Population and Community Ecology
Model population growth, carrying capacity, competition, predation, symbiosis, succession, and biodiversity in ecological communities.
A lesson of Biology, in Homeostasis, Body Systems, and Ecology, about 10 minutes.
By the end of this lesson, you will be able to describe population growth models, explain limiting factors, describe species interactions, and explain how communities are structured.
Population Ecology
A population is all members of one species in a specific area at a specific time.
Population characteristics:
- Density: number of individuals per unit area
- Dispersion: spatial pattern (clumped, uniform, random)
- Growth rate: births − deaths + immigration − emigration
Exponential Growth
$$\frac{dN}{dt} = rN$$
When resources are unlimited, populations grow exponentially (J-curve). $r$ = intrinsic growth rate.
Logistic Growth
$$\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)$$
As population approaches carrying capacity $K$, growth slows (S-curve).
Carrying capacity (K): maximum sustainable population size for a given environment.
Limiting factors: food, water, space, shelter, predation, disease — all reduce population growth.
Ecological Interactions
| Interaction | Species 1 | Species 2 |
|---|---|---|
| Predation | + (predator) | − (prey) |
| Competition | − | − |
| Mutualism | + | + |
| Commensalism | + | 0 |
| Parasitism | + | − |
| Amensalism | 0 | − |
Coevolution: two species evolve in response to each other (arms races).
Example: cheetah and gazelle — faster gazelles survive (predation selects for speed in both).
Competition and Niches
Ecological niche: the role a species plays in its community — its habitat, food, timing, and interactions.
Competitive exclusion principle: two species with identical niches cannot coexist — one will outcompete the other.
Resource partitioning: similar species avoid competition by using different parts of the same resource.
Example: MacArthur's warblers — five warbler species coexist in the same spruce trees by foraging at different heights.