6.5.7 Testing for Distribution and Abundance Flashcards
Why are estimates important?
Measuring all the different levels of biodiversity within an ecosystem could be very time-consuming
Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list
This is possible for areas that are very small or where the species are very large like trees
However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there
When this is the case different samples of the area can be taken and used to make an estimate for the total species numbers in the area
What is ampling?
Sampling is a method of investigating the abundance and distribution of species and populations
There are two different types of sampling:
Random
Systematic
What is random sampling?
In random sampling the positions of the sampling points are completely random or due to chance
This method is beneficial because it means there will be no sampling bias by the person that is carrying out the sampling that may affect the results
What is systematic sampling?
In systematic sampling the positions of the sampling points are chosen by the person carrying out the sampling
There is a possibility that the person choosing could show bias towards or against certain areas
Individuals may deliberately place the quadrats in areas with the least species as these will be easier and quicker to count
This is unrepresentative of the whole area
When a sampling area is reasonably uniform or has no clear pattern to the way the species are distributed then random sampling is the best choice
What are the three main sampling methods when estimating a population size?
There are three main sampling methods used when trying the estimate the size of a population:
Quadrats (for non-motile or slow-moving species)
Transects (for non-motile or slow-moving species)
Mark-release-recapture (for motile species)
How can you investigate the distribution of organisms?
The distribution of a species describes how it is spread throughout the ecosystem
The abundance of a species is the number of individuals of that species
The distribution and abundance of non-motile or slow-moving species in an area can be assessed using two different practical methods:
Frame Quadrats
Belt Transects
What are frame quadrats?
Some ecosystems are very complex with large numbers of different species of different sizes
For the sake of logistics, sampling is often used to estimate the distribution and abundance of species
When carrying out sampling, square frames called quadrats can be used to mark off the area being sampled
Quadrats of different sizes can be used depending on what is being measured and what is most suitable in the space the samples are being made in
Quadrats must be laid randomly in the area to avoid sampling bias
This random sampling can be done by converting the sampling area into a grid format and labeling each square on the grid with a number
Then a random number generator is used to pick the sample points
Once the quadrat has been laid on the chosen sample point the abundance of all the different species present can be recorded
What are the results from quadrat?
The results from the quadrats can be used to calculate the predicted frequency and density of a species within an area
Species frequency is the probability that the species will be found within any quadrat in the sample area
The number of quadrats that the species was present in is divided by the total number of quadrats and then multiplied by 100
For example, if bluebells were found in 18 out of 50 quadrats the species frequency would be (18/50) x 100 = 36%
It can sometimes be difficult to count individual plants or organisms. When this is the case percentage cover of the species within the quadrat can be estimated instead
The quadrat is divided into 100 smaller squares. The number of squares the species is found in is equivalent to its percentage cover in that quadrat
For example, if grass is found in 89 out of 100 squares in the quadrat then it has a percentage cover of 89%
What are belt transects?
Throughout some areas, there can be changes in the physical conditions (i.e. abiotic factors)
For example, there may be changes in altitude, soil pH or light intensity
When investigating the species distribution in these kinds of areas systematic sampling is more appropriate
Methods using transects can help show how the distribution or abundance of a species changes with the different physical conditions in the area
A transect is a line represented by a measuring tape, along which samples are taken
For a belt transect:
Place quadrats at regular intervals along the tape and record the abundance of each species within each quadrat
This produces quantitative data
What is the mark release capture method?
The two sampling methods described above are only useful for non-motile (sessile) organisms
Different methods are required for estimating the number of individuals in a population of motile animals
The mark-release-recapture method is used
For a single species in the area:
The first large sample is taken. As many individuals as possible are caught, counted and marked in a way that won’t affect their survival e.g. if studying a species of beetle, a small amount of brightly coloured non-toxic paint can be applied to their carapace (shell)
The marked individuals are returned to their habitat and allowed to randomly mix with the rest of the population
When a sufficient amount of time has passed another large sample is captured
The number of marked and unmarked individuals within the sample are counted
The proportion of marked to unmarked individuals is used to calculate an estimate of the population size
The formula for the calculation is:
N = (n1 × n2) ÷ m2
Where:
N = population estimate
n1 = number of marked individuals released
n2 = number of individuals in the second sample (marked and unmarked)
m2 = number of marked individuals in the second sample
What are the results from the mark release capture method?
Assumptions
When using the mark-release-capture method, there are a few assumptions that have to be made:
The marked individuals are given sufficient time to disperse and mix back in fully with the main population
The marking doesn’t affect the survival rates of the marked individuals (e.g. doesn’t make them more visible and therefore more likely to be predated)
The marking remains visible throughout the sampling and doesn’t rub off
The population stays the same size during the study period
there are no significant changes in population size due to births and deaths
there are no migrations into or out of the main population
How can you measure abundance and distribution in difficult habitats?
The methods of quadrats, transects and mark-release-recapture work well for estimating the abundance and distribution of a large number of land-based species
However, certain species in their habitats are not so easy to estimate, for example:
The abundance of lichen species growing on tree branches and twigs
The abundance of aquatic species on the sea bed
The ACFOR scale is used by biologists to estimate abundance:
ACFOR = Abundant, Common, Frequent, Occasional, Rare
This is a suitable (though less quantitative) method that can be used for the lichen example above
For seabed analysis, the use of quadrats is carried out, using specialist diving equipment and expertise to analyse and identify the contents of the quadrats
