Communities, interdependence and adaptations
The words of ecology
- 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.
- Feeding: water fleas eat algae; sticklebacks eat water fleas
- A disease kills most of the sticklebacks
- Fewer water fleas are eaten, so the water fleas increase
- 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.
Parasitism and mutualism (Edexcel and OCR)
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 and biotic factors
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
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.
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