What a habitat description should include

A habitat is the place, or set of environmental conditions, where an organism, population, species or community lives. Try not to leave it vague. “Forest” is only a starting point; “shaded, moist leaf litter in a temperate deciduous forest on neutral soil” gives a biologist much more to work with.

An organism is one individual living thing that carries out the processes of life. A population is a group of organisms of the same species living in the same area at the same time. A species is a group of organisms that can interbreed to produce fertile offspring under natural conditions. A community includes all the populations of different species living and interacting in the same area.

A good habitat description can build in three layers of information:

• Geographical location: where on Earth it is found, such as a coastline, mountain range, island or river basin.
• Physical location within that place: the microhabitat, such as rock crevices, exposed sand, shallow water, tree canopy or muddy sediment.
• Type of ecosystem: for example mangrove swamp, sand dune, coral reef, hot desert, tundra or tropical rainforest.

Habitats are more than map references. They include the physical conditions an organism actually experiences: temperature, light, water availability, salinity, soil type, pH, oxygen availability and exposure to wind or waves. Adaptations are matched to those conditions.

Adaptation means form matched to function

An adaptation is an inherited feature of an organism that increases its chance of survival or reproduction in a particular environment. Be precise with the wording: a plant doesn’t “decide” to grow waxy leaves because the dune is dry. Variation is already present, and natural selection favours individuals with features that work better in that habitat.

An abiotic factor is a non-living physical or chemical part of the environment, such as temperature, water availability, light intensity, pH, salinity, oxygen concentration or soil texture. A biotic factor is a living component of the environment, such as competitors, predators, pathogens or pollinators. In extreme habitats, abiotic factors often drive most of the explanation.

Grass adapted to sand dunes

A sand dune is a tough place for a plant. The sand drains rapidly, contains little organic matter, may be salty near the coast, shifts under wind, and can bury shoots. Marram grass, Ammophila arenaria, is a useful named example of a dune grass because its structure almost reads like a checklist of dune problems and solutions.

Marram grass has long, deep and spreading roots that anchor it and reach water below the dry surface. It also produces rhizomes, horizontal underground stems that let the plant spread through shifting sand and send up new shoots after burial. The leaves can roll inwards, which traps humid air near the stomata and reduces the exposed surface area. A thick waxy cuticle reduces evaporation from the leaf surface, and sunken stomata reduce water loss in windy conditions. Tough supporting tissue helps leaves hold their shape when water is scarce.

Tree adapted to mangrove swamps

A mangrove swamp creates two main problems for trees: the mud is salty, and it is waterlogged, so oxygen diffuses very slowly to roots. Grey mangrove, Avicennia marina, is a named mangrove tree adapted to these abiotic conditions.

Grey mangrove roots exclude much of the salt from entering the plant, while salt glands in the leaves remove excess salt that does get in. The tree produces pneumatophores: vertical root branches that stick up above the mud and take in oxygen through small openings. Air spaces inside the root tissues help oxygen move down to living root cells. The roots also stabilize the tree in soft sediment, and buoyant propagules can be dispersed by seawater to new muddy shores.

These examples fit the guiding question well: habitat conditions and adaptations are related because the properties of body parts match the physical problems of the habitat. Hydrophobic cuticle reduces water loss; air-filled root tissue improves gas exchange; salt glands move ions out of leaves. In biological systems, the properties of components matter because they determine what the whole organism can do.

Distribution is limited by conditions

A species distribution is the geographical pattern of places where a species occurs. A distribution map shows more than the places where someone has recorded the organism; it often reflects the abiotic conditions the species can tolerate.

A range of tolerance is the span of values of an abiotic variable within which a species can survive, grow and reproduce. Within that range, there is usually an optimum where the species performs best. Near the edges, survival may still be possible, but growth or reproduction falls. Outside the range, the species is absent unless people protect it artificially.

Abiotic variables for plants

Plant distributions are strongly affected by abiotic variables such as:

• temperature, especially frost, heat stress and length of growing season;
• water availability in soil;
• light intensity and day length;
• soil pH;
• soil salinity;
• availability of mineral ions such as nitrate, phosphate and potassium;
• soil texture, drainage and aeration.

For example, a plant adapted to acidic bog soil may fail on alkaline chalk soil because nutrient availability and root physiology differ. A shade-adapted woodland herb may be absent from open grassland, where high light intensity and dry air increase water loss beyond its tolerance.

Abiotic variables for animals

Animal distributions are also shaped by abiotic variables, including temperature, water availability, salinity, dissolved oxygen concentration, water depth, current speed and substrate type. An aquatic invertebrate that needs well-oxygenated flowing water will not be distributed through stagnant ponds. A desert mammal may be restricted to places where it can avoid daytime heat and conserve water.

Sometimes the limiting abiotic requirement applies to only one life stage. A fish may feed widely as an adult but need shallow, oxygen-rich water with a particular gravel size for eggs. That one reproductive requirement can restrict the whole species distribution.

Adaptations give a species its range of tolerance. Distribution, then, is the ecological result of form and function: what the organism is built to tolerate determines where it can live.

Limiting factors and tolerance curves

A limiting factor is an environmental variable that restricts an organism’s survival, growth, reproduction or distribution when conditions fall outside its tolerance range. For one species, the limiting factor might be soil pH. For another, it could be light intensity, salinity or temperature.

A typical tolerance curve has three useful regions. Abundance or performance is high in the optimum range. In zones of physiological stress, individuals may survive, but they grow or reproduce less successfully. Beyond the limits of tolerance, the species is absent.

Using transects to find correlations

A transect is a sampling line or strip used to measure how organisms and environmental conditions change across a habitat. Transects work especially well where there is a gradient: dry ground to wet ground, shade to full sun, upper shore to lower shore, or unburned woodland to burned woodland.

For this syllabus, you should be able to use transect data to correlate the distribution of a plant or animal species with an abiotic variable. A correlation is a relationship in which two variables change together; by itself, it does not prove that one variable causes the other. It’s a small but important scientific caution.

In a field investigation, collect the data from a natural or semi-natural habitat. A natural habitat is an area dominated by wild species with little direct human alteration. A semi-natural habitat is an area influenced by humans but still dominated by wild rather than cultivated species, such as an old meadow, managed woodland edge or restored pond margin.

There are three common transect approaches:

• Line transect: record each organism touching a tape laid along the ground.
• Belt transect: sample a strip of fixed width, often using quadrats at regular intervals.
• Observational transect: follow a defined route at a steady pace and record sightings of target species.

A quadrat is a frame of known area used to estimate abundance, percentage cover or frequency of organisms. A sensor is a device that detects and records an environmental variable. In this topic, useful sensors include thermometers or temperature probes, light meters and soil pH probes. Data loggers can store repeated sensor readings, which helps when conditions change through the day.

To analyse the data, plot species abundance against distance along the transect and plot the abiotic variable on the same distance scale. You could also use a scatter graph of abundance against the abiotic variable. If a plant is abundant only where soil pH is low, or an animal is present only where light intensity is low, the pattern suggests that the abiotic variable may be limiting its distribution.

Coral reefs as a marine ecosystem

A marine ecosystem is an ecosystem in saltwater, including its organisms and the physical conditions they interact with. A coral reef is a marine ecosystem built from calcium carbonate skeletons deposited by reef-building corals.

Reef-building corals are animals, though many have mutualistic photosynthetic algae living in their tissues. A mutualism is an interaction between two species in which both benefit. The coral receives some products of photosynthesis; the algae get shelter, carbon dioxide and mineral nutrients. Light, for that reason, is central to reef formation.

Coral reefs form only when several abiotic conditions are suitable at the same time:

• Water depth: water must be shallow enough for enough light to reach the photosynthetic algae; reef formation is usually strongest in clear shallow water.
• pH: seawater must be alkaline enough for calcium carbonate deposition; lower pH makes skeleton formation harder.
• Salinity: salt concentration must remain within the tolerance range of the coral and its algae; large freshwater inputs can prevent reef development.
• Clarity: suspended sediment reduces light penetration and can smother coral surfaces.
• Temperature: water must be warm but not excessively hot; prolonged high temperatures can disrupt the coral–algal mutualism.

So the usual setting is tropical or subtropical, shallow, clear, warm seawater with stable salinity. Reefs show adaptation to environment well, because the whole ecosystem depends on the match between coral physiology, algal photosynthesis and abiotic conditions.

Temperature and rainfall set the broad pattern

A biome is a group of ecosystems with similar communities because they occur under similar abiotic conditions. On land, temperature and rainfall do most of the shaping. With a given average temperature and rainfall pattern, one natural ecosystem type is usually more likely to develop than another.

That doesn’t mean every tropical forest has the same species, or that every grassland supports the same animals. Geography and evolutionary history shape the exact species found there. The broad structure, though, is fairly predictable: forests need enough warmth and water for trees; deserts form where rainfall is too low for continuous plant cover; tundra forms where low temperature restricts tree growth.

A biome climate graph shows this pattern. Mean annual temperature goes on the horizontal axis, and annual precipitation goes on the vertical axis. Each biome covers a broad region, not a single sharp point, because ecosystems can tolerate some variation, and because seasonality also matters.

Why distant ecosystems can look similar

Convergent evolution is the independent evolution of similar features in distantly related organisms because they experience similar selection pressures. That helps explain why ecosystems on separate continents can look familiar, even when the species living there are unrelated.

In hot, dry areas, natural selection favours organisms that conserve water, avoid overheating and reproduce when water is available. In cold areas with short growing seasons, it favours low growth forms, insulation, dormancy and rapid reproduction during brief favourable periods. Similar abiotic conditions create similar ecological problems, so natural selection often leads to similar solutions.

This answers the guiding question about terrestrial biomes: ecosystems within a biome are similar because their abiotic conditions are similar, and convergent evolution leads to similar adaptations in the communities found there.

Climate conditions of major terrestrial biomes

A clear example of convergent evolution is the repeated evolution of water-storing, spiny or tough-leaved desert plants in unrelated plant families. These plants are not similar because they recently shared one desert ancestor. They are similar because dry habitats reward similar structures.

Hot desert conditions

A hot desert is a terrestrial biome with very low precipitation and high temperatures, often with intense sunlight and big day-to-night temperature changes. Water is the main limiting factor, though heat and poorly developed soils also play a part.

Creosote bush, Larrea tridentata, is a named hot desert plant. Its small, resin-coated leaves cut down water loss and reflect some sunlight. It has an extensive root system, so after rare rain it can take up water quickly. During drought, it can slow its growth and then become active again when water is available. The wide spacing between shrubs reduces competition for scarce soil water.

Merriam’s kangaroo rat, Dipodomys merriami, is a named hot desert animal. It is nocturnal, so it forages when temperatures are lower. It lives in burrows, where the air is cooler and more humid than at the surface. It gets much of its water from dry seeds through metabolic reactions, produces very concentrated urine and dry faeces, and avoids wasteful evaporative cooling whenever possible.

Tropical rainforest conditions

A tropical rainforest is a terrestrial biome with high rainfall, high temperature and high light intensity above the canopy throughout the year. At ground level, though, the canopy intercepts much of the light, so light can be very limited. In rainforest, competition for light can therefore matter as much as water availability.

The kapok tree, Ceiba pentandra, is a named rainforest plant. It grows very tall, lifting its leaves into bright light above much of the canopy. Buttress roots spread out from the base of the trunk and support the tree in shallow tropical soils. Smooth bark and leaf drip tips help water run off surfaces, which reduces the growth of epiphytes and fungi on tissues where they would block light or damage the plant.

The red-eyed tree frog, Agalychnis callidryas, is a named rainforest animal. It is arboreal, meaning it lives mainly in trees, and it has adhesive toe pads for gripping wet leaves and branches. Its green body colour camouflages it among leaves. It is mostly active at night, when humidity is higher and water loss is lower. Eggs are laid on leaves above water, allowing the tadpoles to drop into pools after hatching.

The word “adaptation” can refer to quite different problems. In hot deserts, many adaptations reduce water loss or heat stress. In tropical rainforest, many adaptations help organisms deal with intense competition, heavy rainfall, wet surfaces, climbing and low light below the canopy.