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 too broad. “Forest” is a start; “shaded, moist leaf litter in a temperate deciduous forest on neutral soil” gives far more biological detail.

An organism is one living individual 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 is all the populations of different species living and interacting in the same area.

A strong habitat description can work at three levels:

• 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. The wording matters here. A plant does not “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 much 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, works well as a named example of a dune grass because its structure matches many of these dune problems.

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

Tree adapted to mangrove swamps

A mangrove swamp creates two major 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 can remove excess salt that does get in. The tree produces pneumatophores: vertical root branches that project 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 help 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 does more than show where someone has recorded the organism; it often shows 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 span, there is usually an optimum where the species performs best. Near the limits, survival may still be possible, but growth or reproduction falls. Outside the range, the species is absent unless humans protect it artificially.

Abiotic variables for plants

Abiotic variables strongly affect where plants grow. Examples include:

• 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 because high light intensity and dry air push water loss beyond its tolerance.

Abiotic variables for animals

Abiotic variables also shape animal distributions, 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 only occur in areas where it can avoid daytime heat and conserve water.

Sometimes a single abiotic requirement limits 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 the variable falls outside the organism’s tolerance range. For one species, soil pH might be the limiting factor; for another, it could be light intensity, salinity or temperature.

A typical tolerance curve is usually split into three useful regions. In the optimum range, abundance or performance is high. 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, such as dry ground to wet ground, shade to full sun, upper shore to lower shore, or unburned woodland to burned woodland.

For this syllabus, you need 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 causes the other. That’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, or 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, that pattern suggests the abiotic variable may be limiting its distribution.

Coral reefs as a marine ecosystem

A marine ecosystem is an ecosystem in saltwater, including the organisms living there 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, although 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, plus access to carbon dioxide and mineral nutrients. That’s why light matters so much in 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 stay 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 pattern is tropical or subtropical seawater that is shallow, clear, warm, and has 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 are the two main climatic variables. With a given average temperature and rainfall pattern, one natural ecosystem type is usually more likely to develop than the others.

That doesn't mean every tropical forest has the same species, or that every grassland has the same animals. Geography and evolutionary history shape which species are actually present. Still, the broad form of the ecosystem is fairly predictable: forests need enough warmth and water for trees; deserts develop where rainfall is too low for continuous plant cover; tundra develops where low temperature restricts tree growth.

A biome climate graph is the standard way to show this. Mean annual temperature goes on the horizontal axis, and annual precipitation goes on the vertical axis. Each biome covers a broad region rather than a single sharp point because ecosystems can tolerate some variation, and because seasonality also matters.

Why distant ecosystems can look similar

Convergent evolution means similar features evolve independently in distantly related organisms because they face similar selection pressures. So ecosystems on separate continents can look familiar, even when the species living there are unrelated.

In hot, dry places, natural selection favours organisms that conserve water, avoid overheating and reproduce when water is available. In cold places 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, and natural selection often arrives at similar solutions.

So for 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

One 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.