Populations and communities describe abundance, sampling, growth limits, species interactions and trophic controls that shape ecological patterns across habitats and ecosystems.
A population is all interacting individuals of the same species living in a defined area at the same time. In a sexually reproducing species, members have opportunities to interbreed.
Two groups can belong to the same species but form different populations when geography, behaviour or timing prevents regular gene flow between them.
| Record | Best when… |
|---|---|
| density | individuals can be distinguished and counted |
| frequency | presence or absence across quadrats matters |
| percentage cover | individuals overlap or cannot be separated easily |
| abundance score | a defined ordinal scale is appropriate |
After marked animals mix back into the population, the marked fraction in the second sample should approximate the marked fraction in the whole population.
If M = 50, N = 40 and R = 10, then the estimated population size = (50 × 40) ÷ 10 = 200 animals. Fewer marked recaptures imply a larger estimated population.
Carrying capacity, K, is the largest population that the current environment can sustain over time without exhausting the resources and conditions on which that population depends.
K is not permanently fixed. Seasonal change, disturbance, habitat management or evolution can alter the limiting environment and therefore alter carrying capacity.
population density rises → competition, pathogen transmission, waste effects or predation intensify → birth rate falls and/or death rate rises → net growth falls → density moves back toward K
| Factor | Density-dependent? | Regulatory pattern |
|---|---|---|
| competition, disease, predation | yes | effect becomes stronger as density rises |
| drought, frost, fire or storm | usually no | can change population size regardless of density |
‘Negative’ describes opposition to change, not harm. The result is often fluctuation around K rather than a perfectly constant population.
| Ecological level | Includes |
|---|---|
| population | one species in one area |
| community | all interacting populations in that area |
| ecosystem | the community plus abiotic conditions and their interactions |
A change in one population can alter food, habitat, disease, pollination or competition for several others. Community structure is therefore a network outcome, not a list of species.
| Relationship | Species A | Species B | Defining transfer or effect |
|---|---|---|---|
| mutualism | + | + | each gains a resource or service |
| competition | − | − | both lose access or spend resources contesting |
| predation | + | − | predator kills and consumes prey |
| herbivory | + | − | animal consumes plant or algal tissue |
| parasitism | + | − | parasite gains resources while living on or in host |
| pathogenicity | + | − | pathogen reproduces while damaging host |
The signs describe effects relative to no interaction. They do not mean every individual receives the same benefit or cost, and the outcome can change with conditions.
| Observation or manipulation | Inference |
|---|---|
| Semibalanus absent high on shore even when competitors are removed | desiccation restricts its fundamental niche |
| Chthamalus normally occupies upper shore | its observed realized niche is narrow |
| removing Semibalanus lets Chthamalus survive lower down | Semibalanus competition excludes Chthamalus from part of its fundamental niche |
The fundamental niche is the range tolerated without biotic exclusion. The realized niche is the range actually occupied after competition and other biotic interactions.
more hare → more food raises lynx survival and reproduction after a delay → more lynx increases hare mortality → fewer hare reduce food for lynx after a delay → lynx decline → reduced predation allows hare recovery
The predator curve lags behind the prey curve because births, maturation and deaths take time. Predation is density-dependent, but weather, vegetation and other predators can also affect the real cycle.
| Control | Initial change | Example cascade |
|---|---|---|
| top-down | predator abundance or predation | fewer predators → more herbivores → fewer producers |
| bottom-up | nutrients, habitat or primary production | fewer nutrients → fewer producers → fewer herbivores → fewer predators |
Identify where the disturbance begins, then follow its direct resource or predation effects one trophic level at a time. Many communities contain both controls, but one may dominate under particular conditions.
Microbes also compete chemically. Penicillium releases penicillin, which inhibits susceptible bacteria nearby and reduces their competition for space and nutrients.
On an agar plate, a clear zone of inhibition around the fungal colony shows that a diffusible substance prevents bacterial growth. Zone size also depends on diffusion and dose, so it is not a direct measure of ecological success by itself.
Humans use antibiotics as medicines, but their original biological role is an interaction among organisms in a community.
