Population Ecology

Introduction

Population ecology is the study of how and why the number of individuals in a population changes over time and space. It examines the factors that affect population size, density, dispersion, and demographics, as well as the models that describe population growth.

Distribution and Abundance of Species

Factors Limiting Species Distribution

The geographic distribution and abundance of species are influenced by both biotic (living) and abiotic (non-living) factors. Understanding these limitations is essential for predicting species' responses to environmental changes.

• Biotic Factors: Interactions with other living organisms that can limit distribution, such as:

  • Predation

  • Parasitism

  • Competition

  • Disease

• Abiotic Factors: Physical and chemical environmental factors, including:

  • Temperature

  • Water availability

  • Oxygen levels

  • pH

  • Soil nutrients

  • Light

• Dispersal: The movement of individuals or gametes away from their origin or from centers of high population density. Dispersal can limit distribution if suitable habitats are inaccessible or if individuals cannot reach them.

Example: A plant species may be absent from an area not because it cannot survive there, but because its seeds have never dispersed to that location.

Population Characteristics

Definition of a Population

A population is a group of individuals of the same species that live in the same geographic area and interbreed, producing fertile offspring.

• Populations are defined by their boundaries, which may be natural (e.g., a lake) or arbitrary (e.g., a study plot).

Emergent Properties of Populations

Populations have several key characteristics that can be measured and analyzed:

• Density: The number of individuals per unit area or volume.

• Dispersion: The pattern of spacing among individuals within the boundaries of the population.

• Demographics: The study of vital statistics of a population and how they change over time, including birth rates, death rates, and age structure.

Measuring Population Density

• Complete Count: Counting every individual in the population (rarely practical for large populations).

• Sampling Techniques: Estimating population size using methods such as quadrats, transects, or mark-recapture.

Example: Mark-recapture is often used for mobile animals. A sample of individuals is captured, marked, and released. Later, another sample is captured, and the number of marked individuals is used to estimate total population size.

Population Dispersion Patterns

Dispersion describes how individuals are spaced within a population. There are three main types:

• Clumped: Individuals are aggregated in patches, often due to resource availability or social behavior.

• Uniform: Individuals are evenly spaced, often due to territoriality or competition.

• Random: The position of each individual is independent of others, and spacing is unpredictable.

Examples:

• Clumped: Starfish clumping where food is abundant.

• Uniform: Penguins nesting at regular intervals due to aggressive interactions.

• Random: Dandelion seeds dispersed by wind.

Demography and Life Tables

Life Tables

A life table is a summary of the survival and reproductive rates of individuals in specific age groups within a population. Life tables are used to study population dynamics and predict future trends.

• Life tables typically include columns for age, number alive at the start of each age, number dying during the age interval, and number of offspring produced.

Example: A life table for Belding’s ground squirrels might show at what age individuals start reproducing, the age of maximum reproduction, and the age at which most deaths occur.

Additional info: Table values are inferred for illustrative purposes.

Key Questions from Life Tables

• At what age does reproduction start? (e.g., age 1-2 years for Belding’s ground squirrels)

• At what age is the greatest number of offspring produced? (e.g., age 2-3 years)

• At what age do the largest number of individuals die? (e.g., age 3-4 years)

Population Growth Models

Exponential and Logistic Growth

Population growth can be modeled mathematically to predict changes over time. Two primary models are used:

• Exponential Growth Model: Describes idealized, unlimited population growth. The population increases by a constant proportion at each time step.

• N = population size

• r = intrinsic rate of increase

• Logistic Growth Model: Describes population growth that is slowed by limiting factors as the population size approaches carrying capacity (K).

• K = carrying capacity (maximum population size the environment can support)

Example: A population of rabbits introduced to a new island may initially grow exponentially, but as resources become limited, growth slows and stabilizes at the carrying capacity.

Summary Table: Biotic vs. Abiotic Factors

Conclusion

Understanding the factors that limit species distribution, the characteristics of populations, and the models of population growth is fundamental in ecology. These concepts help explain patterns of biodiversity and inform conservation efforts.