Communities, interdependence and adaptations

• A population is all the organisms of one species in an area.
• A community is all the populations of different species in that area.
• A habitat is where an organism lives.
• An ecosystem is a community interacting with the non-living parts of its environment.

Species in a community depend on each other for food, shelter, pollination and seed dispersal: this is interdependence. Take one species away and the others feel it. In a stable community the species and the environment are in balance, so population sizes stay about the same.

1. Feeding: water fleas eat algae; sticklebacks eat water fleas
2. A disease kills most of the sticklebacks
3. Fewer water fleas are eaten, so the water fleas increase
4. More water fleas eat more algae, so the algae decrease

Say what changes and why: "less food" or "fewer eaten". The mark is for the reason.

Edexcel and OCR also ask about two close relationships between species.

In parasitism, a parasite lives in or on a host and takes food from it, so the host is harmed. Fleas and head lice feed on blood, a tapeworm absorbs food the host has already digested, and mistletoe takes water and mineral ions from the tree it grows on. A parasite depends on its host staying alive, so it rarely kills it quickly.

In mutualism, both species benefit:

• Oxpeckers eat ticks from the skin of buffalo: the bird gets food and the buffalo loses its parasites.
• Cleaner fish eat parasites from bigger fish.
• Nitrogen-fixing bacteria live in nodules on the roots of clover, peas and beans. The bacteria give the plant nitrogen compounds; the plant gives the bacteria sugars.

Ask who gains and who loses. One gains and one is harmed: parasitism. Both gain: mutualism.

Abiotic factors are non-living: light intensity, temperature, moisture, soil pH and mineral content, wind, carbon dioxide for plants, oxygen for animals in water.

Biotic factors come from living things: the food available, new predators, new pathogens, and competition. Plants compete for light, space, water and mineral ions; animals for food, mates and territory.

A graph of how much of a species grows against an abiotic factor usually rises to a peak and falls: the peak is where conditions suit it best.

Adaptations are features that help an organism survive where it lives.

• Structural: body features, such as small ears and thick fur on an Arctic fox.
• Behavioural: what it does, such as migrating, hibernating or feeding at night.
• Functional: processes inside the body, such as making very concentrated urine.

Extremophiles live in extreme conditions: very hot springs, high pressure on the sea floor, very salty lakes.

Link the feature to the problem: small ears give a small surface area, so less heat is lost.