Population Ecology: Dynamics, Growth Models, and Demographics

Population Ecology

Introduction

Population ecology is a branch of ecology that studies the structure and dynamics of populations—groups of individuals of the same species living in a specific area. It focuses on factors that influence population size, density, distribution, and growth, as well as the interactions between populations and their environments.

Scope of Ecology

Levels of Ecological Organization

• Organism: An individual living entity.

• Population: All individuals of the same species in a given area.

• Community: All populations of different species in an area.

• Ecosystem: The community plus abiotic factors (e.g., sunlight, water).

• Biosphere: All regions of Earth where life exists.

Habitat is the place where an organism lives.

Key Functions of Ecosystems

• Energy flow and chemical cycling are essential for ecosystem function.

• Ecosystems may have ecotones, transitional zones between adjacent ecosystems.

• Modern ecology aims to model and predict the distribution and abundance of organisms.

Demographics of Populations

Key Terms

• Demography: Statistical study of populations (density, distribution, growth).

• Population Density: Number of individuals per unit area or volume.

• Population Distribution: Pattern of dispersal of individuals (clumped, uniform, random).

• Resources: Abiotic (nonliving) and biotic (living) components that support life (e.g., light, water, food).

• Limiting Factors: Environmental factors that restrict population growth (e.g., temperature, oxygen).

Biotic and Abiotic Factors

• Biotic factors (e.g., competition, predation) and abiotic factors (e.g., climate, nutrients) influence population distribution and size.

• Example: The red kangaroo is limited to arid inland areas of Australia due to its feeding adaptations.

Key Features of Populations

1. Size

• Number of individuals in an area.

• Affected by births, deaths, immigration, and emigration.

2. Density

• Population Density = Number of individuals / Unit of space

• Births and immigration increase density; deaths and emigration decrease it.

3. Dispersion

• Clumped: Individuals aggregate in patches (e.g., resources are unevenly distributed).

• Uniform: Evenly spaced, often due to territoriality or competition.

• Random: Position of each individual is independent of others.

Dispersion patterns provide insight into environmental associations and social interactions.

Factors Affecting Dispersion

• Availability of nutrients and limiting factors

• Abiotic factors (water, sunlight, temperature, pH, salinity)

• Competition for resources

• Behavioral characteristics (e.g., territoriality)

Example: Cedar trees are clumped near the parent plant due to limited seed dispersal.

Population Growth

Growth Rate

• Population growth depends on births, deaths, immigration, and emigration.

• Growth Rate = (Number of births - Number of deaths) / Population size

• Biotic Potential: Maximum growth rate under ideal conditions, influenced by reproductive output.

Example: Mice have higher biotic potential than rhinoceroses due to more frequent reproduction.

Factors Affecting Biotic Potential

• Number of offspring surviving to reproductive age

• Competition within the population

• Age and number of reproductive opportunities

• Presence of disease and predators

Population Dynamics

Natality, Mortality, and Survivorship

• Natality: Birth rate in a population over time.

• Mortality: Death rate in a population over time.

• Cohort: Group of individuals born at the same time.

• Life Table: Age-specific summary of survival patterns.

• Survivorship Curve: Graph showing the number of individuals surviving at each age.

Types of Survivorship Curves

• Type I: High survival in early/middle life, steep decline in old age (e.g., humans).

• Type II: Constant death rate over lifespan (e.g., lizards).

• Type III: High mortality in young, survivors live long (e.g., oysters, mosquitoes).

Age Distribution

• Populations have prereproductive, reproductive, and postreproductive age groups.

• Pyramid-shaped diagrams show age structure; a larger prereproductive group indicates population growth.

Population Growth Models

Exponential Growth

Occurs when resources are unlimited, resulting in a J-shaped curve.

• Formula: $\frac{dN}{dt} = r_{max}N$

• $N$ = population size, $r_{max}$ = maximum per capita growth rate

• Example: If $r_{max} = 1.5$ and $N = 3,000$, then $\frac{dN}{dt} = 4,500$ individuals added per year.

Logistic Growth

Occurs when resources are limited, resulting in an S-shaped curve. Growth slows as population approaches carrying capacity ($K$).

• Formula: $\frac{dN}{dt} = r_{max}N \left(\frac{K-N}{K}\right)$

• $K$ = carrying capacity

• Example: For $K = 2,000$, $r_{max} = 1.0$, $N = 1,200$, $\frac{dN}{dt} = 480$ individuals added per year.

Comparison Table: Exponential vs. Logistic Growth

Feature	Exponential Growth	Logistic Growth
Curve Shape	J-shaped	S-shaped
Resource Limitation	Absent	Present
Carrying Capacity	Not considered	Included
Population Size	Unlimited growth	Levels off at K

Reproductive Strategies

Semelparity vs. Iteroparity

• Semelparity: Single reproductive event in a lifetime (e.g., squid, salmon).

• Iteroparity: Multiple reproductive events throughout life (e.g., most vertebrates, trees).

• Semelparity is an adaptation to unstable environments; iteroparity to stable environments.

Practice Problems

• Population growth calculations using exponential and logistic models.

• Interpretation of growth curves and demographic data.

Summary

• Population ecology examines how populations grow, interact, and are regulated by environmental factors.

• Key concepts include population size, density, dispersion, growth models, and reproductive strategies.

• Mathematical models help predict population trends and inform conservation efforts.