bwj31203_chapter3

Chapter 3: Population Dynamics

Lecture Contents

• Distribution

• Abundance

• Structure

• Movement

• Birth/Death Rates

• Intraspecific Population Regulation

• Population Growth

• Logistic Growth

Introduction to Population Dynamics

• Population: A group of organisms of the same species living in a defined area at a given time. (Wootton 1990)

• Population Dynamics: Refers to changes in the number of individuals in a population or the vital rates (births and deaths) of a population over time.

• Also known as population biology or population ecology.

Importance of Studying Population Dynamics

1. Ecosystem Response: Understanding population dynamics is crucial for assessing responses to ecosystem changes (manipulation/perturbation).

2. Endangered Species: Important for population viability analysis (PVA) to assess the risk of extinction.

3. Fisheries Management: Assessing sustainable yield is critical for managing fish population and ensuring long-term viability.

4. Ecosystem Dynamics Understanding: Helps to understand broader ecological processes and interactions.

Populations

• Key Characteristics of Populations:

  • Age Structure: Distribution of individuals among different ages.

  • Sex Ratio: Ratio of males to females in a population.

  • Population Size: Count of individuals in a population.

  • Movement: Includes immigration (in) and emigration (out).

Age Structure

• Can be categorized into three sections:

  • Pre-reproductive: Not yet breeding.

  • Reproductive: Capable of breeding.

  • Post-reproductive: Not capable of breeding.

• Population Growth Shape:

  • Pyramid shape: Indicates increasing population with lots of reproductive and pre-reproductive individuals.

  • Inverted pyramid: Indicates a declining population with more post-reproductive individuals.

  • Stable: Equal representation in all age groups.

Sex Ratio

• Definition: Proportion of males to females in a population.

• Monogamous Species: Require a 50:50 ratio to maintain growth rates.

• Polygamous Species:

  • 1 male:4 females yields higher offspring than a 1:1 ratio.

  • 4 males:1 female results in drastically fewer young produced.

• Distorted sex ratios due to fishing can affect breeding success.

Population Size

• Population undergoes three distinct life cycle phases: Growth, Stability, and Decline.

Population Density

• Definition: Number of members per unit area.

• Growth rate is related to available resources exceeding individuals available to exploit them.

• Populations can grow rapidly or slowly based on resource availability.

Limiting Factors

• Factors that limit population growth leading to stabilization or decline:

  • Density-Dependent: Effects increase with population density (e.g., competition, predation).

  • Density-Independent: Unaffected by density (e.g., natural disasters).

Population Changes

• Growth calculations include:

  • Births (B)

  • Deaths (D)

  • Immigration (I)

  • Emigration (E)

  • Formula: Nt+1 = Nt + B - D + I - E

Fish Growth

• Indeterminate Growth: Fish continue to grow throughout their life.

• Influenced by:

  • Food availability

  • Weather

  • Competition

  • Physical water properties (temperature, pH).

Von Bertalanffy Growth Model

• Used to estimate growth:

  • L∞: mean length of very old fish

  • K: growth rate coefficient.

  • Example calculation for fish growth provides length estimates at given ages.

Fish Mortality

• Major types include:

  • Natural mortality: Caused by predation, disease, weather, competition.

  • Fishery mortality: Caused by fishing and harvesting.

• Recruitment: Number of young-of-year (YOY) fish entering the population.

Estimating Fish Mortality Rates

• Methods include linear regression and calculating survival probabilities.

• Survivorship can be determined based on age structure and sampled fish data.

Measuring Abundance

• Catch Per Unit Effort (CPUE): Index of abundance allowing comparison of populations based on catch records relative to effort made.

• Density estimates used for indices and calculation of population sizes.

Population Productivity

• High productivity is characterized by:

  • Fast growth

  • Early maturation

  • High natural mortality (short-lived species).

• Low productivity populations tend to be slow growing with long replenishment times.

Logistic Growth Model

• Exponential Growth: Population growth rate remains constant regardless of size (J-shaped curve).

• Logistic Growth: Growth rate decreases as population approaches environmental carrying capacity (K) leading to an S-shaped curve.

Summary

• Understanding population dynamics is crucial for conservation, management, and understanding ecological roles. This includes studying the population's structure, growth, and changes over time due to various biological and environmental factors.