Biology 20 · Ecosystems and change
Communities change
Nothing in an ecosystem sits still. A community rebuilds itself after a disturbance, populations rise and crash against their limits, and over much longer stretches the species themselves change. Three timescales, one idea.
- 1. The words, first
- 2. Succession
- 3. Population dynamics
- 4. Variation and natural selection
- 5. What costs marks
The words, first
The idea: Population, community and ecosystem are nested, and the exam expects the right one.
| Word | What it means |
|---|---|
| Population | All the individuals of one species in an area. |
| Community | All the populations of all species in an area. |
| Ecosystem | A community plus the abiotic environment it interacts with. |
| Biotic / abiotic | Living and non-living components. Soil pH is abiotic; a competitor is biotic. |
| Primary succession | Community development on ground with no soil — new lava, bare rock, land exposed by a glacier. |
| Secondary succession | Redevelopment where a community was destroyed but the soil survived, such as after a fire. |
| Pioneer species | The first colonizers. Lichens on bare rock are the standard example. |
| Climax community | The relatively stable stage that persists until a major disturbance. |
| Carrying capacity (K) | The largest population an environment can sustain indefinitely. |
| Exponential growth | Unlimited growth — a J-shaped curve. |
| Logistic growth | Growth that levels off at carrying capacity — an S-shaped curve. |
| Density-dependent factor | A limit whose effect grows with crowding: disease, competition, predation. |
| Density-independent factor | A limit that hits regardless of crowding: fire, drought, frost. |
| Natural selection | Differential survival and reproduction of heritable variants. |
Succession
The idea: Each stage changes the conditions in a way that suits the next stage better than itself. That is the engine of the whole process.
Primary succession starts with no soil. Lichens colonize bare rock, secrete acids that break it down, and add their own dead tissue; mosses follow; then grasses, shrubs and trees as the soil deepens. It takes centuries.
Secondary succession starts with soil already present, so it is much faster — an abandoned field can be shrubland within a decade.
What changes along the way. Soil depth increases, species diversity generally rises then levels off, and the community becomes more stable. Pioneer species are typically fast-growing and short-lived; climax species are slower and longer-lived.
Climax does not mean frozen. Individuals die and are replaced constantly. What persists is the composition, and only until the next major disturbance.
Population dynamics
The idea: A population grows exponentially only while nothing is limiting. Everything interesting happens once something is.
growth rate = (change in population) ÷ (starting population)
Worked example. 1000 grows to 1200 in a year: 200/1000 = 0.20, so 20%. Dividing by the final number is a different quantity and not what is asked.
Exponential (J-curve) happens when resources are effectively unlimited — a newly introduced species, a bacterial culture in fresh medium. It cannot continue.
Logistic (S-curve) is what real populations do: growth slows as the population nears K and levels off. A population can overshoot K briefly, and then crashes.
Density-dependent versus independent. Disease spreads faster in a crowded population, so its effect depends on density. A frost kills much the same proportion whether the population is dense or sparse. Sorting a given factor into the right box is a common question.
Predator–prey cycles. Predators need time to convert abundant prey into offspring, so their peak lags behind the prey's. That lag is why the two oscillate instead of settling.
Variation and natural selection
The idea: Three conditions, and selection follows automatically: there must be variation, it must be heritable, and it must affect survival or reproduction.
What selection does not do. It does not create variation to order. Individuals do not acquire traits because they need them — that is Lamarck's idea, and it is wrong. The variation has to already be there, by chance.
Worked example: insecticide resistance. A few insects were already resistant by chance. Spraying kills the rest, so the resistant ones do all the reproducing, and the next generation is largely resistant. The spray did not cause resistance; it selected for it. This is exactly why partial treatment breeds resistant pests and why unfinished antibiotic courses are a problem.
Evidence. Homologous structures — the same bones in a whale's flipper, a bat's wing and a human arm, arranged for different jobs — point to common ancestry. Analogous structures, like a bird's wing and an insect's, do the same job with different underlying structure and point to convergent evolution instead.
Speciation usually needs populations to stop interbreeding — often because they are separated geographically — so that differences can accumulate until they no longer can.
What costs marks
The idea: Four wordings that markers watch for.
- Saying an organism adapted in order to survive. Adaptation is a result, not an intention.
- Confusing primary and secondary succession. The test is whether soil is present at the start.
- Calling every limit density-dependent. Weather is the classic independent one.
- Describing carrying capacity as a hard ceiling. Populations can overshoot it — and then crash.