the right of the 300 markStep 2: find the temperature on the vertical axis25 °C is near the top, in the warmest bandStep 3: read where they meet
Warm and very wet puts the point in the tropical rainforest region.
Tropical rainforestCheck the units first. 320 mm would be a completely different answer.
WORKED EXAMPLE
Cacti in American deserts and euphorbias in African deserts both have spines and thick fleshy stems, yet they are not closely related. Explain this. [4 marks]
Step 1: name the process
This is convergent evolution.
Step 2: the shared cause
Both deserts have the same abiotic conditions, so the two groups faced the same selection pressures: very low water availability, high temperatures and grazing animals.
Step 3: the mechanism
In each population, individuals that stored water in the stem or had spines instead of leaves survived better and reproduced more, so those alleles became more common through natural selection.
Step 4: the conclusion
The similarity is therefore in morphology only. They independently adapted to similar conditions rather than inheriting the features from a recent common ancestor.
Same selection pressures + natural selection = similar form, unrelated lineages
WORKED EXAMPLE
Explain how two features of the kangaroo rat help it survive in a hot desert. [4 marks]
Feature 1: behavioural
It stays in an underground burrow during daylight. Soil temperature underground is much lower and more stable, so the rat avoids the highest daytime temperatures and loses less water.
Feature 2: physiological
It produces highly concentrated urine, so very little water is lost in excretion, which means it can survive on the water obtained from its food without drinking.
One behavioural adaptation and one physiological adaptation, each explainedNaming the type of adaptation as behavioural or physiological is often worth credit in itself.
💡 Exam tip
Learn the biome definition word for word, including “as a result of environmental factors”.
Read both climograph axes before answering. The axes can be swapped and the temperature scale can be inverted.
If a point sits near a boundary, say so and name both possible biomes. That is a better answer than guessing one.
For convergent evolution, the mark scheme wants selection pressure, natural selection and not closely related.
Classify adaptations as structural, behavioural or physiological when you can.
Never write that an organism “chose” or “tried” to adapt. Variation exists first, and selection acts on it.
⚠ Common mix-up
Biome and ecosystem used as synonyms. A biome contains many ecosystems.
Assuming similar-looking species are related. Similar form can come from similar conditions instead.
Treating climograph boundaries as exact. Real transitions are gradual.
Mixing cm and mm between the climograph and the data table. A factor of ten changes the biome entirely.
Saying species “adapt during their lifetime”. Populations evolve; individuals do not adapt to order.
Naming a biome after its animals. Biomes are named after their dominant vegetation.
That completes Adapting to the Environment. You now have the whole chain: conditions set a range of tolerance, tolerance sets distribution, and distribution scaled up across continents is what we call a biome.
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