Interaction Between Organisms I

Five key levels of organisation in ecology, from the smallest:

• ORGANISM: an individual living thing (one squirrel, one oak).
• POPULATION: all the individuals of ONE SPECIES in a habitat. E.g. all the grey squirrels in Central Park.
• COMMUNITY: all the populations of different species sharing the habitat. Squirrels + birds + insects + trees + ...
• ECOSYSTEM: the community PLUS the non-living surroundings (rocks, water, air, temperature).
• BIOME: a large area of land or sea with similar ecosystems (e.g. rainforest, desert, tundra, ocean).

A HABITAT is the place where an organism lives. Different species in the same ecosystem often have slightly different MICROHABITATS — for example, an oak tree's canopy is one microhabitat, its trunk another, and the leaf-litter another.

Every ecosystem has THREE main feeding roles:

Producers make their own food. Plants and algae do this by photosynthesis — using light energy to combine CO₂ and water into glucose. They sit at the bottom of every food chain.

Consumers eat other living things. Several levels:

• Primary consumers (herbivores): eat producers. E.g. rabbit eats grass.
• Secondary consumers: eat primary consumers. E.g. fox eats rabbit.
• Tertiary consumers: eat secondary consumers. E.g. eagle eats fox.

Decomposers (mostly bacteria and fungi) break down dead organisms and waste, recycling nutrients back into the soil. Without them, dead matter would pile up and minerals would be locked away.

Competition happens when organisms need the same limited resources:

• Animals compete for food, water, mates, territory.
• Plants compete for light, water, minerals, space.

Competition can be WITHIN a species (e.g. two stags competing for mates) or BETWEEN species (two plant species competing for sunlight in a forest).