Population Growth and Regulation

Population

What is a Population?

  • A group of individuals that occur together at one place and time.

Population Properties

  • Populations are influenced by growth, contraction, and distribution of individuals.

1. Range

  • Geographic distribution of a population.

2. Change Through Time

  • How populations expand or contract their range.

3. Spacing

  • How individuals are geographically distributed.

Population Ranges

  • Geographic distribution of a population can vary.
    • Global distribution.
    • Single location.

Examples:

  • Puma concolor
  • Tursiops truncatus
  • Cyprinodon diabolis

Devil's Hole Pupfish

  • Small range.
  • Small range can impacts the population

Population Change Through Time

  • Populations expanding or contracting their ranges.
  • Ranges largely dependent on abiotic factors.
    • Temperature.
    • Humidity.
    • Rainfall, etc.
  • Community interactions also shape ranges.
    • Food availability.
    • Resource competition.

Range Contraction in the American Pika

  • Increase in global temperature.
  • Leads to loss of territory for pikas.

Population Change Through Time

  • Populations expand or contract their range.
  • Can occur rapidly.
    • European Starling
      • Introduced in New York in 1896.
      • Covered North America in less than 100 years.

Dispersal

  • Rate of expansion depends on mechanism of dispersal.
  • Movement of individuals within and beyond a population.
  • Biotic and Abiotic factors.
    • Animal-mediated dispersal.
    • Flying dispersal.
    • Water dispersal.
    • Wind dispersal.

Population Spacing

  • How individuals are geographically distributed.
  • Resource dependent.
    • Uniform.
      • Occurs when resources are limited and evenly distributed.
    • Random.
      • Occurs when resources are abundant and evenly distributed.
    • Clumped.
      • Occurs when resources are patchy

Uniform Spacing

  • Occurs when resources are limited and evenly distributed.

Random Spacing

  • Occurs when resources are abundant and evenly distributed.

Clumped Spacing

  • Occurs when resources are patchy.
  • Other benefits include protection from predators.

Population Types

  • Species population.
    • All individuals of a species.
  • Metapopulation.
    • Group of geographically separated populations of the same species linked by dispersal.
  • Population.
    • A group of individuals that occur in the same geographic range at the same time.

Population Size and Growth

  • A look back at natural selection:

    1. More offspring are produced than can survive.
    2. Natural variation exists within a population.
    3. Some traits better adapted to their environment, so are more successful.
    4. Traits are heritable.

  • More successful individuals have high fitness.

  • If fitness consistently increases through time, population sizes should increase as well.

Theoretical Population Growth

  • Biotic potential.
    • The maximum rate a population can grow without limitations.
  • Formula:
    • r = (b + i) – (d + e)
      • r = Total rate
      • b = Birth rate
      • i = Immigrant rate
      • d = Death rate
      • e = Emigrant rate
  • Most populations do not continuously increase in size exponentially!

What Limits Population Growth?

Maximum Population Size

  • Carrying Capacity (K).
    • Maximum population size of a species in an environment.
    • Determined by available resources:
      • Food
      • Water
      • Shelter
      • Territory
      • Potential mates
  • New populations go through both types of growth

Life History Traits

  • Traits related to the life history of an organism.
  • Correlates with major events in life!
    • Size at birth
    • Growth rate
    • Age of maturity
    • Number/size of offspring
    • Reproductive investment
    • Mortality/length of life

Survivorship Curves

  • Probability of an organism’s survival changes through time.
    • Type I.
      • High mortality near the end of a life span.
    • Type II.
      • Equal mortality throughout a life span.
    • Type III.
      • High mortality at the beginning of a life span.

Life History Strategies

  • Based on life history traits!
  • Two extreme strategies:
    • r-selected.
      • Many offspring with high mortality.
    • K-selected.
      • Few offspring with low mortality.

r-Selection

  • “Live fast and die young” – The James Dean philosophy
    • Maturity and reproduction early in life
    • Short lifespan
    • Large number of offspring with few reproductive events
    • High mortality, low offspring survival
    • Minimal parental investment

K-Selection

  • “Live long and prosper” – The Spock philosophy
    • Maturity and reproduction later in life
    • Long lifespan
    • Few number of offspring with multiple reproductive events
    • Low mortality, high offspring survival
    • High parental investment

r and K Selection

  • Tradeoffs!
  • Both extreme strategies increase fitness with finite resources.
  • Most taxa have traits from both extremes.