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
• 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

  1. 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, increasing nitrates increases protein

  production and increasing phosphates increases phospholipid production 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 the changes which occur in populations of predators & prey

  Populations fluctuate in cycles, the predator population peaking after the prey (lag time):

  1. Prey population increases so predators have more food

  ○ So more predators survive & reproduce

  2. Predator population increases so more prey killed & eaten

  ○ So less prey survive & reproduce

  3. Prey population decreases so predators have less food

  ○ So less predators survive & reproduce

  4. Predator population decreases so less prey killed & eaten

  ○ So more prey survive & reproduce (cycle repeats)
• 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
• 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 so marked organisms are recaptured OR marking is not toxic so doesn’t

  affect chances of survival OR marking not visible (to predators) so doesn't affect 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)
• Describe and explain how primary succession occurs

  Succession = change in a community over time due to change in abiotic factors

  1. Colonisation by pioneer species (first)

  2. Pioneer species (and other species at each stage) change abiotic conditions

  ○ Eg. they die and decompose, forming soil which retains water (humus / organic matter)

  3. So environment becomes less hostile/more suitable for other species AND less suitable for previous species, so better adapted species outcompete

  4. As succession goes on, biodiversity increases

  5. Climax community reached - final stable community
• Describe features of a climax community

  ● 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
• 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 how succession results in a wide variety of fish living on coral reefs.

  • increase in variety of species
  • provides more habitats/niches