Estimating plant and animal populations

There are three main ways of measuring the population of plants living in a habitat:

Density - if plants are large enough to be seen clearly, the number of them is counted in the quadrat (this gives the density per square metre). This is an absolute measure (not an estimate)

Frequency of occurence - used when individual members are hard to count. It is the number of times a plant species occurs in a given number of quadrats. It is often given as a %. It indicates the probability of finding a species in a series of quadrats examined in an area of interest.

Percentage cover - The percentage cover os each species (need a gridded quadrat). Lots of data can be collected easily and is useful when a particular species is abundant or difficult to count - it is an estimate by eye of the area within a quarat a species occurs.

• For each approach samples should be taken at many different points - more reliable results.
• The mean should then be calculated to get an avg value for an organism per square metre.
• To work out the total population in an area that has been sampled, multiply the mean value by the total area.
• Estimate abundance of each species using SACFOR (can be semi-quantative)

Animals are always moving/may be hidden, therefore it is difficult to calculate population size.

Capture-Mark-Release-Recapture is a technique used to estimate population size. (Motile)

• Involves capturing as many individuals of a species in an area as possible.
• The organisms are marked and then released back into the community.
• Time is allowed for the organisms to redistribute themselves.
• A second samples is taken and the proportion of marked individuals is used to estimate population size (greater no. of individuals marked from the recapture = smaller pop.)

The species evenness in an area can then be calculated by comparing the total number of each organism present. Populations of plants or animals that are similar in size or density represent an even population. Species evenness can be expresses as a ratio between the number of each organism present.

Abiotic factors are the non-living conditions in a habitat. They have a direct effect of the living organisms that live there.

e.g. light, water availability - to enable them to draw conclusions about organisms present and the conditions they need for survival, scientists normally measure these conditions at every sampling point.

Many abiotic factors can be measured quickly and accurately using a range of sensors, which are useful for a number of reasons:

• Rapid changes can be detected
• Human error is reduced
• High degree of precision
• Data can be stored or tracked on a computer.