1The idea in one paragraph

Every species lives in a particular place, its habitat, and the physical, non-living conditions of that place, its abiotic factors, set limits on what can survive there. A species can only live where each factor stays within its range of tolerance, and its adaptations decide how wide that range is. That is why species are distributed as they are, and why you can find the link in the field by running a transect and measuring a factor as you go. Scale it up and the same logic explains biomes: a given combination of temperature and rainfall tends to produce one type of ecosystem, and unrelated organisms in that climate on different continents evolve similar forms. Coral reefs, deserts and rainforests are the guide's worked cases.

2Habitat

A habitat is the place where a community, species, population or organism lives. A full description of a habitat says three things:

• the geographical location: where on Earth;
• the physical location: where within that area, and what the conditions are;
• the type of ecosystem.

Take the common limpet (Patella vulgata). Geographically, it lives on the Atlantic coasts of north-west Europe. Physically, it is clamped to bare rock on the shore between the high and low tide marks, exposed to air twice a day and covered by sea water twice a day. The ecosystem is a rocky intertidal shore. A one-word answer such as "the sea" scores nothing: habitat questions reward precision.

3Adaptations to the abiotic environment

The guide asks for two named plants in hard physical environments: a grass of sand dunes and a tree of mangrove swamps.

Marram grass (Ammophila arenaria) on sand dunes. Coastal dunes are dry, because rain drains straight through sand; windy; salty with sea spray; and unstable, because sand is always moving and burying things. Marram grass is the main grass of dunes on European coasts, and its adaptations answer each problem. Figure 1 shows its leaf.

• The leaf rolls up in dry weather, because hinge cells at the base of the grooves lose water and shrink. Rolling encloses the inner surface, where the stomata are, and traps a pocket of humid air.
• Stomata are sunk in grooves on the inner surface, not on the exposed outer surface. Still, moist air over them reduces the water-vapour gradient and so reduces transpiration.
• Hairs on the inner ridges slow air movement and hold humid air near the stomata.
• A thick waxy cuticle on the outer surface, which faces the wind and sun, reduces water loss.
• Long roots reach down to water deep in the dune, and underground stems (rhizomes) spread sideways.
• It grows upward when buried by sand. Burial stimulates growth, so the plant keeps pace with the moving dune. Its spreading rhizomes also bind the sand, which is why marram is planted to stabilise dunes.

Red mangrove (Rhizophora mangle) in mangrove swamps. Mangrove swamps line tropical coasts and estuaries. The trees stand in salt water that rises and falls with the tide, rooted in soft mud that is waterlogged and so almost without oxygen. Figure 2 shows how the red mangrove copes.

• Prop roots (stilt roots) arch out from the trunk and branches into the mud, giving the tree a wide, stable base in soft ground and against waves and tides.
• Lenticels, small pores on the prop roots above the mud, take in air at low tide, supplying oxygen to root tissues that could not get it from the waterlogged mud.
• Salt exclusion: the roots filter out most of the salt as water enters, so the tree takes up mostly fresh water from sea water.
• Thick, waxy leaves reduce water loss; with salty water around the roots, water is hard to obtain.
• Viviparous propagules: the seed germinates while still attached to the parent tree, growing into a long seedling that drops into the mud or floats away ready to root quickly.

4Abiotic variables and the range of tolerance

An abiotic variable is a non-living factor of the environment. Which ones matter depends on the organism.

The adaptations of a species give it a range of tolerance for each variable: the span of values within which it can survive. Figure 3 shows the pattern.

• In the optimum range, individuals grow and reproduce well, and the population is largest.
• In the zones of physiological stress on either side, individuals survive but grow and reproduce poorly, so the population is smaller.
• Beyond the limits of tolerance, in the zones of intolerance, the species cannot survive at all.

When one variable is outside the range a species tolerates, it limits the distribution of the species however good the other conditions are. That variable is a limiting factor. A species is found only where every abiotic variable is within its range, which is why its distribution often has sharp edges where one factor changes.

5Finding the link in the field: transects

The guide asks you to collect your own data from a natural or semi-natural habitat and correlate a species' distribution with an abiotic variable. A semi-natural habitat has been influenced by people but is dominated by wild rather than cultivated species: a hay meadow, a heath, the edge of a managed wood.

A transect is a line across a habitat along which you sample at regular intervals, chosen so that it crosses a change in an abiotic factor. A line transect records what touches the line; a belt transect places a quadrat at intervals along it and estimates the abundance of the species in each, often as percentage cover. At each sampling point you also measure the abiotic variable, by hand or with a sensor: a light meter, a temperature probe, a soil pH meter or a data logger.

Figure 4 shows invented data from a belt transect running 40 m out from inside a wood into open grassland, with a quadrat every 4 m. Light was measured as a percentage of full sunlight; the species is wood sorrel, a small woodland herb.

Read it in three steps, as a Paper 1B question will ask.

1. Describe. As distance from the wood increases, light intensity rises from 8% to 95% of full sunlight, and wood sorrel cover falls from about 45 to 50% to zero, reaching zero beyond 36 m.
2. Correlate. Plotting cover against light, panel (b), gives a strong negative correlation, r = −0.96. The higher the light intensity, the lower the cover.
3. Interpret with care. The data are consistent with wood sorrel being a shade-tolerant plant limited by high light, but correlation is not cause. Other variables change along the same line: soil moisture, temperature, leaf litter, and competition from grasses, which grow better in the light. Any of them could be responsible.

Improving a transect study. Repeat the transect in several places, so that one odd line does not decide the result; take several abiotic readings at each point at the same time of day, because light changes by the hour; and measure more than one abiotic variable, so that alternatives can be tested.

6Coral reefs

Coral reefs are the guide's example of a marine ecosystem whose distribution is set by abiotic factors. Reef-building corals are colonies of small animals that lay down skeletons of calcium carbonate. Inside their tissues live photosynthetic algae, which supply the coral with much of its food. That partnership is why so many of the conditions are about light. Figure 5 sums them up.

• Water depth: shallow, mostly less than about 50 m, because light for the algae fades with depth.
• Clarity: clear water with little suspended sediment, so light penetrates; sediment also smothers coral.
• Temperature: warm, roughly 20 to 29 °C. Above this range corals expel their algae and turn white, which is coral bleaching, and may die.
• Salinity: full-strength sea water, roughly 32 to 42 g of salt per litre; reefs do not form near large river mouths where fresh water dilutes the sea.
• pH: slightly alkaline, about 8.0 to 8.3. Calcium carbonate is laid down more slowly as sea water becomes more acidic, which is why ocean acidification threatens reefs.

Together these confine reefs to shallow, clear, warm seas in the tropics, mostly between about 30° north and 30° south.

7Biomes: why the same climate gives the same ecosystem

A biome is a group of ecosystems with similar communities, found wherever similar abiotic conditions occur. On land the two conditions that matter most are temperature and rainfall. For any given temperature and rainfall pattern, one natural type of ecosystem is likely to develop. Figure 6 shows the relationship the guide asks you to illustrate.

Why biomes on different continents look alike. The deserts of North America and southern Africa have plants that look strikingly similar, thick green stems swollen with water, covered in spines, with no proper leaves. Yet the American ones are cacti and the African ones are mostly euphorbias, from unrelated plant families. Each evolved its form separately because the same conditions selected the same solution. This is convergent evolution: unrelated organisms evolving similar features because they face similar environments. It is the reason a biome is a biome: similar conditions plus convergent evolution give similar communities.

8Adaptations to life in hot deserts

The problems of a hot desert are too little water, too much heat by day and sometimes cold at night.

Saguaro cactus (Carnegiea gigantea), of the Sonoran Desert in North America.

• Spines instead of leaves. The spines are modified leaves with almost no surface for transpiration; they also shade the stem slightly and deter animals that would eat the stored water.
• A thick, pleated green stem stores water and does the photosynthesis. The pleats let it swell like an accordion after rain and shrink during drought.
• A thick waxy cuticle and few stomata reduce water loss.
• A wide network of shallow roots just below the surface absorbs water quickly after brief rain, before it evaporates.
• Stomata open at night (CAM photosynthesis), taking in carbon dioxide when it is cool and humid, and close by day, which cuts water loss sharply.

Fennec fox (Vulpes zerda), of the Sahara.

• Very large ears give a large surface for losing heat and help it hear prey moving under the sand.
• Nocturnal, hunting in the cool of the night and sheltering by day in a burrow, where it is far cooler than at the surface.
• Fur on the soles of its feet protects against hot sand.
• It can go long periods without drinking, getting much of its water from its food, and its kidneys produce concentrated urine, conserving water.

9Adaptations to life in tropical rainforest

In tropical rainforest water and warmth are plentiful. The problems are different: competition for light in a dense canopy, heavy rain, thin soil poor in nutrients, and life in the trees.

Kapok tree (Ceiba pentandra), of the rainforests of Central and South America and West Africa.

• Emergent height: it grows above the main canopy, reaching full sunlight.
• Buttress roots, wide plank-like flanges at the base of the trunk, support a very tall tree whose roots are shallow.
• Shallow roots spread through the thin surface soil, where nutrients from rapidly decaying leaf litter are released, and take them up before heavy rain washes them away.
• Leaves with pointed tips, a feature shared by many rainforest plants, let rainwater run off quickly (drip tips), so the leaf surface dries and is less likely to be colonised by fungi and algae.

Red-eyed tree frog (Agalychnis callidryas), of Central American rainforest.

• Sticky pads on its toes let it climb and cling to leaves and branches in the canopy and understorey.
• Nocturnal, resting by day on the underside of leaves with its green body hidden.
• Thin, moist skin through which it exchanges gases and takes up water, which only works in the constantly humid air of the rainforest.
• It lays its eggs on leaves hanging over water, so the tadpoles drop into the water when they hatch. Only the high humidity of the forest stops the eggs drying out.