Population

Population Studies Overview

Introduction to Population Studies

  • Organism: A single living entity, such as an animal or plant.
  • Population: A group of organisms of the same species living in a particular area at the same time.
  • Community: A collection of different populations that interact with one another.
  • Ecosystem: A biological community interacting with its physical environment.
  • Biosphere: The global ecological system integrating all living beings and their relationships.

Population Characteristics

  • Groups of Species:
    • Density Variations:
    • Species can exhibit varying dispersal patterns:
      • Clumped: Individuals are grouped in patches.
      • Random: Individuals are spaced in an unpredictable manner.
      • Uniform: Individuals are evenly distributed.
    • Variations in Size: Population sizes can differ substantially.
    • Age Composition: The demographic profile of populations can significantly impact their growth and sustainability.
    • Genetic Composition: Genetic diversity within a population can influence its resilience.

Population Examples and R-Selected vs K-Selected Species

  • Fish (Tuna): 6,000 individuals a year.
  • Frog: 200 individuals a year.
  • Hare: 12 individuals a year.
  • Large Cat (Puma): 2 individuals a year.
  • Chimpanzee: 1 every 5 years.

R-Selected vs K-Selected Species

  • These are two strategies for population success:
    • K-Selected Species:
    • Characterized by stable populations, lower reproductive rates, and longer lifespans.
    • Examples include elephants and humans.
    • R-Selected Species:
    • Characterized by high reproductive rates, early maturity, and high juvenile mortality.
    • Examples include small fish and insects.

Survivorship Curves

  • Survivorship Curve: Plots the number of individuals surviving at each age.
  • Types of Survivorship Curves:
    • Type I (Convex Curve):
    • High survivorship throughout life until late stages (e.g. Humans, Elephants).
    • Typical of K-selected species.
    • Type II (Diagonal Curve):
    • Constant mortality rate irrespective of age (e.g. Birds, Hydra).
    • Type III (Concave Curve):
    • High mortality rate early in life, with few survivors reaching maturity (e.g. many fish species).

Population Growth Calculations

  • Population Growth Equation:
    • ext{Growth} = ( ext{Births} - ext{Deaths}) + ( ext{Immigration} - ext{Emigration})
  • Factors Affecting Population Growth:
    • Births and immigration add individuals.
    • Deaths and emigration remove individuals.

Definitions

  • Immigration: Movement of individuals into a population.
  • Emigration: Movement of individuals out of a population.

Population Density

  • Population Density: Defined as the number of individuals per unit area of land.
    • Formula:
    • ext{Population Density} = rac{ ext{Population}}{ ext{Area of Land}}

Practical Example of Exponential Growth

  • A comparison between receiving $1,000,000 versus a penny a day doubling every day for September:
  • Shows the concept of exponential growth and illustrates how rapidly growth can occur over time.

Daily Doubling Example:

  • Day 1: $0.01
  • Day 2: $0.02
  • Day 3: $0.04
  • Day 4: $0.08
  • Day 5: $0.16
  • Day 6: $0.32
  • Day 7: $0.64
  • Day 8: $1.28
  • Day 9: $2.56
  • Day 10: $5.12
  • Day 11: $10.24
  • Day 12: $20.48
  • Day 13: $40.96
  • Day 14: $81.92
  • Day 15: $163.84
  • Day 16: $327.68
  • Day 17: $655.36
  • Day 18: $1310.72
  • Day 19: $2621.44
  • Day 20: $5242.88
  • Day 21: $10,485.76
  • Day 22: $20,971.52
  • Day 23: $41,943.04
  • Day 24: $83,886.08
  • Day 25: $167,772.16
  • Day 26: $335,544.32
  • Day 27: $671,088.64
  • Day 28: $1,342,177.20
  • Day 29: $2,684,354.40
  • Day 30: $5,368,708.80

Logistic Growth vs Exponential Growth

  • Exponential Growth:

    • Characterized by an unlimited rate of increase when the birth rate exceeds the death rate. Initially slow but accelerates as population size increases.

  • Logistic Growth:

    • Growth is limited by environmental factors (limiting factors).
    • Initial exponential growth phase followed by a slowdown as the population approaches its carrying capacity.

Carrying Capacity

  • Definition: The maximum population density that an environment can sustainably support at any given time.
  • Example: Monitoring population trends of white-tailed deer over various years (1985, 1990, 1995).

Limiting Factors

  • Density-Dependent Factors:
    • Factors that vary with population density, e.g. disease spread, competition for resources, availability of food.
  • Density-Independent Factors:
    • Factors that are not influenced by population density, e.g. natural disasters (tornadoes, floods).
  • Examples of limiting factors:
    • Disease/Parasites: More severe impacts in high-density populations.
    • Accidents: Not density-dependent.
    • Starvation and Predation: Usually affected by population density.
    • Natural Factors (fires, floods): Density-independent.

Specific Case Examples of Density-Dependent vs Density-Independent Factors:

  • Is a tornado caused by a large population? No (density-independent).
  • Is a shortage of food caused by a large population? Yes (density-dependent).
  • Is competition for resources worse when many individuals are present? Yes (density-dependent).

Population Pyramids

  • Visualization tool depicting age structure within a population and illustrating patterns of population expansion, stability, or decline.
    • Expanding rapidly: More young individuals.
    • Expanding slowly: Stable age ratios.
    • Stable: Similar birth and death rates with little population change.
    • Declining: Higher mortality rates, particularly in younger demographics.