Siyapath
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Lesson: Chapter 15 — The biosphere

15.1. Ecological balance and levels of organisation 3 of 4

Population, community and ecosystem

Individuals of one species living together make a population; populations of different species together make a community; the community with its physical surroundings makes an ecosystem.

Population

A group of living things of the same species living in a certain geographical area within a definite time period is a population.

  • The population of Sri Lanka in 2014 was 21,866,445.
  • The number of elephants living in Sri Lanka in 2011 was 5,879.

Community

A group of populations of different species that live in an area and interact with one another is a community.

  • The animal community of Yala National Park
  • The mangrove plant community of the Negombo lagoon

Ecosystem

All the communities of living things in an area, together with the physical environment they interact with, make an ecosystem.

  • A pond, a rotting log, a forest, a coastline with coral reefs

Why each level adds something

Each step up includes everything below it and adds something new. A population adds numbers: questions like how fast elephants breed only make sense for a group. A community adds relationships between species — who eats whom, who competes for what. An ecosystem adds the non-living world: the water of the pond, the light, the soil. Notice that an ecosystem can be any size, from a rotting log full of fungi and beetles to a whole forest. What makes it an ecosystem is that the living and non-living parts are counted together.

Population means one species

All the animals in Yala together are a community, not a population. The elephants of Yala alone are a population.

One species, many species, plus the physical world

Population: one species in an area at a time. Community: interacting populations. Ecosystem: the community with its physical environment.