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).

  
• 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
  2. Intraspecific competition - between organisms of the same species
  3. 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
  2. So chances of survival & reproduction decrease → population size decreases
  3. 2. As population size decreases, resource availability per organism increases, so competition decreases
  4. So chances of survival & reproduction increase → population size increases
• 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
  2. So more predators survive & reproduce
  3. Predator population increases so more prey killed & eaten
  4. So less prey survive & reproduce
  5. Prey population decreases so predators have less food
  6. So less predators survive & reproduce
  7. Predator population decreases so less prey killed & eaten
  8. 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

  Note - marking doesn’t have 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.
• Explain how the mark release recapture equation can be derived

  This is for your understanding only. Assuming the proportion of marked individuals in the second sample reflects the proportion of the entire first sample in the population, the following formula can be derived:

  Number (marked) in sample 1/Total population size = Number marked in sample 2/Total number in sample 2

  This just needs to be rearranged to get the formula for population size.

  
• Worked example: mark-release-recapture: A student collected 17 lizards and marked them before releasing them back into the same area. Later, she collected 20 lizards, 10 of which were marked. Calculate the number of lizards in this area.

  (17 x 20)/10 = 34
• 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)
• Describe and explain how primary succession occurs

  1. Colonisation by pioneer species (first to colonise)
  2. Pioneer species (and other species at each stage in succession) change abiotic conditions
  3. ● Eg. they die and decompose, forming soil which retains water (humus / organic matter)
  4. So environment becomes less hostile / more suitable for other species with different adaptations AND less suitable for previous species, so better adapted species outcompete previous species
  5. As succession goes on, biodiversity increases
  6. Climax community reached - final stable community (no further succession)
• What is meant by succession?

  change in a community over time due to change in abiotic factors / species
• 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
• RP12: Describe how you could investigate the effect of an environmental factor on the distribution of a species in a habitat (random sampling in two areas) - steps 1-3: up to counting frequency

  1. Divide two areas into grids / 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. Standardise this eg. only count it if it is more than half in the quadrat
• RP12: Describe how you could investigate the effect of an environmental factor on the distribution of a species in a habitat (random sampling in two areas) - steps 4+5: from counting frequency

  1. 4. Repeat a large number of times (10 or more) and calculate a mean per quadrat for both areas
  2. 5. Measure environmental factor in each area eg. take soil moisture readings with a soil moisture meter
• RP12: Suggest why percentage cover may be used rather than frequency.

  Too difficult to count individual organisms / individual organisms are too small to count
• RP12: Explain why random sampling is used.

  To avoid sampling bias
• RP12: Explain the importance of a large sample size.

  ● Minimises the effect of anomalies

  ● Ensures sample is representative of the population
• RP12: Describe how you could decide the number of quadrats that should be used in order to collect representative data.

  • Calculate a running mean
  • When enough quadrats, this shows little change
  • Enough to carry out a statistical test
• RP12: Describe how you could investigate the effect of a factor on the distribution of a species in a habitat (systematic sampling)

  1. Place a transect line (tape measure) across an area with an environmental gradient eg. tree to full sun
  2. Place quadrats at regular intervals eg. 1m (until end of transect) and record the number of organisms of [named species] and [named environmental factor] eg. light intensity using a light meter
  3. Repeat in other parallel areas and calculate mean number of plants at each point along the transect
• RP12: Explain the limitations of using systematic sampling to estimate the population of a species in a field.

  • ● Not appropriate unless there is an environmental gradient
  • ● Transects run in one direction, but to cover the entire field, they would need placing in multiple directions
• RP12: Which statistical test should be used to determine the relationship between abundance and an environmental factor?

  Correlation coefficient eg. Spearman’s rank