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What are line transects? -

Line transects are lines that cut through a habitat and along which a researcher moves, counting individuals of a population under study. There are minor variations to transect sampling, one involves normal line transects, the other uses a belt transect (which is actually like a long thin quadrat), another involves the use of distance sampling (used for density estimation) along with line transects.

 

Data collected and estimates obtained from transect sampling -

Data collected during transect sampling includes the transect distance covered by the researcher, the species and number of individuals of the species (under study) sighted during the transect.

When using distance sampling (which is a method used for density and abundance estimation that uses line transects and point transects) the perpendicular distance between the line and the individual sighted at the spot it was sighted is also obtained. When used for studying animals that move the angular distance between the researcher and animal is obtained at the spot it was first sighted and the angle of deviation of the view of the researcher from the line when obtaining a distance measurement of the site of the animal is obtained and the perpendicular distance of the line to the site of the animal when spotted is obtained by using the pythagoras theorem. Distances are measured with laser range finders.

Distance sampling is not required when undertaking relative abundance studies or inventory studies and is only necessary when you want to estimate densities and absolute abundance.

Click here (coming soon) for a detailed explanation of Distance Sampling and the theory behind it.

 

 

Assumptions of Line Transect Sampling -

The sampling method has certain assumptions which need to hold true for accurate estimates and results to be derived from this method. The greater the violation of the assumptions the greater the reduction in accuracy of the estimates. While it may not always be possible to undertake any sampling strategies such as this one without the violation of one or more assumptions at times, the degree of violation has to be reduced as much as possible. The assumptions are given below -

  1. The probability of detection of individuals being sampled is 100% when they are on the line. In other words, any individual that is present on the line should not be missed. This assumption can get violated when conducting aerial transects areas having vegetation cover, or transects by boat when the study involves individuals under the surface of the water.

  2. Individuals being sampled are detected at their initial location prior to any movement in response to the observer. This is important in the case of animals, as when sampling is undertaken for animals, they may view the researcher and move away from the original spot before the researcher can take a distance estimate to that spot. In such cases it is best to mark the spot where the animal was before it moved and measure the distance up to that spot.

  3. All distances are measured accurately (when distance sampling is used).

  4. Individuals are detected independently. This means that the detection of one individual should not be related or influenced by the detection of another. This assumption is often violated to an extent, especially if the population under study consists of individuals living and moving in groups.

5. Transects are laid either randomly or systematically.

 

What biodiversity groups can be sampled using transects?

 

Transects can effectively be used for all groups of biodiversity that are terrestrial and large enough to see with the naked eye, and large marine organisms like whales, that can often be found on the surface of the water.

 

 

Deciding the length of a transect -

 

The length of the transect depends upon a multitude of factors such as the size of the biodiversity under study, the area that needs to be covered and the effort or labour that goes into covering the transect distance. Researchers choose a length depending upon these factors when planning a study. What is more important is that all the transects that are laid in a study are standardized with respect to their size (that is they should all have the same size) and they should be placed randomly in a manner that would cover a significant percentage of the study area.

 

 

How is sampling undertaken using line transects -

 

All transects should be laid either systematically or randomly depending on the strategy chosen for the study. For smaller organisms and studies conducted in a relatively small space (like when transects are used for sampling plants) a thin rope can be tied between stakes at the two end points of the transect to serve as the transect. For sampling across larger areas (such as sampling done for ungulates like deer) the transects can cover a distance of several kilometres and have trees or other large objects on the path of the transect marked with paint to indicate the direction of the transect, or GPS devices with way points serving as the endpoints and a track connecting the two can also be used. Data is collected by a researcher that travels along the length of the transect on foot, by air, road or by boat.

Click here  to go to learn how to lay line transects in a systematic or random manner.

 

road transects

 

Transects sampling can be done along roads, where roads are the transect.

 

line transects

 

Distance sampling which uses line transects requires the perpendicular distance from transect to individual for each individual to calculate densities using a mathematical formula. When sampling animals, the angular distance from researcher to individual and angle of view are used to calculate the perpendicular distance as the animal will move in response to the approaching researcher.

 

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References

Southwood T. , and P. Lenderson. Ecological Methods - Published by Blackwell Science Ltd.