Investigating Populations Flashcards
How can we investigate the population growth of bacteria?
1) grow bacteria in a liquid broth (broth culture).
2) When light is passed through a sample of broth culture, some of it is scattered because bacteria are present - this reduces how much light passes through the culture. 3) A machine called a spectrophotometer (colorimeter) can measure light passing through a sample of the culture and produce an absorbance value.
The more bacteria present the more light absorbed. A high absorbable value suggest there is a high number of bacteria.
What is a liquid broth?
A liquid containing nutrients the bacteria need to grow
What do you get if you plot absorbance value against time when growing bacteria?
You get an exponential graph that shows the bacteria doubling at certain intervals of time. We use a logarithmic scale on the y-axis as the bacteria increase in large increments
The graph on the left shows an exponential curve with the number of bacteria increasing rapidly, whereas the graph on the right shows a logarithmic scale on the y-axis, producing a straight line graph
Why do we use logs?
1) to compare values that have a large range (comparing very small values to very large values )
2) makes it easier to plot on a graph
3) instead of numbers you present the values in order of magnitude
What is the definition of abundance and how can it be measured ?
Abundance is the number of individuals of one species in a particular area (population size)
Can be measured by:
1) frequency - the number of samples a species is recorded in
2) percentage cover - how much of the area you are investigating is covered by the species, can only be used for non-motile or slow moving organisms, eg plants or limpets
What is distribution in reference to populations?
How a species is spread across a habiat. A species could be:
1) evenly spread throughout the whole habitat
2) clumped - they could favour a specific area within an ecosystem
3) randomly spread
What is the method of random sampling ?
1) choose an area to sample
2) devise a method that would allow random samples to be taken at random locations such as a random number generator to generate coordinates on a grid
3) use an appropriate sampling technique
4) take as many samples as possible (minimum 20)
5) carry out statistical tests on the data collected
The larger the sample size the more representative the data collected is
What are two methods that can be used for investigating non motile/slow moving populations?
1) frame quadrats - sample organisms on the ground in an area that is relatively uniform
2) transects - lines through the study area, along which samples are taken
What 3 things can a quadrat be used to find ?
1) species frequency - the likelihood of a particular species occurring within the quadrat eg if a species appears in 15 squares out of the total 30 squares the frequency is 50%
2) number of individuals - the number of each species that is recorded in each quadrat
3) percentage cover - count the number of squares of the quadrat that is covered by the species (include a square it is more than half covered ) and use this information o calculate a percentage
What are the two type of transects?
1) belt transects - quadrat are paced net to each other along the line of study, this can be used to calculate the species frequency and percentage cover
2) interrupted belt transects - place quadrats at regular intervals along the line of study, eg place 1m apart
Transacts can be used to help find out how2 organisms are distributed across an area. This is particularly useful if there is a change in an abiotic factor across an area eg shade
What methods can we use on investigating motile populations?
We can use nets or traps
Flying insect you could use a sweep net
Terrestrial insects you could use a pitfall trap
What is the mark-release-recapture method?
1) capture sample, mark them so they can be easily identified and release
2) should be a harmless method of marking eg paint/ attaching a tag
3) take second sample and count marked organisms
4) use following equation to estimate population size:
What assumptions does the mark-release- recapture method depend upon?
1) the marked sample has had enough time and opportunity to mix with the non-marked population
2) the marking hasn’t affected the individuals chance of survival
3) the mark is still visible
4) there are no changes in population size due to births, deaths and migration
What are some ethical issues associated with capturing organisms?
1) organisms could be stressed after being captured which could lead to a reduced change of survival after release
2) some organisms may be more likely to enter a trap again in they have been previously caught
