B4.1 Adaptation to environment

A habitat is the place where an organism lives. The word is used most often for a species, but you can also describe the habitat of a single organism, a population or a whole community.

A good description of a habitat answers three questions:

A second example is the red mangrove (Rhizophora mangle). It lives on tropical and subtropical coasts of the Americas and West Africa (geographical location), in the intertidal zone of sheltered shores and estuaries, which is flooded by seawater at every high tide (physical location), in a mangrove swamp (type of ecosystem).

Physical and chemical conditions

A fuller description of a habitat adds the conditions found there. Marram grass grows in loose, dry sand that is low in nutrients and exposed to strong wind and salt spray. The red mangrove grows in soft, waterlogged mud that is salty and has almost no oxygen. These conditions are what the species’ adaptations must cope with (next section).

Habitat of a community

The habitat of a community is the area that the whole group of interacting species occupies. For example, the habitat of a coral reef community is warm, shallow, clear tropical sea.

The environment of an organism includes biotic factors (other living things) and abiotic factors (non-living conditions such as temperature, water, light and salinity). An adaptation is a heritable characteristic that suits an organism to its environment; adaptations arise by natural selection (A4.1 Evolution and speciation). They are easiest to see in plants of extreme habitats.

Marram grass on sand dunes

A coastal dune is hard to live in: sand drains quickly and holds little water, the wind is strong and dries leaves, there is salt spray, and moving sand can bury plants. Marram grass (Ammophila arenaria) is a xerophyte (a plant adapted to dry conditions).

• Rolled leaves: in dry weather hinge cells on the inner surface lose turgor and the leaf rolls into a tube, enclosing a pocket of humid air.
• Stomata on the inner surface only, sunken in grooves between ridges, so water vapour diffuses out slowly.
• Hairs on the ridges trap moist air and reduce air movement next to the stomata.
• Thick waxy cuticle on the exposed outer surface, which has no stomata.
• Sclerenchyma fibres keep the leaf stiff when it is short of water.
• Rhizomes (underground stems) grow sideways and upwards, and burial by sand stimulates new shoots to grow up through it; long roots reach water deep in the dune.

Mangrove trees in tidal swamps

Mangrove mud is waterlogged and anaerobic (it contains almost no oxygen), very salty, soft and regularly flooded by the tide.

• The red mangrove (Rhizophora mangle) has arching prop (stilt) roots that anchor it in soft mud. Lenticels on the roots above the mud let air in, and air spaces in the roots carry oxygen to the parts buried in mud. Its roots exclude most of the salt as they take up water. Its seeds germinate on the tree (vivipary); the seedlings drop, float and root in the mud.
• The grey mangrove (Avicennia marina) has pneumatophores: vertical roots that grow up out of the mud from shallow cable roots and take in air at low tide. Salt glands in its leaves secrete excess salt.
• In both, cells hold high concentrations of solutes, so their water potential is lower than that of seawater and water still enters the roots by osmosis.

The distribution of a species is where it is found. Each species is found only where abiotic conditions suit its adaptations. Examples of abiotic variables that limit distribution:

Adaptations give a range of tolerance

A species’ adaptations work only over a certain range of each factor. Its range of tolerance is the range of values in which it can survive.

• In the optimum range, individuals grow, survive and reproduce best, so populations are largest.
• On each side is a zone of physiological stress, where individuals survive but grow and reproduce poorly, so they are few.
• Beyond the limits of tolerance are the zones of intolerance, where the species cannot survive at all.

A factor that is outside, or close to the limits of, a species’ range of tolerance acts as a limiting factor, restricting where the species is found or how well it grows there. Species with a wide range of tolerance can live in many habitats; species with a narrow range are restricted to a few.

Biotic factors matter too

Abiotic factors set where a species could live. Whether it actually lives there also depends on biotic factors. On rocky shores, for example, the upper limit of many seaweeds is set by drying out at low tide (abiotic), but their lower limit is often set by competition with other seaweeds (biotic).

The range of tolerance of a species can be investigated by experiment (growing it at different levels of a factor) or by looking for correlations in the field. In the field the usual method is a transect: a line across a habitat along which samples are taken, chosen so that it crosses a gradient in an abiotic variable, for example from a woodland edge into the wood, up a sand dune system or down a rocky shore.

Collecting the data

You need two sets of data at each sampling point:

1. The abiotic variable. Electronic sensors with data loggers measure light intensity, temperature, soil moisture or pH quickly and precisely, can record repeatedly or over long periods, and store the readings digitally for analysis.
2. The distribution of the species. In a belt transect, quadrats are placed at regular intervals and abundance is recorded as percentage cover or number of individuals. In a line transect, organisms touching the line are recorded at intervals.

The IB guide expects you to collect transect data yourself in a natural or semi-natural habitat: one influenced by humans but dominated by wild species, such as a grazed meadow or managed woodland.

Analysing the data

Plot a scatter graph of abundance against the abiotic variable and describe the trend. A correlation coefficient such as Pearson’s r measures how close the points are to a straight line.

• In the worked example, meadow grass shows a strong positive correlation with light (r = 0.99).
• The woodland herb shows a hump: it is absent in full light, most abundant at about 20% light and less abundant in deep shade. Its overall r (−0.69) hides this pattern; this is what a tolerance curve looks like in field data.
• Repeat the transect (or use several) so that the pattern is not an accident of one line.

Coral reefs are marine ecosystems built by reef-building (hard) corals. A coral is a colony of small animals (polyps, which are cnidarians) that deposit a skeleton of calcium carbonate. Over centuries, these skeletons build up the rock of the reef. Inside the cells of the polyps live zooxanthellae, single-celled photosynthetic algae. The relationship is a mutualism: the zooxanthellae supply most of the coral’s food as sugars from photosynthesis, and the coral provides carbon dioxide, nutrients and a protected place in the light (see C4.1 Populations and communities).

Because of this partnership, reefs only form where five conditions are met:

These conditions explain where reefs are found: in shallow, clear, warm tropical seas, mostly between about 30° N and 30° S. They also explain why reefs are threatened. The average pH of the surface ocean has fallen from about 8.2 to about 8.1 since pre-industrial times, as the oceans absorb carbon dioxide (ocean acidification), and marine heatwaves are causing more frequent mass bleaching.

On land, the type of ecosystem that develops depends above all on two abiotic factors: temperature and precipitation (rainfall and snow). For any combination of mean annual temperature and annual precipitation, one natural type of ecosystem is likely to develop. This is shown on a biome graph (often called a Whittaker diagram), with temperature on one axis and precipitation on the other.

How to read the graph

• Find the temperature on the x-axis and the precipitation on the y-axis; the region where they meet is the most likely biome.
• Temperature sets the main sequence from left to right: tundra, taiga, temperate biomes, tropical biomes.
• Precipitation decides, at each temperature, whether the result is desert, grassland or forest.
• The boundaries slope upwards to the right. Warmer air evaporates more water, so a hot climate needs more rain than a cool one to support the same vegetation. About 100 cm of rain supports temperate forest at 10 °C, but only savanna at 25 °C.
• The graph has an upper envelope: very cold climates are never very wet, because cold air holds little water vapour.

Why the graph is only a guide

The boundaries are not sharp lines. Real biomes merge into each other over wide transition zones, and other factors also matter: how rainfall is spread through the year (a long dry season favours savanna over forest), soil type, fire, grazing animals, altitude and human activity.

A biome is a group of ecosystems that resemble each other because they have similar abiotic conditions, even though they may be on different continents. Tropical rainforests in the Amazon, the Congo basin and Borneo, for example, are separate ecosystems with mostly different species, but they form one biome.

Why distant ecosystems look alike

Similar climates exert similar selection pressures. Unrelated organisms in each region have therefore evolved similar adaptations: convergent evolution, producing analogous structures (A4.1 Evolution and speciation).

• Cacti in American deserts and euphorbias in African deserts belong to unrelated families, but both have swollen, green, water-storing stems with spines and use CAM photosynthesis.
• Kangaroo rats (North America) and jerboas (Africa and Asia) are desert rodents from different families that both hop on long hind legs, live in burrows and produce very concentrated urine.

The six biomes to know

Hot deserts

Hot deserts get very little rain, often with long droughts. Days are very hot and sunny, but nights can be cold because clear skies let heat escape. Soils are thin and contain little organic matter.

Saguaro cactus (Carnegiea gigantea), Sonoran Desert of the USA and Mexico:

• A pleated stem that swells to store water after rain and shrinks during drought; the green stem carries out photosynthesis.
• Leaves reduced to spines, which have almost no surface for water loss, shade the stem and deter animals that would eat it for its water.
• A thick waxy cuticle and CAM photosynthesis: stomata open at night, when it is cooler and more humid, and stay closed in the heat of the day.
• Shallow roots spreading widely just below the surface absorb rain quickly before it evaporates; a taproot anchors the plant.

Fennec fox (Vulpes zerda), Sahara:

• Very large ears with many blood vessels radiate heat; they also detect prey moving under the sand.
• Nocturnal: it spends the hot day in a cool burrow dug in the sand.
• Pale, sandy fur reflects sunlight; thick fur insulates against the cold nights; furry soles protect the feet from hot sand.
• It can go for long periods without drinking, getting most of its water from food and producing concentrated urine.

Tropical rainforests

Tropical rainforests are hot and wet all year, with no cold season and intense light at the top of the canopy. Most nutrients are held in living things, and the soils are thin and poor because heavy rain washes nutrients out (leaching).

Yellow meranti (Shorea faguetiana), Borneo and the Malay Peninsula:

• An emergent tree: its crown rises above the main canopy into full sunlight, so it is not shaded by neighbouring trees. One individual in Sabah, Borneo, is 100.8 m tall, the tallest tropical tree known.
• Buttress roots support the tall trunk in shallow soil, and shallow roots absorb nutrients from the thin topsoil.
• Evergreen leaves photosynthesize all year round.
• Winged fruits spin as they fall and drift away from the parent tree. Like many dipterocarps it fruits in mast years, every few years, producing so many seeds at once that seed-eaters cannot eat them all.

Geoffroy’s spider monkey (Ateles geoffroyi), Central America:

• Long arms and legs and very flexible shoulders let it swing beneath branches and reach fruit at the tips.
• Hook-like hands with tiny thumbs hook over branches as it swings.
• A prehensile tail with a bare, ridged gripping pad can support its whole weight, freeing the hands for feeding.
• It eats mainly ripe fruit, available all year in the rainforest, and is active by day in the upper canopy.