Populations in ecosystems (A-level only) — AQA A-Level Biology

A population is all the organisms of one species living in the same place at the same time that can interbreed. A community is all the populations of all the different species living in that same place at the same time, and the point of the word is that those populations interact: they compete for the same resources, eat one another, pollinate one another, parasitise one another. Community is a purely biotic term, so soil, temperature and water are not part of it. Communities are described by sampling them. Species richness counts how many different species are present; an index of diversity goes further and uses the number of individuals of each species as well, so it distinguishes a community with one dominant species from one where individuals are spread evenly.

An ecosystem is the community plus the abiotic components of its environment, working as one interacting unit. Abiotic components are non-living, physical and chemical conditions: light intensity, temperature, water availability, pH, oxygen and carbon dioxide concentration, soil type and wind exposure. Two things link the living and non-living parts. Energy flows one way: light is captured by producers, passes along food chains and is lost as heat at every transfer, so the system needs a constant input. Nutrients such as carbon and nitrogen cycle repeatedly between the abiotic reservoir, producers, consumers and saprobionts.

An ecosystem is defined by what it contains, not by how big it is. A rotting log, a rock pool, a puddle in a tree hole or the film of water around a cushion of moss each contains a community of interacting species together with its own abiotic conditions, so each is an ecosystem. So is a lake, an oak woodland, a coral reef or the North Atlantic. What sets the boundary is the question being investigated, so the investigator chooses it. Small ecosystems sit nested inside larger ones: a rock pool within a rocky shore within a coastal ecosystem. No ecosystem is closed. Organisms migrate across the boundary, energy enters as light and leaves as heat, and nutrients are carried in and out by water, wind and animals, so what happens outside affects what happens inside. Scale matters most when you sample.

A habitat is where a species lives; a niche is its role, governed by adaptation to abiotic and biotic conditions. A niche includes tolerated abiotic conditions (temperature, pH, light intensity, water availability) and biotic relationships (food, predators, competitors). For example, a red squirrel's niche includes feeding on pine cones in coniferous woodland and being active during the day; the woodland itself is its habitat, not its niche. Because a niche is defined by adaptation, each species has its own. Two species cannot occupy exactly the same niche in the same habitat. One will exploit the shared resource more efficiently, outcompete the other, and the less successful species will be excluded.

Carrying capacity is the maximum population size an ecosystem can support over time. Abiotic factors set the limit: temperature and light affect photosynthesis, while water and pH affect growth. Biotic factors cause fluctuations around this limit. Intraspecific competition (between members of the same species) intensifies as numbers rise, reducing reproduction and causing population drops. Interspecific competition (between different species) occurs when they share resources; the less competitive species may decline or be outcompeted. Predation also drives fluctuations: as prey numbers rise, predators have more food and increase, which then reduces prey numbers, causing predators to starve and decline in a lagging cycle.

For non-motile or slow-moving organisms, count in quadrats. Where the habitat looks uniform, place quadrats randomly: lay two tapes at right angles as axes and generate coordinates with random numbers, which removes bias. Record the number of individuals, the percentage cover where individual plants cannot be counted, or the frequency. Scale up with the mean number per quadrat divided by the quadrat area and multiplied by the total area. Where there is an environmental gradient, random placement would hide the pattern, so use a belt transect: a tape along the gradient with quadrats at regular intervals. For motile organisms use mark-release-recapture. Capture a first sample, count and mark the individuals harmlessly, release them, allow time for mixing, then capture a second sample.

Dynamic means constantly changing rather than fixed. Populations within an ecosystem rise and fall around their carrying capacities as abiotic factors change with the season and as competition and predation vary. Energy enters continuously as light, is fixed by producers, is transferred along food chains and is lost as heat at every transfer, so the system can never be static and always needs a fresh input. Nutrients such as carbon and nitrogen are cycled repeatedly between the abiotic reservoir, producers, consumers and saprobionts. Over longer timescales the community itself changes through succession, so even the species list is not permanent. Most of these changes are held within limits by negative feedback: a rise in prey supports more predators, and the predators push the prey back down.

Primary succession begins on newly exposed land with no soil, like bare rock or blown sand. The first colonisers are pioneer species adapted to harsh abiotic conditions. For example, lichens colonise bare rock, while marram grass colonises sand. Pioneers weather rock and, upon death, saprobionts decompose their remains. This dead organic matter (humus) mixes with rock particles to form a thin soil. Soil retains water and mineral ions, allowing mosses (using rhizoids) and small rooted ferns to establish. Their remains deepen the soil, supporting grasses, shrubs, and eventually trees. Each stage is a seral stage, making the environment less hostile for new species, culminating in a stable climax community determined by the climate.

The engine of succession is that organisms change their own abiotic environment. Lichens on bare rock release acids that break the rock down and hold moisture at the surface; when they die, decomposition by saprobionts adds humus, so a thin soil forms. That soil holds water and mineral ions, reduces the extremes of surface temperature and gives an anchorage for roots, which are exactly the conditions mosses and small herbs need and which they could not have tolerated earlier. Those plants are larger, so they produce more dead organic matter and a deeper soil, which in turn supports grasses, then shrubs, then trees. The species arriving at each stage have different adaptations from the last: drought-tolerant pioneers with no true roots, such as lichens and mosses with rhizoids, give way to deeper-rooted species with higher water and mineral ion demands.

Succession removes species as well as adding them, because the same change that opens a habitat to new colonisers can close it to the ones that made it possible. Mosses growing on the thin soil formed by lichens shade those lichens and hold water at the surface; taller herbs then overtop the mosses; shrubs and finally trees form a canopy that cuts light intensity at ground level to a small fraction of full sunlight. Light-demanding pioneers cannot photosynthesise fast enough in that shade, so they lose the interspecific competition for light, their population falls and they are excluded from the area. The same happens below ground, where deeper-rooted, faster-growing species absorb water and mineral ions more effectively, so the shallowly anchored pioneers are outcompeted for those resources too.

Abiotic conditions on newly exposed ground are hostile: there is no soil, so no store of water or mineral ions, temperatures swing widely, and exposure to wind and light is high. Colonising organisms change this. Dead remains decomposed by saprobionts add humus, which holds water and releases mineral ions. Plant cover shades the surface, narrowing the temperature range and reducing evaporation. Roots bind the substrate and reduce erosion; on a dune, marram grass traps sand and adds humus. Each change makes the abiotic environment less hostile. More species can tolerate the conditions, so biodiversity rises, measured as species richness and an index of diversity.

Many valued habitats are early or mid-successional: heather moorland, chalk grassland, coppiced woodland, reedbed and open dune. Left alone, each would continue through succession to the climax community, usually woodland, and the species adapted to the open stages would be outcompeted and lost. Conservation therefore deliberately arrests or deflects succession, producing what is called a plagioclimax. The practical methods all remove the larger competitors and keep light reaching ground level: grazing by sheep, cattle or rabbits, mowing, rotational burning of heather, coppicing, clearing invading scrub and trees, and scraping ground bare to restart colonisation.

Conservation maintains natural resources sustainably but often conflicts with human needs like food, timber, and housing. Managing this requires sustainable practices like coppicing or fishing quotas. When evaluating conservation data, identify evidence both supporting and opposing a claim, alongside limitations like small sample sizes. Population size is estimated using mark-release-recapture. The formula is: Estimated population = (Number in first sample × Number in second sample) / Number marked in second sample. For example, if 20 are marked, and a second sample of 30 contains 5 marked individuals, the estimate is (20 × 30) / 5 = 120. This assumes no births, deaths, or migration.

This practical investigates how an environmental factor affects species distribution. Choose a measurable factor: light intensity, soil moisture, pH, or temperature. Where the factor changes along a gradient, use a belt transect by running a tape measure along the gradient and placing quadrats at regular intervals to capture the pattern. At each quadrat, record species abundance (e.g., percentage cover or frequency) and measure the environmental factor, taking repeat readings to calculate a mean. If comparing two uniform areas without a gradient, sample randomly within each area using coordinates generated by a random number calculator to avoid bias.