lecture recording on 12 February 2025 at 12.44.28 PM

Understanding Populations in Ecology

Definition of Population

• A population is defined as a group of individuals of the same species that occupy a specific area at a specific time.

• Example: The population of white-tailed deer in Iowa today versus in the year 1900.

• The population size can vary geographically; white-tailed deer in Iowa may differ from those in Montana due to distance and habitat differences.

Characteristics of Populations

1. Population Size

   • Refers to the total number of individuals in a population.

   • Example: Approximately 300,000 deer in Iowa today.

2. Population Density

   • Density is expressed as the number of individuals per unit area (e.g., deer per square mile).

3. Distribution Patterns

   • Uniform Distribution: Individuals are evenly spaced.

   • Random Distribution: Individuals are spaced unpredictably.

   • Clumped Distribution: Individuals are grouped together.

   • Clumped distributions are most common in nature, especially among social animals (e.g., elephants).

4. Natality (Birth Rate)

   • Number of young born within a specific time period, reported as births per thousand individuals.

5. Mortality (Death Rate)

   • Number of individuals that die in a specific timeframe, often reported similarly to natality.

6. Sex Ratio

   • Ratio of males to females in a population; often around 50:50 at birth but may change with age due to differing life spans.

   • Many species experience higher female survival rates in older age classes.

7. Age Structure

   • The distribution of individuals within different age categories, often illustrated in population pyramids, indicating growth trends.

   • Shapes can indicate the nature of population growth: expanding (pyramid), stable (rectangle), or declining (inverted pyramid).

Factors Influencing Population Growth

• Biotic Potential: The maximum reproductive capacity of a species under optimal conditions.

• Limits on Growth: Populations cannot grow exponentially forever due to environmental resistance and limiting factors.

Population Growth Models

1. Exponential Growth

   • Characterized by rapid increase (J-shaped curve), where populations grow without limits.

   • Often observed in conditions of abundant resources.

   • Example: Bacteria multiplication.

2. Logistic Growth

   • Takes into account environmental limits (S-shaped curve). Growth slows as population approaches the carrying capacity (K) of the environment.

   • Carrying capacity is the maximum population size that the environment can sustain.

Density-Dependent and Density-Independent Factors

• Density-Dependent Factors: Affect population size based on density (competition, predation, disease, waste accumulation).

• Density-Independent Factors: Affect populations regardless of density (weather events, natural disasters).

• Examples of density-independent events include catastrophic events like fires and floods, which can eliminate significant numbers of individuals regardless of population size.

Overshooting Carrying Capacity

• Populations can exceed carrying capacity temporarily leading to resource depletion and potential population crashes.

• Examples include:

  • Reindeer on St. Paul Island: Rapid growth led to resource overexploitation and near-extinction.

  • Kaibab Plateau Deer: Predation removal led to overpopulation and subsequent starvation due to resource depletion.

Top-Down vs Bottom-Up Control of Populations

• Top-Down Control: Predators regulate prey populations.

• Bottom-Up Control: Abundance of resources regulates population sizes.

• Both mechanisms interact, influencing population dynamics in complex ways.

Reviewing Key Concepts for Exam

• Understand population distribution types: uniform, random, clumped.

• Recognize differences between exponential and logistic growth curves.

• Define carrying capacity and differentiate between density-dependent and density-independent factors.