Biology 9.1 - 9.4 - Communities and interdependence

Levels of ecological organisation

Ecologists can study the same place at different scales. Each scale includes the level before it, but adds more organisms or more of the environment.

The sequence is:

individual organism → population → community → ecosystem

An ecosystem is not just "everything alive" in a place; that describes the community. It is also not just the place itself. The defining idea is the living community interacting with the abiotic environment.

[DIAGRAM: asset_name: Biology 9.1-9.4 - Ecosystems, communities and factors - diagram 01; asset_slug: biology_9_1_9_4_ecosystems_communities_and_factors__diagram_01; recommended_method: image_gen; description: A clean monochrome four-stage pond infographic on white, read left to right with visible right-pointing arrows: Individual organism shows one pond snail; Population shows several pond snails of the same species; Community shows pond snails plus pondweed, stickleback fish and a dragonfly; Ecosystem shows that same living community together with directly labelled sunlight, pond water and temperature. Exact stage labels must be Individual organism, Population, Community and Ecosystem. Use #6A6B6E linework and labels; no colour coding, food-chain arrows, trophic levels, sampling equipment, biomass, cycles or decorative frame.]

Abiotic factors and communities

An abiotic factor is a non-living physical or chemical part of the environment. A change in an abiotic factor can alter survival, growth or reproduction. That changes the abundance of one or more populations and therefore changes the composition of the community.

These are not four automatic rules. For example, a temperature rise does not make every population fall: each species has a different suitable range. Likewise, pollution may change the relative abundance of species rather than remove all life.

Worked example: interpreting abiotic-factor data

The following original dataset shows pondweed abundance at four light intensities.

1. Identify the quantities. Light intensity is the abiotic factor. Mean pondweed shoots per $\mathrm{m}^{2}$ is the measured population abundance.
2. Describe the overall pattern with data. As light intensity rises from 20 to 80 arbitrary units, mean abundance rises from 6 to 19 shoots per $\mathrm{m}^{2}$.
3. Notice the shape, not just the direction. From 40 to 60 units, the increase is $18 - 11 = 7$ shoots per $\mathrm{m}^{2}$. From 60 to 80 units, it is only $19 - 18 = 1$ shoot per $\mathrm{m}^{2}$. The response is levelling off at the highest values shown.
4. Explain with biology. More light can support a higher rate of photosynthesis, so more new plant material can be made and more shoots can grow. At high light, a different factor may be limiting further growth.
5. Limit the conclusion. The data show an association between light and abundance. Unless other factors were controlled, they do not prove that light alone caused the difference.