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The concept of an ecosystem

Ecological Terms
  • Ecosystem: all the abiotic and biotic factors in an area/environment surrounding a species
  • Community: "all the organisms present in an area/habitat/ecosystem"1
  • Population: "all the individuals of one species in an area"2
  • Environment: sum of all conditions in the ecosystem outside the organism
  • Habitat: place within an ecosystem where a particular population is found
  • Niche: species' function in its habitat
    • Two species occupy different niches when they NOT compete
      • Feeding at different times on different organisms
    • "Two species NO LONGER occupy the same niche when
      • one species displaces other species (better adapted)/one species survives/none survive due to
      • competition (for environmental resources)/insufficient
      • food, territory, mates (animals); light, CO2, H2O, mineral ions (plants)"2

Investigating Numbers and Distribution
Investigating Variation
  • Several samples eliminate chance and anomalies (eg three averaged readings)
  • Random sampling eliminates bias (favour of one type)
    • Random coordinates achieved by calculator
  • Any differences must reflect real differences in the population of sampled data

Frame Quadrats
  • Three measurements can be taken / population size of a particular species occupies in a quadrat
  • Assumptions
    • Stationary organisms / must not move
    • Quadrats must be chosen randomly
    • Sample represents whole population
  • To investigate an area
    • Area is divided into a grid by measuring tapes
    • Quadrats are chosen at random by using random numbers from a calculator
    • Organisms under investigation are counted in each quadrat
  • To investigate a volume
    • Container of known volume is immersed at random points in the pond
    • Number of tadpoles are counted each time
  • Limitation: Area being studied is much larger than the small quadrats
POPULATION SIZE = (Sorganisms per quadrat * A of field)/A of quadrat)
S = Sum of; A = Area;

Line Transects
  • Measures distribution of species in a straight line across a habitat
  • Useful for identifying changes in a habitat
  • Records all species which touch a tape stretched across a habitat
    • Belt transect records species between two lines
    • Interrupted belt transect records species present in a number of quadrats placed at fixed points along a line stretched across the habitat
  • Limitation: one transect may not cross typical areas

  • Animals are marked and released back into the community (N1)
  • Second sample is collected
    • Total size of sample (N2) and
    • Number of marked animals (n) are counted
  • Population size = (N1*N2)/n
  • Marking must not affect organisms' behaviour / non-toxic marking, survival rate must not be affected / would make it not more vulnerable to predators
  • Animals must not die, reproduce, migrate into study area population size must not change
  • Marked animals in population must mix

  • Measure of the number of species and their success within an area
  • Shows stability of an ecosystem
  1. Use large number of traps in each of two habitats
  2. Place traps at random / by random coordinates using a calculator
  3. Keep factor (size/length of time/time of day) constant
  4. Count number of organisms of each type
    1. N = number of organisms of all species present in the community
    2. n = number of organisms of each individual species
  5. Calculate index of diversity d = N(N-1) / ∑n(n-1) for both habitats
  6. Takes account of number of individuals (as well as number of species)

Abiotic (non-living) Factors
  • Climatic factors: seasonal changes in temp, humidity, daylength, rainfall
  • Physical conditions: pH, soil particles, ions, availability of H2O
  • Organism must have physiological adaptations to live in abiotic conditions
  • Lack of inorganic ions (nitrate) often limit plant growth
  • Plants are primary producers, affecting all populations in a community

Biotic (living) Factors - Interaction Between Organisms
  • Intraspecific competition / members of the same species compete for resources
    • Space, patch of soil to grow on, nesting site, food, ...
  • Interspecific competition / different species need same resources at same trophic levels
    • Plants compete for light
    • Herbivore species compete for plants
    • Carnivore species compete for prey (predation)

Table 5-14-1: The environment influences diversity
Less Extreme/Harsh Environments Extreme Environments
Higher diversity more plant species more niches greater variety of food less competition for resources more food available less harsh environment (abiotic) in forest Lower diversity less plant species less
niches smaller variety of food higher competition for resources less food available more harsh environment (abiotic) in forest
Stable ecosystem, if the population of one species changes, alternative food sources are available Unstable ecosystem, change in the population of one species affects populations of other species
Biotic factor(s) dominate Abiotic factor(s) are extreme and dominate
Many species have adaptations that allow them to survive, including many producers Only a few species have adaptations that allow them to survive
Higher solar energy input gives more light for photosynthesis (and higher temperatures)  

  • Process in which different species make up a community over time
  • Autogenic succession: brought about by plants only
  • Allogenic succession: external factor (eg flooding) alters development of community
  • 1° succession / succession where no living organisms have been found before
  • 2° succession / community of living organisms have already been there / human activity damaged vegetation and stopped succession
  • CLIMAX COMMUNITY: final most complex stage of succession / affected by abiotic factors
    • Succession can stop before a tree community
      • Valley / top of a high mountain
    • Climate climax: climate affects succession and complexity
    • Grazing climax: grazing animals stop succession
      • Grazing sheep and cattle prevent grassland to revert to woodland
  • SERAL STAGES change the environment to decrease abiotic factors (whole succession = sere)
    • Different types of vegetation enter the area
    • Increases amount and depth of soil
    • Allows other plants to enter
    • Will create more niches / more complex food webs / higher diversity
    • More animals will enter the area
    • Species diversity and complexity of food webs increases until climax is reached

Table 5-14-2: Succession










References and Further Reading
1) AQA (2002) Mark Scheme January 2002 GCE Biology/Human Biology A Unit BYA5, [PDF]
2) AQA (2002) Mark Scheme June 2002 GCE Biology/Human Biology A Unit BYA5, [PDF]
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]

BYA5 SECTION: 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10