Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 53.5 Density-Dependent Factors Regulate Population Growth
Populations cannot grow forever. As density increases, environmental limits emerge. Density-dependent factors—such as competition, disease, and predation—act as negative feedback to stabilize populations near their carrying capacity, influencing long-term population dynamics.
1. Population Regulation by Density
- Regulated populations decrease when large and increase when small due to density-dependent factors
- Density-independent factors (e.g., storms) affect populations regardless of size
- Example: dune fescue grass death is density-independent (sand burial), but birth rate drops with density (nutrient limits)
2. Mechanisms of Density-Dependent Regulation
- Negative feedback slows growth as density rises, through:
- Competition for resources (e.g., nutrient-limited crops)
- Territoriality (e.g., cheetahs marking territory)
- Disease spreads more in crowded populations
- Predation increases when prey density overwhelms hiding spots
- Toxic waste buildup (e.g., ethanol limits yeast growth)
- Intrinsic physiological factors (e.g., hormonal changes reduce reproduction in crowded mice)
3. Stability and Fluctuation
- Some populations are stable; others show dramatic fluctuations
- Example: moose on Isle Royale fluctuate with winters, food limits, and wolf predation
- Changes in one population affect others (e.g., seabirds and fish)
4. Population Cycles: The Case of Hares and Lynx
- Snowshoe hare and lynx follow 10-year population cycles
- Hypotheses:
- Food limitation: extra food raised hare numbers but didn’t eliminate cycles
- Predation: blocking predators prevented crashes → predation is the main driver
- Hares face many predators, and overexploitation drives the cycles
5. Immigration, Emigration, and Metapopulations
- Immigration/emigration shape population size and genetic flow
- Metapopulation: network of populations across habitat patches
- Example: Glanville fritillary butterflies in Finland occupy 500 of 4,000 meadows—colonizing and disappearing continuously
- Mobility is genetically influenced (e.g., Pgi gene improves butterfly dispersal and success)
In a Nutshell
Density-dependent factors regulate population growth through negative feedback, stabilizing populations around carrying capacity. Competition, disease, predation, and territoriality all play roles. Understanding these dynamics—including immigration and metapopulation structure—is essential for managing both endangered and overabundant species.