- Populations are regulated by density-dependent factors, which become stronger as population density increases.
- These factors act as negative feedback mechanisms, returning populations to equilibrium around their carrying capacity (K).
- In contrast, density-independent factors (e.g., droughts, floods, volcanic eruptions) affect populations regardless of their size, but do not regulate them long-term.
Density-Dependent Factors
Definition
Density-dependent factors
Density-dependent factors are biotic elements that intensify as population density increases.
They create negative feedback loops, stabilizing populations around the carrying capacity.
Key Density-Dependent Factors
1. Competition for Limited Resources
- As population density increases, individuals compete for limited resources (e.g., food, water, nesting sites).
- This reduces individual growth, reproduction, and survival rates.
2. Predation
- Higher prey density attracts more predators or increases the predator success rate.
- This acts as a self-regulating mechanism that keeps the prey population in balance.
3. Disease and Pathogen Transmission
- Close contact in dense populations promotes the rapid spread of infectious diseases.
- This increases mortality and lowers reproductive success.
- Remember that density-dependent factors act as negative feedback mechanisms.
- As the population increases, limiting factors intensify and bring the population back toward equilibrium.
Negative Feedback Mechanisms
Definition
Negative feedback loops
Negative feedback loops are processes where the output of a system acts to reduce or reverse changes, helping the system maintain stability.
- Negative feedback occurs when an increase in population size triggers mechanisms that reduce growth rates and bring the population size back towards equilibrium.
- These mechanisms ensure that populations do not exceed the carrying capacity of their environment.
- As populations exceed carrying capacity:
- Resource scarcity reduces reproduction rates.
- Predation and disease increase mortality.
- These processes act as stabilizing forces to return the population toward equilibrium (K).
- Increased density → competition for resources → decreased birth rate and increased death rate → population decreases.
- Decreased density → more resources per individual → higher birth rate and lower mortality → population increases.
- In a coral reef ecosystem, increased small fish density leads to more predator fish activity.
- As predation reduces prey numbers, the predator population also decreases, restoring balance, a classic negative feedback loop.
Density-Independent Factors
- These are abiotic factors that affect populations regardless of density.
- These include:
- Natural disasters (fires, floods, droughts).
- Temperature extremes or long-term climate change.
- Volcanic eruptions, earthquakes, hurricanes.
- These can cause sudden population crashes, but do not regulate around K.
- Volcanic eruption: A volcanic eruption may indiscriminately wipe out both small and large populations.
- Weather change: A cold snap can cause widespread mortality among insect populations, regardless of their density.
Rabbit Population in Australia
- European rabbits were introduced to Australia and multiplied rapidly due to a lack of natural predators.
- The introduction of the myxomatosis virus (1950) drastically reduced their numbers, an example of a density-independent event.
- Later, calicivirus further regulated dense populations, demonstrating density-dependent disease transmission.
Population Growth
- Population growth follows two primary models:
- Exponential growth (J-curve): Growth in the absence of limiting factors.
- Logistic growth (S-curve): Growth limited by carrying capacity.
- These models describe how populations expand and stabilize over time depending on resource availability.
Exponential Growth (J-Curve)
- Exponential growth occurs when a population grows at a constant rate without any limitations.
- The population increases rapidly over time, resulting in a J-shaped curve.
- Growth is unlimited, with no environmental constraints.
- The rate of change (growth rate) remains constant as long as resources are available.
When placed in a nutrient-rich environment in a petri dish, bacteria reproduce exponentially until the nutrients are depleted.
Phases of Exponential Growth
- Lag Phase: Slow initial growth due to few reproducing individuals.
- Exponential Phase: Rapid population increase; birth rates exceed death rates.
- Crash Phase: Population overshoots K, leading to resource depletion and sudden decline (a “boom and bust” pattern).
Logistic Growth (S-Curve)
- Logistic growth occurs when a population’s growth rate slows down as it approaches the carrying capacity of the environment.
- The growth curve starts exponentially but levels off as resources become scarce, leading to an S-shaped curve.
- Population growth is initially fast (rapid increase), then gradually slows, and eventually stabilizes at the carrying capacity.
- Density-dependent factors (e.g., food availability, competition, predation) slow the growth rate as the population increases.
Deer populations in forests: Initially, deer population increases rapidly, but as resources (food, space) are consumed, the population stabilizes near the carrying capacity.
Phases of Logistic Growth
- Lag Phase: Slow growth due to small population size.
- Exponential Phase: Rapid increase as conditions are favorable.
- Transitional Phase: Growth slows due to competition and limiting factors.
- Plateau Phase: Population stabilizes at K, fluctuating slightly with environmental changes.
