Notes on Population Ecology and Distribution of Organisms

Chapter 40: Population Ecology and the Distribution of Organisms

Lecture Presentations by: Kathleen Fitzpatrick, Simon Fraser University
Nicole Tunbridge, Kwantlen Polytechnic University
Copyright © 2025, 2020, 2016 Pearson Education, Inc. All Rights Reserved

Overview of Population Ecology

• Population Ecology
  • Study of how biotic and abiotic factors affect population size and distribution over time.
  • A population is defined as a group of individuals of the same species living in a defined area.

What Determines Species Distribution?

• Climate has the strongest effect on where terrestrial organisms live.
• In aquatic environments:
  • Light and Nutrient Availability: These factors strongly influence where aquatic organisms live.
• Dispersal: Movement of species and their interactions with biotic and abiotic factors affect their population size and location.
• Population size is dynamic and can change over time.

Ecology Defined

• Ecology: The scientific study of interactions between organisms and their environment.
• Interactions can be organized into a hierarchy:
  • Individual organisms
  • Populations
  • Communities
  • Ecosystems
  • Landscapes
  • The biosphere

Types of Ecology

1. Organismal Ecology:
   • Examines how structures, physiology, and behavior of organisms meet environmental challenges.
   • Includes physiological, evolutionary, and behavioral ecology.
2. Population Ecology:
   • Studies factors that affect population size and change over time.
3. Community Ecology:
   • Looks at interactions between species affecting community structure and organization.
4. Ecosystem Ecology:
   • Focuses on energy flow and chemical cycling among organisms and their environment.
5. Landscape Ecology:
   • Investigates exchanges of energy, materials, and organisms across multiple ecosystems.
6. Global Ecology:
   • Analyzes how energy and material exchanges at regional scales influence global patterns and distribution of organisms.

Influences of Earth’s Climate on Terrestrial Biomes

• Climate: Long-term prevailing weather conditions, prominently defined by:
  • Temperature
  • Precipitation
  • Sunlight
  • Wind
• Abiotic Factors: Nonliving aspects of the environment.
• Biotic Factors: Living organisms that make up the environment.

Global Climate Patterns

• Determined by solar energy and the planet's movement.
• Sunlight Intensity: Affects temperature; more intense at the equator than at poles.
  • Curved Earth contributes to variation; intensity decreases with latitude.

Global Air Circulation and Precipitation

• Tropical Rainfall: Warm, moist air rises and cools, leading to precipitation.
• Dry Areas: Descending dry air creates arid conditions, contributing to desert formation.

Seasonal Variation

• Caused by Earth's tilt and orbit around the sun, resulting in seasonal temperature changes.
• Variation is most pronounced at middle to high latitudes.
• Patterns of sunlight affect ecosystems and blooming periods in habitats.

Effects of Geography on Climate

• Bodies of Water: Moderate coastal climates through heat exchange.
  • Example: Labrador Current vs. Gulf Stream effects on North American coasts.
• Mountains: Influence rainfall and sunlight; create rain shadows.
  • South-facing slopes in the Northern Hemisphere receive more sunlight.

Vegetation and Climate Influence

• Forests absorb solar radiation and affect local climate through transpiration, impacting temperatures.

Major Terrestrial Biomes

• Defined by climate and vegetation types.
  • Biomes are influenced by latitude, temperature, and precipitation.
  • Climographs illustrate average temperature and precipitation patterns of various regions, indicating how biomes can change based on seasonal patterns.

Examples of Terrestrial Biomes

1. Tropical Forest:
   • Found near equatorial regions, high rainfall year-round.
   • High biodiversity and vertical structure.
2. Savanna:
   • Occurs in equatorial/subequatorial regions, seasonally variable precipitation.
   • Fires and drought-resistant plants.
3. Desert:
   • Low precipitation, variations between hot and cold deserts.
   • Adaptations to conserve water are common.
4. Chaparral:
   • Midlatitude coastal regions, seasonal rainfall.
   • Dominated by shrubs adapted to drought.
5. Temperate Grassland:
   • Occurs at midlatitudes, moderate precipitation.
   • Dominated by grasses and adapted to fire.
6. Temperate Broadleaf Forest:
   • Midlatitudes with sufficient moisture year-round; significant precipitation throughout.
   • Deciduous trees are dominant.
7. Northern Coniferous Forest (Taiga):
   • Largest biome, snowfall and conifers dominate.
8. Tundra:
   • Low precipitation; significant cold temperatures and permafrost presence.
   • Low diversity of plants.

Aquatic Biomes

Features of Aquatic Biomes

• Temperature, light, and nutrient availability are significant determinants of aquatic habitats.
• Aquatic biomes are categorized as:
  • Freshwater (lakes, streams, wetlands)
  • Marine (oceans, coral reefs, intertidal zones).

Vertical Zonation in Aquatic Environments

• Divided into photic (light-available) and aphotic (light-deprived) zones.
• Benthic Zone: Rocks and sediments at the bottom.
• Pelagic Zone: Open water.

Population Density and Distribution

• Density: Number of individuals per unit area/volume.
• Dispersion: Pattern of spacing individuals within a population (clumped, uniform, random).

Demographics and Life Tables

• Demography: Study of birth, death, and migration rates over time.
• A life table provides a summary of the survival and reproductive rates within a population, typically focusing on females' reproductive output.

Population Models

Exponential Growth Model

• Describes ideal conditions of population growth.
• Exponential Growth Equation: $N<em>t = N</em>o e^{rt}$ where
  $N<em>t$ = population size at time t, $N</em>o$ = initial population size,
  $e$ = base of the natural logarithm,
  $r$ = intrinsic growth rate.

Logistic Growth Model

• Considers environmental limitations and carrying capacity (K).
• Logistic Growth Equation: $dN/dt = rN(K - N)/K$
  • When populations near K, growth slows.

Density-Dependent vs. Density-Independent Factors

• Density-Dependent: Factors that change in intensity as a function of population size (e.g., predation, competition, diseases).
• Density-Independent: Factors affecting population size regardless of density (e.g., natural disasters).

Summary of Key Concepts

• Ecological processes unite life and physical sciences.
• Biotic and abiotic factors influence population dynamics.

Conclusion

• Population ecology is essential for understanding the interactions among organisms and their environments, which is crucial for conservation efforts and managing natural resources effectively.