Adaptations, Interdependence and Competition

Ecologists use a hierarchy of terms — each one bigger and broader than the last. Learn the definitions and the order because AQA examiners ask for them directly.

Organism. A single living thing. One robin in your garden is an organism. So is one oak tree, one earthworm, one bacterium.

Population. All the individuals of one species living in the same place at the same time. Every grey squirrel in Richmond Park is a population. Every bluebell in an English woodland is a different population.

Community. All the populations of all the different species living in the same place at the same time. A UK oak woodland community includes oak trees, bluebells, squirrels, deer, owls, fungi, bacteria, beetles and so on — all interacting.

Ecosystem. A community PLUS the non-living (abiotic) parts of the environment it interacts with — soil, water, sunlight, air, temperature. A pond, a hedgerow, a rocky shore and the Amazon rainforest are all ecosystems, just at different sizes.

Biosphere. Every single ecosystem on Earth added together — the thin layer of land, water and air where life exists. There is only one biosphere.

Why the hierarchy matters. When you answer an exam question, choose the right level. If the question asks about all the bluebells in a wood — that's a population. All the species in the wood — community. Wood + soil + climate — ecosystem.

In a community, every species relies on other species to survive. This web of dependence is called interdependence.

Why species need each other:

• Food. Almost every animal is eaten by something else, or eats something else. Remove rabbits from a heath and foxes lose a major food source.
• Shelter and habitat. Oak trees provide nesting holes for woodpeckers, surfaces for lichens and homes for thousands of insect species.
• Pollination. UK wildflowers depend on bees, hoverflies, butterflies and moths. Without pollinators, flowering plants cannot reproduce sexually.
• Seed dispersal. Many UK trees use animals to spread their seeds — squirrels bury acorns; blackbirds eat berries and excrete the seeds elsewhere.

Why this matters for the exam. If a question describes a change — a disease wipes out one species, or a new species arrives — you should explain the knock-on effects through these dependencies.

Example: UK pollinator decline. Declining bee populations affect crops (apples, strawberries, oilseed rape) AND wildflowers AND the animals that depend on those plants. A single change ripples through the community.

Example: ash dieback. Hymenoscyphus fraxineus fungus is killing UK ash trees. Ash supports many lichens, fungi, birds and insects that struggle to find alternative hosts — interdependence in action.

Resources are limited. When more than one organism needs the same resource, they compete — and only those that get enough survive long enough to reproduce.

Plants compete for:

• Light — needed for photosynthesis. Taller plants shade smaller ones (e.g. an oak canopy reducing the light reaching bluebells).
• Water — needed for photosynthesis, support and transport. Roots from different plants can drain the same patch of soil.
• Mineral ions (nutrients) — nitrate, phosphate, potassium and magnesium are absorbed from the soil.
• Space — for roots to spread, leaves to fan out and seeds to germinate.

Animals compete for:

• Food — herbivores compete with other herbivores for the same plants; carnivores compete for prey.
• Mates — usually within the same species, often males competing for females.
• Water — drinking water in habitats where it's scarce (e.g. dry summer chalk grassland).
• Territory — space to feed, nest and raise young. Robins are famously aggressive over UK garden territories.

Why it matters. Competition shapes which species succeed in a habitat. The 'winners' get more resources, survive better and produce more offspring; the 'losers' die or move on. Over time this contributes to natural selection.

A stable community is one in which all the species and the environmental factors are in balance, so that population sizes remain roughly constant from year to year. Mature UK oak woodland and tropical rainforest are classic examples.

Features of a stable community:

• Population numbers fluctuate only slightly around an average level.
• The mix of species stays broadly the same over decades.
• Abiotic factors (temperature, water, soil nutrients) are reliable enough to support the species there.
• Disturbances (fire, storm, disease) are usually small enough that the community recovers.

Why stability matters. Stable communities support high biodiversity and provide reliable 'ecosystem services' — pollination, clean water, carbon storage. Unstable communities lose species, change rapidly and provide fewer services.

Cross-link. Stable communities are vulnerable to environmental change (4.7.2.4) — climate change, pollution and habitat loss can push a stable community out of balance, sometimes irreversibly. UK conservation work (e.g. SSSI protection) tries to keep stable communities stable.

An abiotic factor is a non-living physical or chemical feature of an environment that affects which organisms can survive there and how well they thrive. The word means literally 'not biological'.

Abiotic factors set the physical limits of a habitat. A species can only live where the temperature, moisture, light and chemistry are all within the range its body can cope with. Push any one factor outside that range and the species declines or disappears.

Why abiotic factors matter on the spec.

• Examiners give you a graph or table showing a change in one abiotic factor (e.g. light dropping after a hedge grows) and ask you to predict the effect on a population.
• You must link the factor to a biological process (photosynthesis, respiration, enzyme activity, osmosis, gas exchange) to score the explanation marks.

The AQA list (memorise it):

1. Light intensity
2. Temperature
3. Moisture levels (water availability)
4. Soil pH
5. Soil mineral content
6. Wind intensity and direction
7. Carbon dioxide levels (for plants)
8. Oxygen levels (for aquatic animals)

Light intensity. The brightness of light reaching a habitat. It affects:

• Photosynthesis rate — more light = more glucose = more plant growth.
• Flowering time — many UK plants flower in spring/summer when day length increases.
• Shade-tolerant vs sun-loving species — wood anemones thrive in shaded woodland; gorse needs full sun.

Reduce light (e.g. a new building shades a garden) → photosynthesis falls → plants grow less, herbivores have less food, the whole food web is affected.

Temperature. Affects:

• Enzyme activity — too cold and enzymes work slowly; too hot and they denature.
• Species distribution — Arctic species cannot survive UK summer heat; tropical species cannot survive UK winters.
• Reproduction timing — frogs spawn when ponds warm to a critical temperature.

Warmer UK springs (climate change) are pushing some species northwards and shifting flowering dates earlier — an example of an abiotic factor altering a community.

Moisture (water availability). Affects:

• Photosynthesis — water is a reactant.
• Turgor pressure — water keeps plant cells firm; drought makes plants wilt.
• Transport — water carries minerals up the xylem.
• Animal survival — animals need water for cell function, cooling and digestion.

Habitats range from wet (peat bog, river margin) through moist (oak woodland soil) to dry (chalk grassland, sand dune). Each has its own community adapted to that water level.

Soil pH. UK soils range from acidic peat (pH 3–4) through neutral garden loam (pH 6.5–7) to alkaline chalk (pH 8). pH affects which mineral ions are available and which plants can grow:

• Acid-loving plants: heather, bilberry, rhododendron.
• Alkaline-loving plants: many orchids, traveller's joy, chalk grassland flowers.

Soil mineral content. Plants need nitrate, phosphate, potassium and magnesium ions. Low nitrate is the most common cause of stunted growth and yellowing leaves. Some plants (legumes — clover, peas) cope with low nitrate using nitrogen-fixing root bacteria.

Wind intensity and direction. Wind:

• Dries leaves (increases transpiration).
• Mechanically damages stems and branches.
• Reduces tree height on exposed UK coasts (you see 'flag-shaped' coastal trees leaning away from the prevailing south-westerly).
• Aids seed and pollen dispersal for some plants.

Carbon dioxide concentration (for plants). CO₂ is a reactant in photosynthesis. Inside a dense crop canopy on a bright day, CO₂ can be the limiting factor — local levels drop because plants are using it faster than diffusion replaces it.

Oxygen concentration (for aquatic animals). Fish, mayfly nymphs and other freshwater animals breathe dissolved oxygen. Pollution (sewage, fertiliser run-off) feeds bacteria that use up oxygen, leaving levels too low for trout and salmon. Fast-flowing UK upland streams have higher dissolved O₂ than slow lowland rivers — and a different community to match.

A biotic factor is a living influence on a community — something one organism does to another. The word means literally 'biological'. Biotic factors sit alongside abiotic factors (4.7.1.2) and the two together decide which species thrive in any habitat.

The four AQA biotic factors:

1. Availability of food.
2. New predators arriving.
3. New pathogens (disease) arriving.
4. One species outcompeting another so numbers fall too low to breed successfully.

Why biotic factors matter on the spec. Examiners describe a change — a new species arrives, a disease spreads, a food source declines — and ask you to predict the effect on populations and explain why. A good answer names the biotic factor and traces the cause-and-effect chain.

Availability of food. Every consumer depends on a food source. If that food becomes scarce, the consumer's population falls.

Examples:

• A poor acorn year in UK oak woodland → fewer wood mice survive winter → fewer mice for tawny owls to eat → owl population drops the following year.
• Heather burning (intensive grouse moor management) reduces moorland plants → fewer insects → fewer ground-nesting birds.
• Loss of wildflowers → fewer pollinators → fewer plant species reproduce → spiral of decline.

New predators arriving. Often the most dramatic biotic effect, especially when the prey has no evolved defences against the newcomer.

Examples:

• Domestic cats in UK gardens kill an estimated 100+ million wild animals a year — robins, frogs, slow worms — that didn't evolve with cats as predators.
• American mink escaped from fur farms in the 20th century and devastated UK water vole populations (which had never faced an aquatic predator that could follow them into their burrows).
• Lionfish (not UK, but a classic exam case) introduced to the Caribbean — native reef fish have no escape behaviour, populations crash.

The pattern: prey co-evolved with their original predators and have escape behaviours suited to them. A NEW predator bypasses those defences.

New pathogens arriving. A pathogen is a disease-causing microorganism (bacterium, virus, fungus or protist). When a pathogen reaches a population with no immunity, infection rates and mortality can be enormous.

UK examples:

• Ash dieback — the fungus Hymenoscyphus fraxineus arrived in the UK in 2012. It is killing 60–90% of UK ash trees. Many other species (lichens, fungi, insects) depend on ash — a textbook interdependence cascade.
• Squirrelpox virus — carried by grey squirrels with no harm to themselves; lethal to red squirrels with no immunity.
• Crayfish plague — a water mould carried by introduced American signal crayfish, lethal to native white-clawed crayfish.

Interspecific competition (one species outcompeting another). When two species compete for the same resources and one is better at getting them, the weaker species can fall below the number needed to find mates and reproduce. Even if some individuals survive, the population effectively dies out.

UK examples:

• Grey squirrel vs red squirrel. Greys are larger, eat a wider range of food (including unripe acorns) and tolerate human-altered habitats better. Reds are squeezed into shrinking strongholds in Scotland, Northern Ireland, the Lake District and Anglesey.
• Rhododendron (introduced ornamental) shades out native UK woodland plants beneath it.
• Himalayan balsam outcompetes native UK riverbank flowers and reduces biodiversity.

Why low numbers stop a species recovering. Below a critical population size, individuals can't find mates, inbreeding becomes a problem, and chance events (a bad winter) can finish the species off — sometimes called the 'Allee effect' (term not required for AQA).

An adaptation is any feature that helps an organism survive and reproduce in its environment. Adaptations evolved by natural selection over many generations.

AQA expects you to classify adaptations into three types — and you'll often need to give an example of each:

Structural adaptations — physical features of the body. The shape, size or material of body parts.

• Polar bear: thick white fur, layer of blubber, small ears.
• Cactus: spines (reduced leaves), waxy cuticle, swollen stem.
• Camel: long eyelashes, hump of fat, broad feet, slit nostrils.
• Earthworm: streamlined cylindrical body for burrowing.

Behavioural adaptations — things the animal does.

• Polar bear: hunts on sea ice in cooler hours.
• Swallows: migrate from UK to Africa for winter.
• Hedgehogs: hibernate through cold UK winters.
• Wolves: hunt in packs.
• Bees: dance to communicate flower locations.

Functional adaptations — internal body processes (chemistry, physiology).

• Camel: produces very concentrated urine to conserve water.
• Polar bear: low metabolic rate when food is scarce.
• Antifreeze proteins in Arctic fish blood.
• Enzymes in thermophilic bacteria that work at 80 °C.
• Cactus: CAM photosynthesis that opens stomata only at night.

The point of the classification. Examiners often ask you to identify which type an adaptation is, or to provide one example of each. Practising the three categories until they feel automatic is the key to scoring.

Polar bear (Arctic cold).

• Structural: thick fur and blubber for insulation; white fur for camouflage on ice/snow; small ears to reduce heat loss; large paws spread weight on snow and act as paddles in water.
• Behavioural: hunts seals on sea ice; mothers den up in snow to give birth.
• Functional: relatively low metabolic rate when food is scarce; rich diet of seal blubber maintains energy reserves.

Camel (desert dry and hot).

• Structural: hump stores fat (energy + metabolic water); long eyelashes keep sand out of eyes; broad feet spread weight on sand; slit nostrils close in sandstorms; thick lips for thorny plants.
• Behavioural: rests in shade during heat of day; tolerates large daily body-temperature swings to reduce sweating.
• Functional: kidneys produce very concentrated urine; gut reabsorbs water from faeces; can lose up to 25% of body water without dying.

Cactus (desert dry and hot).

• Structural: spines instead of leaves reduce surface area for water loss (and deter herbivores); thick waxy cuticle on swollen stem; deep tap root reaches groundwater AND wide shallow roots catch any rainfall.
• Behavioural (plants): not really applicable in the animal sense, but some open flowers at night for nocturnal pollinators.
• Functional: stem is the main photosynthetic organ; CAM photosynthesis (some species) opens stomata at night to lose less water; stores water in fleshy tissues.

UK-relevant example: hedgehog (temperate woodland).

• Structural: spines for defence against UK predators (foxes, badgers).
• Behavioural: hibernates through cold UK winter when invertebrate food is scarce.
• Functional: lowers body temperature and metabolic rate during hibernation to save energy.

Extremophiles are organisms — usually bacteria — that thrive in conditions most other life forms find lethal. AQA's named examples include high temperature, high salt concentration and high pressure.

Thermophiles — live at very high temperatures.

• Example: bacteria in hot springs at Yellowstone (e.g. Thermus aquaticus) and around deep-sea hydrothermal vents at 80 °C+.
• Adaptation: heat-stable enzymes that don't denature; membranes that stay intact at high T.
• Useful to humans: the enzyme Taq polymerase, taken from T. aquaticus, runs the PCR test used in COVID-19 and DNA forensics worldwide.

Halophiles — live in very salty water (Dead Sea, salt pans).

• Adaptation: cell chemistry tolerates concentrations that would shrivel ordinary cells by osmosis.

Barophiles / piezophiles — live at very high pressure (deep ocean trenches).

• Adaptation: cell membranes and enzymes work under pressures thousands of times higher than sea level.

Acidophiles — live in very acidic conditions (volcanic pools, mine drainage).

• Adaptation: maintain neutral internal pH despite acidic surroundings.

Why this matters for the spec. Extremophiles show how natural selection can adapt life to almost any environment — and that life is more flexible than we once thought. They also support theories about life elsewhere in the solar system (e.g. on Mars or Europa's icy oceans).