7.4 - Populations in ecosystems

Question 1

Describe how you could estimate the size of a population of sundews in a small marsh. (5)

Answer 1

• Use a grid / Divide area into squares/sections
  ‘accept use of tape measures/map/area with coordinates / Belt transect’
• Method of obtaining random coordinates/numbers e.g. calculator/computer /random numbers table/generator
  ‘accept quadrats at regular intervals or current mark point’
• Count number/frequency in a quadrat/section
  ‘accept % cover in quadrat/section’
• Large sample and calculate mean/average number (per quadrat/section)
  ‘accept large sample and calculate mean %’
• Valid method of calculating total number of sundews, e.g. mean number of plants per quadrat/section/m2
  multiplied by number of quadrats/sections/m2 in marsh

Question 2

Describe how the mark-release-recapture method could be used to determine the population of mosquitoes at the start of the investigation. (3)

Answer 2

• Capture / collect / sample, mark and release
• Leave time for mosquitoes / Aedes to disperse before second sampling / collection
• (Population =) number in first sample × number in second sample divided by number of marked in second sample

Question 3

Apart from availability of water, describe and explain how two abiotic factors may have caused differences in the species of algae growing at sites A and B. (4)

Answer 3

• Wave action - firmer grip on rock is necessary (at either site)
• Wind/air movement/less humid - more evaporation at site A / more (physical) damage
• Light – (linked to) photosynthesis (at either site)
• Temperature – (linked to) photosynthesis/respiration/enzymes/evaporation (at either site)
• pH – (linked to) enzymes/proteins

Question 4

What is a species? (1)

Answer 4

• (Organisms that) can breed together / interbreed and produce fertile offspring

Question 5

What is meant by uniformly distributed? (1)

Answer 5

• Same number (of organisms) in each region / (organisms) equally spread

Question 6

How do the assumptions made in proportional sampling differ from those made in mark-release-recapture? (2)

Answer 6

• In mark–release-recapture no assumption that organisms are uniformly distributed
• In mark–release-recapture size of total area / size of sampled region not required

Question 7

What is meant by niches?

Answer 7

• The role of a species plays within its habitat

Question 8

What is meant by a community?

Answer 8

• Several/multiple populations (of different species) living in the same area/habitat

Question 9

What is meant by a population?

Answer 9

• All the members of a single species which live in the same area

Question 10

It is not possible for two species to occupy the same role within an ecosystem. Explain why this is the case. (2)

Answer 10

• They will compete / be in competition with each other
• One species will outcompete the other / one of the species will die out (in the local area) / be forced to leave

Question 11

Define the term succession. (1)

Answer 11

• A directional change in an ecosystem over time / an ecosystem changing from being simple to complex over time / a change in the species that make up a community over time