Offwell Wetland Survey
Why use line transects?
transects are used when you wish to illustrate a particular gradient or linear pattern
along which communities of plants and, or animals change. They provide a good way of being
able to clearly visualise the changes taking place along the line. Depending on how
detailed the line transect is, they can usually be accomplished fairly quickly.
Line transects do not produce as much information on
the relative densities of individual species as a belt transect
would do. A line transect tells you what is there, but gives limited information on how
much of it is present. If detailed density information is required then a belt transect
must be carried out instead.
||The transect diagram on the left illustrates the type
of data which is collected using a continuous line
transect . This notes every individual which
touches the line. The data is displayed in the form of a diagram, using symbols for
different species, which are drawn to scale. This is a useful way of being able to clearly
visualise what changes are taking place along the line. It enables patterns of zonation in
species along the line to be picked out. The photographs in the left-hand diagram
are purely there for illustrative purposes and would not normally be included.
|These patterns of zonation can then be
demonstrated more clearly in charts such as the distribution diagram on the right.
Where there is a great deal of plant cover, continuous line
transects are useful only over relatively short distances. If there are a great many
plants touching the line, it is very time-consuming to survey over long distances. It is
also difficult to adequately display the data so that it can be shared with others. This
is illustrated by the continuous line transect diagrams drawn for Transect 1 and 2 through
the wetland (see the 'Results' link at the foot of the page). The original paper transect
diagrams from which these are redrawn required 5 x A4 sheets of graph paper stuck
together to order to adequately display the entire length of the transect! However, where
individual organisms are sparsely distributed along the line (for example in a desert, or
on sand dunes), continuous line transects become workable over longer distances.
As continuous line transects note every individual touching the line, they
can also become very tedious where separating individuals of a species is a problem. An
example of this is plants such as grasses or mosses, which spread through vegetative
growth. Where does one plant stop and another begin? This does not just apply to grasses
and mosses, but to a wide variety of other plants and also some animals. The Yellow Iris and Branched Bur-reed plants
involved in the Wetland Survey are another example. More here
If a particular area does not suit the continuous line transect
method, interrupted line transects may be the answer. These note individuals touching
regular points along the line, for example at every metre mark. These can be carried out
reasonably quickly over long distances and may allow an area to be surveyed fairly
rapidly. Displaying the data collected in line transect diagrams also becomes easier
because the horizontal scale can be adjusted to fit smaller sheets of paper. The
disadvantage in this case is that many of the species present may be overlooked if the
interval selected is too large. To illustrate this, the number of species recorded on each
of Line Transects 1 and 2, using three different line transect methods is shown in the
|No. of Species
||Line Transect 1
||Line Transect 2
Every other metre
In both Transects 1 and 2, the interrupted line transects with an interval of 2 metres
failed to pick up more than 60% of the species which were actually there.
It is important to bear in mind that
the type of method which is most suitable will depend entirely on the habitat under
investigation and on the purpose of the survey. If an interrupted line transect, with
measurements taken at every other metre, had been the sole method of surveying the Offwell
Wetland habitat, it would have been less than ideal. However, in a different habitat it
might be perfectly acceptable. Survey methods have to be decided on an individual habitat
More information on the plus and minus
points of the different types of transect, as related to the Wetland Survey, can be found
on the individual transect diagrams presented in the results section.