Ecology 1: Species Commonness

• Population: a group of individuals of the same species living in the same area
• Ecology: interactions between organisms and their environments
• The most common species is unknown - it could be humans, a type of grass, an insect, a fish, or a microbe
• What makes a species common


  • Range size
  • Local vs global abundance
  • Temporal scales
  • Local vs global biomass
• Restricted distribution


  • Bristol whitebeam - only found in the Avon Gorge, within the city of Bristol, UK
• Global distribution


  • Fire moss - found on every continent, including Antarctica
  • Brown rat - found on every continent, except Antarctica, thanks to human activity
  • Peregrine falcon - has a global distribution with year-round residents, summer visitors, winter visitors, and passage migrants
• Metapopulations


  Interconnected populations across a species' range, where not all available patches are occupied at once and site occupation changes yearly depending on local extinction and migration

  
• There is 30x more carbon in humans than in wild birds, even though there are 40x more wild birds than humans
• Humans and our livestock dominate estimates of global mammal biomass, replacing wild mammals and converting other sources of biomass into mammals

  
• Most wild land mammal species are rodents, most individuals are bats, but most biomass is in ungulates (especially the white-tailed deer)
• Population


  A group of individuals of the species that live in the same area and interact and interbreed
• Population structure


  • Density
• Patterns of dispersion


  • Clumped
  • Uniform
  • Random
• How populations change


  1. Inflow (births + immigration)

  2. Outflow (deaths + emigration)

  3. Change in numbers = Inflow - Outflow
• Intrinsic population growth factor


  The magnitude of population change per unit time
• If a population starts with half as many individuals (500), the population will grow by 93 in 1 year
• (deaths + emigration)


  Change in numbers
• Change in time


  Change in numbers
• Trinity College Dublin, The University of Dublin
• Simplifying the equation


  1. Assume immigration is equal to emigration

  2. Define new value representing the magnitude of change
• Exponential growth


  • Population grows by a set proportion at each time step
  • Larger populations grow at an ever-faster rate
• Exponential growth requires unlimited resources, so cannot occur in nature
• Carrying capacity (K)


  The number of individuals that can be supported in the environment
• As population size approaches carrying capacity

  Growth rate declines
• Snowshoe hare and Canadian lynx populations cycle as food available and predation risk fluctuate
• Predator-prey interactions can lead to boom-and-bust cycles
• Life history traits


  Traits which make up an organism's schedule of reproduction and survival
• Examples of life history traits

  • Number of offspring
  • Age of first reproduction
  • Life span
• Risk of death increases with age for most species
• Types of survival rate patterns

  • Type I - mortality risk low when young
  • Type II - mortality risk even throughout life
  • Type III - mortality risk highest when young
• Semelparity


  Breed just once in life
• Iteroparity


  Breed repeatedly throughout life
• 1. r selected species

  Focus on reproductive rate (many offspring, high mortality)
• 1. K selected species

  Focus on survival (few offspring, low mortality)
• Life history traits combine to create different survival/reproductive strategies but none intrinsically better than the others