Ecosystems, Niches and Population Dynamics Revision Notes | A Level AQA Biology

1. From Individual to Ecosystem: Building the Picture

Picture a woodland in autumn. A robin hunts earthworms in the leaf litter, oak trees compete for light, and fungi break down dead wood. Every organism, every fallen leaf, every raindrop is part of one interconnected system. That system — living and non-living parts together — is what ecologists call an ecosystem.

To talk precisely about ecosystems, you need four terms that AQA examiners use with strict meaning. A population is all the individuals of one species in a given area. Several populations living together form a community. The physical place where an organism lives is its habitat. Put the community together with all the non-living (abiotic) components of that habitat — temperature, light intensity, water availability, soil pH — and you have an ecosystem.

The scale point matters for exam questions. AQA explicitly states that ecosystems can range from the very small to the very large. A rock pool on a Cornish beach and the entire North Sea are both ecosystems — they differ only in scale, not in the principles that govern them.

2. The Niche: Every Species Has a Role

Knowing where an organism lives tells you its habitat. Knowing how it lives — what it eats, when it is active, what conditions it tolerates, what eats it — tells you its niche. The niche is the functional role a species plays within its ecosystem, and it is shaped by two sets of factors working simultaneously.

• Abiotic factors: temperature range tolerated, light intensity required, water availability, soil pH, salinity — the physical and chemical conditions a species can survive and reproduce in.
• Biotic factors: food sources exploited, predators avoided, competitors interacted with, parasites tolerated, mutualistic partners relied upon — the living interactions that shape behaviour and survival.

Consider the red fox in a British urban environment. Its abiotic niche includes tolerance of a wide temperature range and the ability to exploit fragmented, disturbed habitats. Its biotic niche includes an omnivorous diet (earthworms, small mammals, fruit, discarded food), avoidance of domestic dogs, and competition with urban badgers for some food sources. Change any one of those factors — remove the food waste, introduce a new predator — and the niche shifts.

A critical principle follows directly from this: no two species can occupy exactly the same niche in the same habitat at the same time. This is the competitive exclusion principle. If two species attempt to occupy identical niches, one will outcompete the other until only one remains. In practice, closely related species partition resources — feeding at different heights, being active at different times, or exploiting slightly different prey sizes — so their niches overlap but are never identical.

Exam questions on niches often ask you to explain rather than just describe. The key is to link the adaptation to both the abiotic and biotic conditions. For example: the barn owl's asymmetric ear placement (adaptation) allows it to locate prey by sound in darkness (biotic factor — prey detection) in open farmland habitats where light levels are low at night (abiotic factor — low light intensity). That is a complete niche explanation.

3. Carrying Capacity and What Controls Population Size

Resources in any ecosystem are finite. Food, water, nesting sites, light — there is only so much to go around. This places a ceiling on how large any population can grow, and that ceiling has a name: the carrying capacity.

When a population is well below carrying capacity, resources are plentiful, competition is low, and the population grows rapidly. As numbers increase and resources become scarcer, growth slows. Eventually the population stabilises around the carrying capacity — fluctuating above and below it rather than sitting at a fixed point. This produces the characteristic S-shaped (sigmoidal) growth curve you will have seen in your notes.

The carrying capacity is not a fixed, permanent value. It can shift upwards or downwards as conditions change. A run of mild winters might increase food availability and raise the carrying capacity for a herbivore population. A drought might reduce plant biomass and lower it. Understanding this variability is central to the AQA specification point that ecosystems are dynamic systems.

3a. Factors That Cause Population Size to Vary

Population size fluctuates around the carrying capacity because of a range of interacting factors. AQA groups these into two broad categories: abiotic factors and biotic factors.

The distinction between intraspecific competition (within one species) and interspecific competition (between different species) is a frequent exam target. Intraspecific competition is a key density-dependent regulating mechanism: as a population grows, individuals of the same species compete more intensely for the same resources, which reduces survival and reproductive success, pulling the population back towards the carrying capacity.

3b. Predator–Prey Dynamics: A Classic Case Study

One of the most instructive examples of population fluctuation is the predator–prey cycle. The classic dataset is the Canadian lynx and snowshoe hare, recorded through Hudson's Bay Company fur-trapping records over nearly a century. The two populations oscillate out of phase with each other, and the pattern reveals how tightly linked population dynamics can be.

Worked example: Interpreting a Predator–Prey Population Cycle

A graph shows hare numbers peaking at time T, then declining. Lynx numbers peak shortly after T. Explain the sequence of events that produces this pattern.

1. Hare population is high (at peak, time T). Food for hares (vegetation) is abundant. — High prey availability means lynx have plentiful food, so lynx survival and reproduction rates increase.
2. Lynx numbers rise after the hare peak — there is a time lag because it takes time for increased food availability to translate into higher lynx reproductive success and survival of cubs. — The time lag is a key feature examiners test. Lynx do not respond instantaneously; population growth takes at least one breeding season to register.
3. As lynx numbers rise, predation pressure on hares increases. Simultaneously, the large hare population has been overgrazing vegetation, reducing food availability — intraspecific competition intensifies. — Both biotic factors (predation, intraspecific competition) act together to reduce hare numbers. This is why real population dynamics are rarely caused by a single factor.
4. With fewer hares available, lynx face food shortage. Lynx survival and reproduction fall, so lynx numbers decline — again with a time lag after the hare decline. — The cycle then resets: reduced predation pressure allows hare numbers to recover, and the oscillation continues.

Answer: Hare peak → lynx numbers rise (time lag) → hare numbers fall (predation + intraspecific competition) → lynx numbers fall (food shortage, time lag) → hare numbers recover → cycle repeats.

4. Ecosystems as Dynamic Systems

Everything covered so far points to one overarching principle: ecosystems are not static. They are dynamic systems — constantly changing in response to the interactions between organisms and between organisms and their abiotic environment.

Change operates at multiple timescales. Day-to-day fluctuations in temperature alter the activity of ectotherms. Seasonal cycles shift food availability and trigger migration or dormancy. Over years, a population of herbivores can overgraze an area, reducing plant cover, which then reduces the herbivore population, allowing vegetation to recover — a feedback loop. Over decades, climate shifts can alter the carrying capacity of entire biomes.

The dynamic nature of ecosystems also means that the carrying capacity itself is not fixed. A disease outbreak that reduces a competitor species can temporarily raise the carrying capacity for another species. Human activity — habitat destruction, introduction of non-native species, pollution — can shift carrying capacities dramatically and rapidly, often with cascading effects through the community.

5. Exam Technique: Getting Full Marks on Ecology Questions

Ecology questions reward precise language and structured reasoning. The most common reason students lose marks is using vague phrasing where the mark scheme demands specific terminology. Here is how to approach the most frequently tested question types.

For describe questions about population graphs, work through the graph in sequence: state what happens to the population (rises, falls, fluctuates), give approximate values or time points if the graph provides them, and note any time lags between linked populations. Do not explain — the command word is describe, so causes are not required unless the question also says explain.

For explain questions about population changes, always link cause to effect using the word "because" or "therefore". For example: "The prey population decreases because predator numbers have risen, increasing predation pressure." A bare statement of what happened scores no marks for explanation.

For evaluate questions about factors affecting population size, present evidence for more than one factor, acknowledge limitations or counter-arguments, and reach a conclusion. A common 6-mark structure: identify the main factor with evidence → identify a second factor → compare their relative importance → conclude which is more significant in the given context.