Populations in ecosystems notes

In a nutshell

This subtopic zooms out from single organisms to whole systems: how populations of different species live together as a community, how the living and non-living environment together make an ecosystem, and what sets the size of any one population.

You need to be able to define the key terms precisely, estimate a population size from real field data (quadrats and mark-release-recapture), explain how a community changes over time by succession, and discuss the conflict between conservation and human needs.

Assumed knowledge: Populations, Biodiversity within a community.

Core content

Population, community, ecosystem

These three terms sit inside one another, and AQA credits the precise wording, so learn the boundaries exactly.

A population is all the organisms of one species living in a habitat at a given time.
A community is all the populations of all the different species living in the same habitat at the same time.
An ecosystem is a community together with the non-living (abiotic) components of its environment, interacting as a unit.

A habitat is the place where an organism lives. Ecosystems range in size from the very small (a rock pool, a rotting log) to the very large (a forest, an ocean).

Every ecosystem has two sets of factors acting on it:

Abiotic factors are the non-living, physicochemical conditions: temperature, light intensity, pH, water and humidity, oxygen or mineral ion availability.
Biotic factors are the effects of other living organisms: competition, predation, disease.

Niche

A niche is the role of a species within its habitat, including its position in food webs and all of its interactions with the biotic and abiotic conditions.

The spec is precise here: a species occupies a niche governed by its adaptation to both abiotic and biotic conditions. A limpet grazing algae on a wave-battered rock at a particular tidal height occupies a different niche from a barnacle filter-feeding on the same rock.

A consequence you must be able to state: two species cannot occupy exactly the same niche in the same habitat. If they try, they compete for identical resources and one is outcompeted, so only one survives there. This is why the number of distinct niches limits how many species a habitat can hold.

Carrying capacity and what changes population size

The carrying capacity is the maximum population size of a species that an ecosystem can support over a long period.

A population colonising new space typically follows an S-shaped (sigmoid) growth curve: a slow start, then rapid near-exponential growth while resources are plentiful, then a levelling off as the population fluctuates around the carrying capacity.

Population size can vary because of:

Abiotic factors. When an abiotic factor is closer to a species' optimum (for example a warmer temperature within its tolerance), more individuals survive and reproduce, so the carrying capacity is higher. When conditions are harsher, fewer survive. Abiotic factors affect every individual in the same way.
Interspecific competition (between species).
Intraspecific competition (within a species).
Predation.

The last three are biotic and are covered next.

(Microorganisms grown in a broth culture grow so fast that their numbers are often plotted on a logarithmic scale, which turns exponential growth into a straight line and lets a huge range of numbers fit on one axis.)

Intraspecific competition

Intraspecific competition is competition between individuals of the same species for the same resources (food, water, light, breeding sites, territory).

It is the process that pulls a population back towards its carrying capacity:

When the population is large, resources per individual are scarce, so intraspecific competition is intense.
Fewer individuals get enough to survive and reproduce, so the population falls.
With fewer individuals, competition eases, more survive and reproduce, so the population rises again.

The population therefore fluctuates around the carrying capacity rather than sitting at a fixed value.

Interspecific competition

Interspecific competition is competition between individuals of different species for the same resource.

Its effects:

Both competing populations are limited: neither reaches the size it would in the absence of the other.
If one species is a better competitor for the shared resource, the other's population and distribution shrink, and it may be excluded from the area entirely.

You must name the resource being competed for (light, water, a named nutrient, space). "They compete" on its own does not earn the mark.

Predation

Predation is where one organism (the predator) kills and eats another organism (the prey). Predator and prey population sizes are linked and oscillate in a repeating cycle.

The causal loop, in order:

When prey are plentiful, predators have abundant food, so more predators survive and reproduce: the predator population rises.
More predators eat more prey, so the prey population falls.
With less food, predators starve, so the predator population falls (its peak comes after the prey peak).
With fewer predators, more prey survive, so the prey population recovers, and the cycle repeats.

Still don't get it? · why the predator peak comes after the prey peak

Think of a shop that only restocks by selling. When the shelves are full (lots of prey), the shop hires more staff (predators multiply). But hiring takes time, so the extra staff only arrive after the shelves were already full, and by then they are selling stock faster than it comes in. The shelves empty (prey crash), staff get laid off (predators crash), the shelves slowly refill, and it starts again.

Rebuilding it one step at a time: predators cannot respond instantly. Being well fed lets them raise young, but those young take time to be born and grow before they hunt. That delay is the whole reason the two curves are out of step rather than rising and falling together. The predator numbers always "chase" the prey numbers a little behind.

In exam terms: prey numbers rise first; the predator population rises after a lag because it depends on the food supply; predators then reduce the prey; the predator peak therefore lags the prey peak. Note that predation is usually only one factor acting on the prey, so real data rarely show a perfect cycle.

Estimating population size: quadrats (Required practical technique)

For slow-moving or non-motile organisms (plants, algae, limpets), you count them in quadrats, small sample frames, and scale up.

Where the habitat looks uniform and you want overall abundance, place quadrats using random sampling:

Lay out a grid and generate random coordinates with a random number generator, so placement is unbiased. You do not throw the quadrats.
Use a large number of quadrats so the sample is representative.
Calculate a mean per quadrat, then scale to a density per m².

Abundance inside a quadrat can be recorded as:

Frequency, the proportion (or percentage) of quadrats in which the species is found.
Percentage cover, the estimated percentage of the quadrat area the species covers, useful when individuals cannot be counted (dense grass, algae).

Where you are studying how a species changes along an environmental gradient (up a shore, from a path into a wood), use a belt transect: lay a line across the gradient and record abundance in quadrats placed at regular measured intervals along it. This shows distribution, not just overall abundance.

Mark-release-recapture (motile organisms)

Random quadrats do not work for animals that move, so for motile organisms you use mark-release-recapture:

Capture a first sample, count and mark the individuals in a way that does not harm them or make them more visible to predators, then release them.
Leave enough time for the marked individuals to disperse and mix back into the whole population.
Capture a second sample and count how many of it carry a mark.

The population is then estimated from the proportion of the second sample that is marked:

$\text{population estimate} = \frac{\text{(number in first sample)} \times \text{(number in second sample)}}{\text{number marked in the second sample}}$

Still don't get it? · why the mark-release-recapture formula works

Imagine a jar of dried beans and you want to count them without tipping them all out. You take a scoop of 60 beans, paint them red, tip them back and shake the jar. Now 60 beans in the whole jar are red. You take a second scoop and find that a quarter of it is red. If a quarter of your scoop is red, it is a fair bet that about a quarter of the whole jar is red too, so the whole jar holds about 60 divided by one quarter, which is 240 beans.

Step by step: marking a known number (say 60) means the fraction of the whole population that is marked is 60 divided by the true total. When you take a second sample, the fraction of it that is marked should be that same fraction, because the marked ones mixed back in evenly. Setting the two fractions equal and rearranging gives the formula above.

This is exactly why the two assumptions matter: you must leave time for mixing (so the marked fraction really is spread evenly), and there must be no significant births, deaths or migration (so the marked fraction does not change between the two samples). In the exam, the "number marked in the second sample" is the recaptured-and-marked count, not the size of the whole second sample.

Assumptions of mark-release-recapture (each is a place the estimate can go wrong):

The marking does not affect survival (it is not toxic and does not make individuals easier for predators to catch), and the mark is not lost.
There is no significant immigration or emigration, and no significant births or deaths, between the two samples.
Marked individuals have enough time to mix randomly back into the population before the second sample.
Sampling is carried out in the same way both times.

Required practical 12: an environmental factor and distribution

The required practical investigates how a named abiotic factor affects the distribution of a species, for example how light intensity affects the distribution of a woodland plant, or how soil pH or shore exposure affects a named species.

Method in outline:

Lay a belt transect along the gradient of the factor.
At regular intervals, record the species' abundance (frequency or percentage cover) in a quadrat and measure the abiotic factor with the right apparatus (a light meter for light intensity, a pH probe for soil pH).
Look for a relationship between the factor and the abundance along the transect.

Succession

Ecosystems are dynamic: the community in an area changes over time. Succession is the directional change in the species that make up a community over time. Primary succession starts on newly exposed land with no soil (bare rock, sand, cooled lava).

The mechanism is the part AQA marks, so learn it as a chain:

Pioneer species colonise first. They are adapted to the harsh abiotic conditions (for example lichens that tolerate bare rock and little water).
The pioneers change the abiotic environment: they die and decompose, adding organic matter and forming soil, retaining water and nutrients.
This makes the environment less hostile, so new species with different adaptations can now colonise and survive.
Each new set of species may change the environment further and often outcompetes the earlier species, so the earlier species are replaced.
Biodiversity increases as succession proceeds, and the ecosystem becomes more stable, until a climax community is reached: a stable community where no further change occurs.

Conservation and managing succession

Conservation of habitats frequently involves the management of succession. Left alone, many valuable habitats (heathland, chalk grassland, sand dunes) would undergo succession to woodland and lose their characteristic species. Conservationists therefore deliberately halt succession at an earlier stage, for example by grazing animals, mowing, or controlled burning, to keep an earlier community (sometimes called a plagioclimax).

Conservation also means managing the conflict between human needs and protecting species and habitats, so that the use of natural resources is sustainable (resources are used in a way that meets present needs without stopping future generations meeting theirs).

When a question gives you data or evidence about a conservation issue, your job is to use that data: quote the trend, weigh the conflicting evidence, and reach a supported judgement. Generic textbook lines earn nothing here.

Worked examples

Worked calculation: mark-release-recapture

A student estimates the number of woodlice in a leaf-litter plot. In the first sample she catches, marks and releases 60 woodlice. Two days later she catches a second sample of 80 woodlice, of which 24 are marked. Estimate the population.

Identify the three numbers:

number in first sample (marked) = 60
number in second sample = 80
number marked in the second sample = 24

Substitute into the formula:

$\text{population estimate} = \frac{60 \times 80}{24} = \frac{4800}{24} = 200 \text{ woodlice}$

Two things earn or lose the marks: putting the right numbers in the right places (the 24, not the 80, is the "marked in the second sample"), and giving the whole-number answer with its unit.

Model long answer: "Describe how primary succession leads to a climax community."

A five-mark "describe" needs five distinct, linked points in sequence, not one idea restated. Number them as you plan:

Pioneer species colonise the bare rock or sand.
Pioneers die and decompose, adding organic matter and forming soil.
The abiotic environment becomes less hostile, so new species with different adaptations can colonise.
Later species outcompete earlier species, which are replaced, and biodiversity increases.
A stable climax community is reached, in which no further change occurs.

Common exam mistakes

Defining a community vaguely as "a group of organisms" or "the organisms in an area". Examiners want all the populations of all the different species in the same habitat at the same time.
Poor quadrat method. Saying you "throw" the quadrats (this is not random and is rejected), using too few quadrats, or describing one quadrat without saying you take a large number and calculate a mean.
Belt transect placed wrongly. Describing the line as running "across and along" the shore. The transect is laid along the environmental gradient, with quadrats at regular intervals.
Mark-release-recapture slips. Forgetting to say the individuals are released; forgetting that the mark must not harm them or make them more visible to predators; and, most commonly, not leaving time for the marked individuals to mix back in before the second sample. In the calculation, using the whole second sample as the "marked" number instead of the recaptured-marked count.
Calling a herbivore eating a plant "predation". Predation is one organism killing and eating another organism; grazing on a producer is not treated as predation in these answers.
Not naming the resource in competition answers. "They compete" scores nothing; you must name what they compete for (light, water, a named nutrient, space).
Rote succession answers. Only mentioning the pioneer species, or describing succession in general when the question gives data to interpret. You must say the pioneers change the abiotic environment and make it less hostile, and, in a data question, refer to the species and figures given.
Dismissing given data. Answering a "use the data" question with rote criticisms such as "correlation does not show causation", "not enough data" or "need repeats". If the data are provided for you to analyse, use them rather than reject them.
Vague conservation answers pitched at GCSE level (for example a general account of polar bears and melting ice) instead of specific, data-supported management of a named habitat.
Saying "the results are significant". All results are what happened; the claim that earns credit is that the difference is significant.

Key definitions

Population - all the organisms of one species living in a habitat at a given time.
Community - all the populations of all the different species living in the same habitat at the same time.
Ecosystem - a community together with the non-living (abiotic) components of the environment, interacting as a unit.
Habitat - the place where an organism lives.
Niche - the role of a species within its habitat, including its position in food webs and its interactions with the biotic and abiotic conditions.
Abiotic factor - a non-living, physicochemical component of the environment (for example temperature, pH, light intensity).
Biotic factor - a living component of the environment (for example competition, predation, disease).
Carrying capacity - the maximum population size of a species that an ecosystem can support over a long period.
Intraspecific competition - competition between individuals of the same species for the same resources.
Interspecific competition - competition between individuals of different species for the same resource.
Predation - where one organism (the predator) kills and eats another organism (the prey).
Succession - the change in the species making up a community over time.
Pioneer species - the first species to colonise a new or bare area in succession, adapted to the harsh abiotic conditions.
Climax community - the stable community reached at the final stage of succession, in which no further change occurs.

Specification

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