B15.1 The Importance of Communities

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: populations, communities and ecosystems; competition, interdependence, predator–prey cycles and stable communities. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define population, community and ecosystem, and put them in order of size
• Describe interspecific and intraspecific competition, and what plants and animals compete for
• Describe how organisms in a community depend on each other (interdependence)
• Explain predator–prey cycles and use a food web to predict the effect of a change
• Describe what makes a community stable

Key words

• Population: All the organisms of one species living in a particular area at the same time.
• Community: Two or more populations of different species living in the same area at the same time.
• Ecosystem: The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.
• Competition: Organisms trying to get the same limited resource. Interspecific = between different species; intraspecific = within one species.
• Inter- dependence: Species in a community depend on each other for food, shelter, pollination, seed dispersal and more.
• Stable community: A community where all the species and environmental factors are in balance, so population sizes stay roughly constant.

From individual to ecosystem

To survive and reproduce, organisms need materials from their surroundings and from other organisms. Ecologists study this at different levels; each level contains the one before it.

• Individual; Meaning One organism; Example on a rocky shore one limpet
• Population; Meaning All the organisms of one species; Example on a rocky shore all the limpets on the shore
• Community; Meaning All the populations of different species; Example on a rocky shore limpets, crabs, seaweeds, barnacles, starfish …
• Ecosystem; Meaning The community plus the non-living environment; Example on a rocky shore the community plus the rock, sea water, temperature, light and waves

Key fact: Population = one species. Community = many species. Ecosystem = community + abiotic factors.

Competition

Resources are limited, so organisms compete for them. Competition between different species is interspecific (e.g. owls and foxes both hunting mice). Competition within one species is intraspecific (e.g. two oak seedlings growing side by side). Darwin called this the “struggle for existence”; it drives natural selection.

• light (for photosynthesis); Animals compete for food
• space; Animals compete for mates
• water; Animals compete for territory
• mineral ions from the soil; Animals compete for

Interdependence

No species lives on its own. Species in a community depend on each other for food, shelter (birds nest in trees), pollination (bees carry pollen) and seed dispersal (birds eat berries). A food web shows many linked food chains; the arrows point from the food to the organism that eats it.

Worked example: a change in the food web

A disease kills most of the rabbits. Suggest what happens to the wood mice and the tawny owls.
Foxes have fewer rabbits, so they eat more wood mice. The wood mouse population falls.
Tawny owls eat wood mice, so they now have less food. The owl population is likely to fall too.

Answer: wood mice fall (eaten more by foxes), so owls also fall

Exam tip: In “what happens if…” questions, follow every arrow into and out of the organism that changes, and give a reason for each step: “…because…, so…”.

Predator–prey cycles

The numbers of predators and their prey rise and fall in cycles. When prey are plentiful, more predators survive and breed, so the predator population rises – after a delay. More predators eat more prey, so the prey population falls. With less food, the predator population then falls, and the prey can recover.

Stable communities

In a stable community all the species and environmental factors are in balance, so population sizes stay roughly constant (they cycle within a narrow range). Tropical rainforests, ancient woodlands and coral reefs are examples. Removing even one species can upset the balance of the whole community.

Worked example: is the population stable?

A wildlife group counts the hedgehogs in a park each autumn: 34, 37, 35, 33, 36.
Mean = (34 + 37 + 35 + 33 + 36) ÷ 5 = 175 ÷ 5 = 35. Range = 37 − 33 = 4.
The numbers vary only a little around the mean, so the population looks stable.

Answer: mean 35 hedgehogs; small range of 4

Common mistakes

• Mixing up population (one species) and community (all the species).
• Saying an ecosystem is “just the plants and animals”. It includes the non-living parts too.
• Mixing up interspecific (different species) and intraspecific (same species) competition.
• Saying predators peak at the same time as prey. Predator numbers rise after prey numbers, because breeding takes time.

Revision questions

Try these, then check your answers below each one.

Question 1. What is a community? (Choose from: Two or more populations of different species living in the same area at the same time, All the organisms of one species in an area, The non-living parts of a habitat, The place where an organism lives.)

Answer: Two or more populations of different species living in the same area at the same time

Working: A community is made of the populations of different species that live and interact in the same place at the same time.

Question 2. Use the meadow food web. A disease kills most of the rabbits. What is most likely to happen to the number of voles, and why? (Choose from: It falls, because the foxes eat more voles, It rises, because voles no longer compete with rabbits for clover only, It stays the same, because voles do not eat grass, It falls, because owls stop eating voles.)

Answer: It falls, because the foxes eat more voles

Working: Foxes eat rabbits and voles. With fewer rabbits, foxes eat more voles, so the vole population is likely to fall. (Later, less grazing by rabbits might leave more food for voles.)

Question 3. The graph shows the populations of lemmings and of stoats, which eat them. Why does each peak in the stoat population come after a peak in the lemming population? (Choose from: With plenty of lemmings to eat, more stoats survive and breed, but this takes time, Stoats eat lemmings only when stoat numbers are low, Lemmings eat stoats when lemming numbers are high, The two populations are not linked.)

Answer: With plenty of lemmings to eat, more stoats survive and breed, but this takes time

Working: When prey increase, predators have more food, so more survive and breed – after a delay. Then more predators eat more prey, so prey fall, and later predators fall too. This is predator–prey cycling.

Related lessons

• Previous lesson: B14.6 New Systems of Classification
• Next lesson: B15.2 Organisms in Their Environment
• In this chapter: B15.3 Distribution and Abundance
• In this chapter: B15.4 Competition in Animals
• In this chapter: B15.5 Competition in Plants

Continue in the full course

This lesson is section B15.1 (AGCSC1501) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.