Explain the factors that lead to each phase in the sigmoid curve of population growth, making reference, where appropriate, to the role of limiting factors

The sigmoid curve of population growth illustrates how populations grow in environments with limited resources, characterized by four distinct phases: lag phase, exponential (log) phase, stationary phase, and death phase. Each of these phases is influenced by various factors, particularly limiting factors that can either enhance or restrict population growth.

1. Lag Phase

• Description: This initial phase is marked by slow population growth as individuals acclimate to their new environment.
• Factors Leading to This Phase:
  • Adaptation: Individuals are adjusting to environmental conditions, which limits reproduction and growth.
  • Low Population Density: With few individuals, there is less competition for resources, but the overall reproductive output remains low due to the lack of established populations.
• Limiting Factors:
  • Biotic Factors: Limited availability of mates and low reproductive rates contribute to the slow growth.
  • Abiotic Factors: Environmental conditions such as temperature and nutrient availability may not yet be optimal for rapid growth.

2. Exponential (Log) Phase

• Description: In this phase, the population experiences rapid growth as conditions become favorable.
• Factors Leading to This Phase:
  • Resource Abundance: Ample food supply and suitable habitat allow for high reproductive rates.
  • Low Competition: With resources plentiful, individuals face little competition, leading to increased survival and reproduction rates.
• Limiting Factors:
  • Initially, limiting factors are minimal. However, as the population grows, density-dependent factors (like competition for food) may start to emerge but have not yet significantly impacted growth.

3. Stationary Phase

• Description: Growth stabilizes as the population reaches its carrying capacity, where birth rates equal death rates.
• Factors Leading to This Phase:
  • Resource Limitation: As the population approaches the carrying capacity, resources such as food, space, and water become limited.
  • Increased Competition: Higher population density leads to increased competition for available resources, which can reduce individual growth rates and reproductive success.
• Limiting Factors:
  • Density-Dependent Factors: Competition for limited resources increases mortality rates and decreases birth rates. Predation may also increase as more individuals attract predators.
  • Disease Spread: Higher density can facilitate the spread of diseases, further impacting population stability.

4. Death Phase

• Description: In some contexts, this phase may occur when the population begins to decline after exceeding the carrying capacity.
• Factors Leading to This Phase:
  • Resource Depletion: Overuse of available resources can lead to starvation and increased mortality.
  • Increased Mortality Rates: As conditions worsen (e.g., food scarcity), death rates may exceed birth rates.
• Limiting Factors:
  • Density-Dependent Factors: Increased competition and predation can lead to higher mortality rates.
  • Density-Independent Factors: Environmental changes such as natural disasters or climate shifts can also impact population size regardless of density.