12.2 Population Dynamics, Succession, and Biodiversity
Key Takeaways
- Density-dependent limiting factors such as competition, predation, disease, and waste accumulation intensify as crowding increases, while density-independent factors such as drought, freeze, and fire act regardless of population density.
- Predator and prey populations oscillate with the predator peak lagging behind the prey peak, because predators can only increase after their food supply has already increased.
- A niche is a species' entire role — habitat, food, activity timing, and interactions — and resource partitioning lets similar species coexist by using the same resource in different ways or at different times.
- Primary succession begins on bare rock with no soil and is initiated by lichens and mosses, while secondary succession begins where soil and a seed bank survive a disturbance and therefore proceeds much faster.
- A keystone species has an effect on community structure disproportionate to its abundance, so removing it can collapse an ecosystem even when the species was numerically rare.
What Regulates a Population
The growth curves themselves — exponential J-shaped and logistic S-shaped, bounded by carrying capacity — are developed in the Domain I resources section. This section takes the ecological next step the framework asks for: identifying which factors do the regulating, and what happens to a community over time.
Limiting Factors
| Type | How it acts | Examples | Typical effect |
|---|---|---|---|
| Density-dependent | Intensifies as the population becomes crowded | Competition for food, water, nesting sites; predation; disease and parasitism; waste accumulation | Regulates the population near carrying capacity; produces the leveling of a logistic curve |
| Density-independent | Acts with the same intensity regardless of crowding | Drought, hard freeze, flood, wildfire, hurricane, habitat destruction | Causes abrupt crashes unrelated to population size |
The distinction is a favorite item because students assume any bad event is density-dependent. A February freeze kills the same fraction of a sparse population as a dense one — that is what "density-independent" means. Disease, by contrast, spreads faster when hosts are crowded, so it is density-dependent.
Species Interactions
| Interaction | Effect on species A | Effect on species B | Example |
|---|---|---|---|
| Competition | − | − | Two grass species drawing on the same soil water |
| Predation | + | − | Coyote and jackrabbit |
| Parasitism | + | − | Tick and white-tailed deer |
| Mutualism | + | + | Bees and flowering plants; mycorrhizal fungi and tree roots |
| Commensalism | + | 0 | Cattle egret feeding on insects stirred up by grazing cattle |
Predator-prey cycles are the classic quantitative pattern. Prey numbers rise; abundant food lets predators reproduce, so predator numbers rise afterward; heavy predation then drives prey numbers down; predators decline from starvation; prey recover. The predator peak always lags the prey peak, and on a graph the two curves trace offset waves. Reading that lag correctly — rather than assuming the curves rise together — is the tested skill.
Niches and Resource Partitioning
A niche is the total role a species occupies: its habitat, food sources, activity timing, tolerance ranges, and its interactions with other species. The fundamental niche is everything a species could tolerate without competitors; the realized niche is the smaller portion it actually occupies once competition and predation are present.
The competitive exclusion principle holds that two species cannot occupy exactly the same niche indefinitely in the same place — one outcompetes the other. Coexistence therefore requires resource partitioning: dividing a shared resource in space, in time, or by type.
- Spatial partitioning — warblers feeding at different heights in the same conifer.
- Temporal partitioning — hawks hunting rodents by day, owls by night.
- Dietary partitioning — seed-eating birds with different beak depths taking different seed sizes.
Adaptive characteristics create these divisions. A beak shape, a tolerance of saline soil, a nocturnal activity pattern — each narrows a species' niche and reduces direct competition, which is precisely the framework's descriptive statement about "adaptive characteristics that result in unique niches."
Ecological Succession
Succession is directional, predictable community change over time.
| Feature | Primary succession | Secondary succession |
|---|---|---|
| Starting point | Bare rock or new surface with no soil | Existing soil after a disturbance removed vegetation |
| Trigger examples | New volcanic island, retreating glacier, fresh lava flow | Wildfire, flood, abandoned farm field, clear-cut |
| Pioneer species | Lichens and mosses that chemically and physically break rock into soil | Fast-growing grasses and weeds from the surviving seed bank |
| Soil formation | Must be built from scratch | Already present, with seeds and roots intact |
| Timescale | Centuries to millennia | Decades |
A climax community is the relatively stable end stage for a region — in East Texas a mixed pine-hardwood forest, on the Blackland Prairie a tallgrass grassland. Modern ecology treats "climax" as a useful simplification rather than a fixed endpoint, since periodic disturbance keeps many real communities in earlier stages indefinitely. Fire-maintained prairie is the standard Texas example: suppress fire and woody species invade, so the prairie is not a stage on the way to forest but a community sustained by recurring disturbance.
Populations also modify their own ecosystems. Beavers dam streams and convert running water to ponds and then to meadows. Lichens generate the soil that makes their own replacement inevitable. Humans reshape water tables, nutrient cycles, and species distributions at continental scale.
Biodiversity and Sustainability
Biodiversity operates at three levels:
- Genetic diversity — variation within a species; it supplies the raw material for adaptation and buffers against disease.
- Species diversity — the number and relative abundance of species in a community.
- Ecosystem diversity — the variety of habitat types across a region.
Higher biodiversity generally increases resistance (the ability to withstand disturbance without changing) and resilience (the speed of recovery afterward). A monoculture pine plantation loses most of its canopy to a single beetle outbreak; a mixed-species forest loses only the susceptible species, because the pest cannot spread easily between unrelated hosts.
A keystone species exerts influence far out of proportion to its abundance. Sea otters are numerically rare but keep sea urchin populations in check; remove the otters and urchins overgraze the kelp, collapsing the entire kelp-forest community. Prairie dogs perform a comparable role in Texas grasslands, creating burrows and grazing patterns that dozens of other species depend on. Contrast this with a dominant species, which shapes a community simply by being abundant — the distinction is influence per individual, and exam items test it directly.
A graph shows snowshoe hare and lynx populations over 40 years. Both rise and fall in waves, but each lynx peak occurs roughly two years after the corresponding hare peak. What does the lag indicate?
Which scenario best illustrates a density-independent factor limiting a population?
After a forest fire removes the trees but leaves the soil intact, grasses and weeds grow first, then shrubs, then pine trees, and finally oak and hickory dominate decades later. This sequence is best described as: