Ecosystems and Interdependence

An ecosystem is the interaction of a community of living organisms with the non-living parts of their environment. The living parts are called biotic; the non-living parts are called abiotic.

To make sense of any ecosystem, biologists describe it at increasing scales. Each level contains the ones before it.

Level	Definition	Example
Individual	A single organism	One oak tree
Population	All the organisms of one species in a habitat	All the oak trees in a wood
Community	All the populations of different species in a habitat	The oaks, foxes, beetles and fungi
Ecosystem	The community plus the abiotic environment they interact with	The whole woodland: organisms + soil, light, water

A habitat is the place where an organism lives. A community is all the populations of all the different species living together in that habitat.

Notice the key step: a population is one species, while a community is many species interacting together.

To survive and reproduce, every organism needs a supply of materials from its surroundings and from the other living organisms around it.

When a resource is in short supply, organisms must compete for it. The better-adapted competitors get more of the resource, survive, and reproduce; the rest may not.

Plants and animals compete for different things, because they obtain energy and materials in different ways:

Group	Competes for	Why it is needed
Plants	Light	Powers photosynthesis to make glucose
Plants	Space	Room for roots and leaves to grow and reach resources
Plants	Water	Needed for photosynthesis and to keep cells turgid
Plants	Mineral ions (from soil)	E.g. nitrate ions to build proteins for growth
Animals	Food	Source of energy and materials
Animals	Mates	Needed to reproduce and pass on genes
Animals	Territory	Space containing food, shelter and breeding sites

Plants compete for light, space, water and mineral ions. Animals compete for food, mates and territory. Learning these two lists in full is worth easy marks.

A common exam task gives you a habitat and asks you to suggest the factors organisms are competing for. Read the scenario, identify the organism type, then choose from the correct list.

Scenario — A dense patch of bluebells grows on a woodland floor. In early spring the bluebells flower and grow quickly, then die back. By late spring the oak trees above them are in full leaf, and the woodland floor becomes deeply shaded.

Step 1 — Identify the organisms. Bluebells are plants. So they compete for light, space, water and mineral ions.

Step 2 — Match the scenario to the resource. The trees coming into leaf block the sunlight reaching the floor. The limiting resource here is light.

Step 3 — Explain the outcome. By flowering and growing early, the bluebells get the light they need for photosynthesis before the oak canopy closes over. This is an adaptation that lets them avoid losing the competition for light.

Answer: The bluebells compete with the oak trees for light. They are adapted to grow and flower early in the year, so they photosynthesise before the canopy shades the floor.

When asked to suggest a competed-for factor, only name factors from the correct plant/animal list — and link your choice directly to a detail in the scenario.

Within a community, each species depends on other species — for food, shelter, pollination, seed dispersal and more. This web of dependencies is called interdependence.

Because the species are linked, if one species is removed it can affect the whole community. A change to one population ripples outward to others.

Interdependence: within a community each species depends on other species. Removing or changing one species can affect many others.

A stable community is one where all the species and environmental factors are in balance, so that population sizes remain fairly constant over time.

Worked example — removing one species from a community:

Consider a simple meadow community:

Now suppose a disease wipes out the bees. Trace the effect step by step:

1. With no bees, the clover is not pollinated, so it produces far fewer seeds and the clover population falls.
2. Less clover means less food for the field mice, so the mouse population falls.
3. Fewer mice means less prey for the owls, so the owl population falls too.

One removed species has reduced three others. The community is no longer in balance — it is no longer stable. This is interdependence in action.

Abiotic factors are the non-living conditions of an environment that can affect a community. A change in any of them can change the size of a population.

Abiotic factor	Example effect of a change
Light intensity	Less light → slower photosynthesis → plants grow less → fewer plants and the animals that eat them
Temperature	Affects enzyme activity and growth rate; many species breed only within a temperature range
Moisture levels	Too little water limits plant growth; some species cannot survive in dry or waterlogged ground
Soil pH and mineral content	Wrong pH or low minerals (e.g. nitrate) → plants grow poorly → fewer plants
Wind intensity and direction	Strong wind increases water loss (transpiration) from plants and can damage them
Carbon dioxide levels (plants)	More CO₂ can raise the rate of photosynthesis, increasing plant growth
Oxygen levels (aquatic animals)	Low dissolved oxygen in water → aquatic animals such as fish cannot respire enough → numbers fall

Note two factors with a specific organism group: carbon dioxide matters for plants (photosynthesis), and oxygen levels matter for aquatic animals (respiration in water).

In an exam you may be given a graph or table and asked to explain how a change in an abiotic factor affects a community — to gain full marks, link the factor to a process (photosynthesis, respiration, enzyme activity) and then to population size.

Biotic factors are the living factors that can affect a community. The AQA specification lists four:

Biotic factor	Example effect of a change
Availability of food	Less food → animals starve or fail to breed → population falls
New predators arriving	A new predator eats a prey species that has no defence → prey numbers fall sharply
New pathogens	A new disease spreads through a population with no resistance → many die
One species outcompeting another	A better competitor takes the resources, so the other species' numbers drop too low to breed

The fourth factor is precise: one species outcompetes another so its numbers are no longer sufficient to breed. Outcompeting reduces a population to the point where breeding fails.

Worked example — classify the factor, then predict the effect:

Scenario — Grey squirrels are introduced to a woodland already home to red squirrels. The greys are larger, eat a wider range of food, and carry a virus that is harmless to them but deadly to red squirrels. Over several years the red squirrel population collapses.

Step 1 — Classify each factor. Both the grey squirrels and the virus are living, so these are biotic factors.

Step 2 — Name the specific biotic factors. (a) One species outcompeting another — greys take more of the shared food. (b) A new pathogen — the virus the greys carry.

Step 3 — Predict the effect. The red squirrels get less food and are killed by a disease they have no resistance to. Their numbers fall until too few remain to breed, so the population collapses. The community shifts towards grey squirrels.

To decide abiotic vs biotic, ask one question: is the factor alive (or made by a living thing)? Predators, food, pathogens and competitors are biotic. Light, temperature, water, pH, wind, CO₂ and oxygen are abiotic.

1. Confusing population, community and ecosystem

A population is one species; a community is all the populations of different species; an ecosystem is the community plus the abiotic environment. Marks are lost for using "community" when you mean "population".

2. Mixing up what plants and animals compete for

Plants compete for light, space, water and mineral ions. Animals compete for food, mates and territory. Writing "plants compete for food" or "animals compete for mineral ions" loses the mark.

3. Swapping the carbon dioxide and oxygen factors

Carbon dioxide is the abiotic factor that matters for plants (photosynthesis). Oxygen levels matter for aquatic animals (respiration in water). Do not state them the other way round.

4. Calling a factor abiotic when it is living

A new predator, a new pathogen, food availability and a competing species are all biotic — they are living or come from living things. Only non-living conditions are abiotic.

5. Vague definition of a stable community

A stable community is not just "one that does not change". State it precisely: all the species and environmental factors are in balance, so population sizes remain fairly constant.

6. Forgetting to follow the knock-on effect through the community

When asked how removing one species affects others, do not stop at the first link. Trace the chain of interdependence (food, pollination, predation) through each affected population to show the full effect.