StemUp: AQA A level Biology 3.7.4 Populations in ecosystems

What is meant by a community? (1)

Populations of different species

What is meant by a ecosystem? (1)

Formed by a community and the non-living components of its environment

What is meant by a species' niche? (2)

- An organism's abiotic requirements in the habitat and its position in the food web

- It is a description of how the organism fits into the ecosystem

What is meant by an ecosystem's 'carrying capacity'? (1)

The size of population of a species that an ecosystem can support

How would you carry out a random sampling technique? (4)

1. Study area is split up into grids

2. Random numbers are used to generate sampling coordinates

3. A quadrat is then placed at the intersection of each pair of coordinates

4. The number of individuals of each species present or the percentage cover for plants is recorded

How are random numbers generated for sampling coordinates? (2)

Either:

- From a table

- From a computer program

What is the importance of random sampling? (1)

Ensure there is no bias in the collection of results

Why do we use a large number of quadrats?

To ensure our results are representative of the population

Why do we use the belt transect technique? (2)

- Useful when there is a transition in habitats and communities ‎‏‏

- Through an area

How you carry out the belt transect technique? (3)

1. Tape is run along the ground

2. At regular intervals, a frame quadrat is laid down along the tape

3. The percentage cover or the frequency of the species can be recorded

Why is percentage cover sometimes used? (2)

- To 'count' species that are hard to separate individuals

- E.g. a bunch of flowers

What is mean density? (2)

- The number each species found in the study area

- Per square metre

What equation do you use to estimate the mean density? (2)

How you would use sampling methods to work out population size? (2)

1. Estimate the mean density

2. Multiply the total area by the estimated density to estimate population size

Describe how you would carry out the mark-release-recapture method (4)

1. Capture a representative sample of organisms from the population

2. Record the number captured and mark the individual organisms

3. The marked organisms are released back into habitat and left for a suitable period of time to enable the organisms to mix randomly within the population

4. After a suitable period a second sample of organisms is captured and the number of marked individuals is counted

What is the equation for the mark-release-recapture method? (4)

(Used to estimate population size)

What are the three abiotic factors that affect population size? (3)

- Temperature

- Light

- pH

Why may a population size vary from an estimate size? (3)

- Due to the effect of abiotic factors

- Due to interactions between organisms, interspecific and intraspecific competition

- Predation

How does temperature affect population size? (3)

- Metabolic processes i.e. respiration and photosynthesis are controlled by enzymes

- Low temperatures are a major limiting factor for photosynthesis

- High temperatures can cause enzyme denaturation

How does light affect population size? (2)

- Effects photosynthesis and so effects the number and diversity of plant species

- Consequently this will effect the number and type of consumers in an ecosystem

How does pH affect population size? (2)

- Effects activity of enzymes

- Acidic and alkaline soils favour the growth of specific plant species

What is primary succession? (2)

- The gradual change in plant and animal communities from the initial colonization

- By pioneer species

- To the formation of a climax community.

Who are the pioneer species in primary succession? (1)

The first organisms to colonize an area during primary succession

How do pioneer species influence the environment during primary succession? (3)

- Change the abiotic environment in various ways

- Such as by developing soil

- Which makes the habitat more suitable for new species

What happens as new species colonize an area during primary succession? (3)

- New species may alter the environment

- In such a way that it becomes less suitable for the previous species

- Often leading to the earlier species becoming less abundant as the new species are better competitors

What are the typical outcomes of changes in the abiotic environment during primary succession? (2)

- Result in a less hostile habitat

- Usually leading to an increase in species diversity

What is a climax community in the context of primary succession? (3)

- The final stable community that forms at the end of primary succession

- Characterised by increased stability and diversity

- As the food webs become more complex

What are the factors that affect succession? (3)

- Natural events

- Grazing

- Ploughing

Why might climax communities not be established in certain areas? (2)

- Due to agricultural practices such as grazing, crop production, and ploughing

- Natural events like fires and floods

How does grazing affect succession? (2)

- Prevent the succession beyond grassland

- By continually eating the seedlings of herbaceous plants and shrubs

What impact does ploughing have on succession? (2)

- Breaks up and buries seedlings

- Which prevents their germination

How do natural events like fires and floods affect succession? (2)

- Disrupts the natural sequence of succession

- Which prevents the establishment of climax communities