There are many reasons that people would need to keep track of populations, like conservation or pest control, and there are many ways to do it.
Sampling involves using a quadrat to discover the number of a species in at a given point. There are two main types of quadrat: the frame quadrat is a square divided up into sections, like a grid; the point quadrat is a standing frame which allows for needles to be dropped through it so they touch the ground (which ever species the needle touches is recorded).
Sampling can be random to avoid bias, this involves dividing an area up on a grid and the using a random number generator to create coordinates which pick out an area to use a quadrat in. Alternatively sampling may need to be systematic for instance if you are looking for a pattern. In much the same way the given area is divided up but this time the quadrats are at regular intervals.
Transects are a form of systematic sampling where the data are taken at equal spaces along a single straight line; this is useful if you are looking for a change over distance e.g. distance from sea. There are two types of transect: a line transect is where you count everything touching the tape measure at a given point; belt transect is where you put a quadrat down at a given point.
The data gained from sampling can be used to calculate abundance in multiple ways:
Frequency: this is looks at how many times the species was present out of all the samples taken.
For example, if there are ten squares in your quadrat and there is grass in five of the squares then the frequency is 5/10.
Percentage cover: this focuses specifically on the percentage of space on the ground that a species covers. It is useful if the plant is very abundant or it is difficult to distinguish between different plants e.g. grass.
For example: there are ten squares in my frame quadrat, daisies cover 3 whole square and two half squares
0.4 *100= 40%
For more mobile species, so animals, the previously stated methods of data collection are not very effective: the same animal could walk around and be counted in every single quadrat you do, alternatively all the animals might leave an area you are sampling from at that moment. The way to get round this is by using a method called mark-release-recapture:
Mark: spend an allotted amount of time capturing as many of a species as you can, every time you mark one you must put a mark on it.
Release: put all the members of the species back where you found them.
Recapture: go back at a time with the exact same conditions (e.g. time of day/season/temperature) and spend the exact same amount of time capturing as many of the species as you can. Record the number of animals you caught and the number that had marks on then put the figures into the Lincoln index:
number in first sample * number in second sample
/number of marked individuals in second sample
The marks may cause a disadvantage to the animals, they may die which will affect your data, but, also, it is a moral issue. Other ethical issues include taking removing animals from their natural habitat and disturbing a habitat by entering it