As part of research projects, researchers often need to have an estimate for the number of animals of various species in an area. When studying carnivores for instance they need to know both the populations of the carnivores being studied, but also the populations of the various prey species in the area.
Actually counting every animal in a large survey area would be extremely difficult, time consuming and expensive, so a number of techniques have been developed which allow researchers to survey a smaller area and then use the results to provide an estimate for the number of animals in the entire study area. A brief overview of some of these techniques is given below. It is not intended to go into much detail on the techniques, and the statistical methods of analysis used but to provide a simplified overview.
In this technique, the study area is divided into a grid. A number of grid squares are chosen at random and these squares are surveyed with the numbers of plants or animals of the species being counted that are spotted within the squares being recorded. These results can then be used to estimate a total figure for the entire study area.
As an example, if the study was to determine the number of acacia trees in an area, the area could be divided into a 10x10 grid (100 squares in total). If 5 squares were chosen at random then these grid squares would be surveyed to count the number of acacia trees in each square. If the survey found a total of 20 trees spread across the 5 squares surveyed then this gives an average of 4 trees per grid square. Multiplying this by the number of squares (100) gives an estimate of 400 acacias in the study area.
This technique allows an estimated population figure to be obtained without the time and expense of surveying the entire study are, but is less effective with animals than with plants or trees as animals move in and out of survey squares and thus distort the results.
This technique uses the same principles as Quadrat sampling but instead of dividing the area up into squares to survey it uses long narrow strips. The person doing the survey travels along the centre of the strip and counts the number of animals of the relevant species on either side, within the width of the strip. Any animals outside the strip are ignored and not recorded.
As with strip transect sampling, the observer travels along a line. Whereas in strip transect sampling the observer only records sightings within the width of the strip, in line transect sampling, all sightings are recorded along with the perpendicular distance from the survey line to where the animal was sighted. By using mathematical equations and statistical analysis, the population density and overall population count for the area can be obtained.
In some cases the animals being surveyed may be difficult to spot - for instance if they are secretive in their habits or are nocturnal. In this situation alternative methods of counting can be used. In the case of apes, a researcher might choose to count the number of their nests. Another option for a number of species is to count dung. For this to be effective it is necessary to first have information about the frequency that the animal deposits fresh dung, to know how long the dung will still be visible in the particular environmental conditions of the study site, and to be able to distinguish between the dung of the target species and that of other species found in the area. By using this data and a count of the amount of dung found in the survey, then an estimated figure for the number of animals of that species in the study area can be obtained.
Surveys can be carried out either on the ground (on foot or in a vehicle) or by using aircraft. Each method has its own advantages and disadvantages and these have to be considered when designing a survey to choose the best option
Aerial surveys
Ground surveys
Aerial surveys require the aircraft to have certain special modifications. Standard aircraft instruments measure the aircrafts height above sea level, but for aerial surveys (particularly strip transect surveys) it is important to keep the aircraft at a fixed height above the ground rather than above sea level. A radar altimeter accurately measures the exact height above the ground when flying at low level, which allows the pilot to maintain a constant height above the ground.
Normal doors and windows in a light aircraft tend to have very limited visibility when looking down and to the sides of the aircraft. To make survey work more practical, special observer doors and windows can be installed to improve visibility.
For strip transect surveys, the observer needs to be able to know which animals are inside or outside the strip being surveyed. Struts can be attached to the aircraft which means the observer can easily identify the upper and lower limits of the strip for the survey. The image below shows two struts attached to a maule aircraft for survey work, and a group of animals is just visible between the two struts.
When an strip transect sampling aerial survey is planned, the width of the strip has to be decided and this is done in conjunction with the positioning of the marker struts and the aircraft height - the higher the aircraft flies, the wider the strip that is visible, but equally flying higher makes it harder to identify animals, so a balance has to be found
© Predator Conservation Trust.
