Biology - 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 abiotic (non-living) components of its environment

Dynamic systems (populations rise /fall over time)

What is a niche?

• Specific role of a species within its habitat (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 a species occupying different niches

• Less competititon 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

• Light intensity, temperature, soil pH + mineral content, humidity

Interactions between organisms

• Interspecific competition - between organisms of different species

• Intraspecific competition - between organisms of the same sepcies

• Predation

Explain how abiotic factors may affect population size / carrying capacity

• If conditions are favourable, organisms more likely to survive and reproduce → increasing carrying capacity

• E.g. increasing light intensity increases rate of photosynthesis in plants

  • Increasing carrying capacity in a variety of plants species

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

  • Increasing carrying capacity of a variety of animal species

Explain how interspecific competition may affect population size

• Reduces (resource) available to both species, limiting their chances of survival and 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 competititon may affect population size

1. As population size increases, resource availability per organism decreases, so competition increases

   • So chances of survival and reproduction decrease → population size decreases

2. As population size decreases, resource availability per organism increases, so competition decreases

   • So chances of survival and reproduction increase → population size increases

Explain the cahnges which occur in populations of predators and 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 and reproduce

2. Predator population increases so more prey killed and eaten

   • So less prey survive and reproduce

3. Prey population decreases so predators have less food

   • So less predators survive and reproduce

4. Predator population decreases so less prey killed and eaten

   • So more prey survive and reproduce (cycle repeats)

Describe how the size of a population of slow-moving or non-motile organisms can be estimated

1. Divide an area into grid / squares (e.g. place 2 tape measures at right angles)

2. Generate a pair of coordinates using a random number generator

3. Place a quadrat there and count number / frequency of (species)

4. Repeat a large number of times (10+) 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 / doesn’t affect survival

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

• Population = (no. in sample 1 x no. in sample 2)/ no. marked in sample 2

What assumptions does the mark-release-recapture method make?

1. Sufficent time for marked individuals to mix evenly with the population

2. Marking not removed and doesn’t affect chances of survival / predation

3. No immigration / emigration

4. No change in population size

Suggest why the mark-release-recapture method can be unreliable in very large areas

• Unlikely that organisms will distribute randomly / evenly

• Less chance of recapturing organism (that were marked intitally)

Describe and explain how primary succession occurs

Succession: Change in a community over time due to change i abiotic factors / species

1. Colonisation of pioneer species

2. Pioneer species (+ others at each stage of succession) change abiotic conditions

   • E.g. die and decompose forming soil which retains water

3. 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

4. As succession goes on, biodiversity increases

5. Climax community reached - final stable community (no further succession)

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)

Describe 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

• Preserves an ecosystem at a certain pont / at its stage of succession

• 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 (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.