7.4 Populations in ecosystems

• What is a community?

  All the populations of different species living in the same place (habitat) at the same time
• What is an ecosystem?

  A community and the non-living (abiotic) components of its environment Ecosystems can range in size from very small to very large. They are dynamic systems (populations rise / fall over time)
• Draw the difference between a population, community and ecosystem

  Picture

  
• What is a niche?

  ● The specific role of a species within its habitat, eg. what it eats, where and when it feeds ● Governed by its adaptation to both abiotic (non-living) and biotic (living) conditions
• Explain the advantage of species occupying different niches

  ● Less competition for food / resources ● If two species tried to occupy the same niche, one would outcompete the other
• What is carrying capacity?

  The maximum (stable) population size of a species that an ecosystem can support
• List the factors that influence carrying capacity

  • Abiotic factors

  Eg. light intensity, temperature, soil pH & mineral content, humidity

  • Interactions between organisms

  a. Interspecific competition - between organisms of different species

  b. Intraspecific competition - between organisms of the same species

  c. Predation (predators kill and eat other animals, called prey)
• Explain how abiotic factors may affect population size / carrying capacity

  ● If conditions favourable, organisms more likely to survive & reproduce → increasing carrying capacity

  ● Eg. increasing light intensity increases rate of photosynthesis in plants

  ○ This increases carrying capacity of a variety of plant species

  ○ So increases the number and variety of habitats, niches and food sources for animals

  ○ So increasing carrying capacity of a variety of animal species
• Explain how interspecific competition may affect population size

  ● Reduces [named resource] available to both species, limiting their chances of survival & reproduction ○ So reduces population size of both species

  ● If one species is better adapted, it will outcompete the other

  ○ So population size of less well adapted species declines, potentially leading to extinction
• Explain how intraspecific competition may affect population size

  1. As population size increases, resource availability per organism decreases, so competition increases ● So chances of survival & reproduction decrease → population size decreases 2. As population size decreases, resource availability per organism increases, so competition decreases ● So chances of survival & reproduction increase → population size increases
• Explain changes of predators & prey


  1. Prey population increases predators have more food ● More predators survive & reproduce
  2. Predator population increases more prey killed & eaten ● Less prey survive & reproduce
  3. Prey population decreases predators have less food ● Less predators survive & reproduce
  4. Predator population decreases less prey killed & eaten ● More prey survive & reproduce

  
• What does the predator and prey cycle show

  Populations fluctuate in cycles, predator population peaking after prey (lag time)
• Describe how the size of a population of slow-moving or non-motile organisms can be estimated

  1. Divide area into a grid / squares eg. place 2 tape measures at right angles
  2. Generate a pair of coordinates using a random number generator (eg. on a calculator)
  3. Place a quadrat here and count number / frequency of [named species]
  4. Repeat a large number of times (10 or more) and calculate a mean per quadrat
  5. Population size = (total area of habitat / quadrat area) x mean per quadrat
• Describe how the mark-release-recapture method can be used to estimate the size of a population of motile organisms

  ● Capture sample of species, mark and release

  ● Ensure marking is not harmful / does not affect survival

  ● Allow time for organisms to randomly distribute before collecting second sample

  ● Population = (number in sample 1 x number in sample 2) / number marked in sample 2
• Why doesn’t marking need to be physical 


  It could be recording the base sequence, for example. Recapturing an organism with an identical base sequence would show the organism has been caught (‘marked’) before
• What assumptions does the mark-release-recapture method make?

  1. Sufficient time for marked individuals to mix / distribute evenly within the population
  2. Marking not removed and doesn’t affect chances of survival / predation
  3. Limited / no immigration / emigration
  4. No / few births / deaths / breeding / change in population size (or birth & death rate are equal)
• Suggest why the mark-release-recapture method can produce unreliable results in very large areas

  ● Unlikely that organisms will distribute randomly / evenly

  ● Less chance of recapturing organisms (that were marked initially
• What is Succession


  A change in a community over time due to change in abiotic factors / species
• Primary succession


  1. Colonisation pioneer species first colonise
  2. Pioneer species + other species at each stage in succession + change abiotic conditions ● Eg. they die + decompose, forming soil which retains water (humus / organic matter)
  3. Environment becomes less hostile / more suitable for other species with different adaptations and less suitable for previous species, better adapted species outcompete previous species
  4. As succession goes on, biodiversity increases
  5. Climax community reached
• Describe features of a climax community

  Stable community (no further succession)

  ● Same species present / stable community over a long time

  ● Abiotic factors (fairly) constant over time

  ● Populations (fairly) stable (around carrying capacity)
• Explain how conservation of habitats involves management of succession

  ● Further succession can be prevented to stop a climax community forming

  ○ By removing or preventing growth of species associated with later stages eg. by allowing grazing

  ● This preserves an ecosystem at a certain point / in its current stage of succession (plagioclimax)

  ● So early species are not outcompeted by later species and habitats / niches are not lost
• Describe the conflict between human needs and conservation as well as the importance of managing this

  ● Human demand for natural resources (eg. timber) is leading to habitat destruction / biodiversity loss

  ● Conservation is needed to protect habitats / niches / species / biodiversity

  ● Management of this conflict maintains the sustainability of natural resources

  ○ Meeting current needs without compromising the ability of future generations to meet theirs