Selecting Random Samples
Units must be selected without bias. A fairly
easy way to do this is to divide a given area into relatively small quadrats
(eg. 4m x 10m). Set up a grid with axes, and lable points along the
axes. Use a random number generator to pick a suitable number of x coordinates
and y coordinates (nuber of coordinates depends on number of samples one
wishes to take).
One can also select an arbitrary starting point, and
use a random number generator to find a distance, and a direction to travel
from that starting point.
In order to locate these points once they have been identified,
one can use flags to form a grid, or tape, or pacing. Perhaps the best
option, however, is GPS.
Monitoring Specific Species
The majority of current monitoring is focused on birds.
Complete population counts for any species are only effective for small
areas, and conspicuous animals. A better option is sample counts.
Indices refer to relative abundance of species. A
common problem is that as populatiion density increases, an index can become
"saturated." For example, one way to index an animal with fairly low
population density is to record the percentage of plots occupied by that animal.
As the population density increases, however, all plots may be occupied,
and the index is no longer useful. It is usually after about 70% that
such an index begins to become saturated.
Field Methods for Fish
- one can simply use a fishing pole, and record changes
in the length of time required to catch a certain number of fish
- mesh nets are often used
- electroshocking temporarily stuns the fish, allowing
them to be sorted by species and replaced without significant harm
- a problem is that fish are less evenly distributed
than most land animals; this requires less extensive monitoring at many
sites, rather than more extensive
monitoring at fewer sites.
Field Methods for Amphibians and Reptiles
A good way to monitor amphibians and reptiles
is to combine two monitoring methods.
- Visual encounter
surveys along a transect line. This involves travelling along a specified
line, and noting the number
of individuals seen, heard, or simply seen evidence of their presence. This
method is particularly good for
- One should also
count the total number of individuals using quadrats
Amphibians in aquatic areas:
- the best method
in this case is auditory surveys. For example, some frogs can be heard
up to 500 m away.
Field Methods for Birds
- Birds can be monitored using "point counts." This
involves remaining in one place and counting the number of birds seen or heard
for a particular duration of time. This, unfortunately, will not work
well for secretive birds, or for large, soaring birds such as hawks. It
also will not work for aquatic birds.
- Another possible method is to broadcast a particular
bird call and record any response.
- In some cases, droppings can be counted.
Field Methods for Large Mammals
In this case, line transect sampling is best.
Elzinga, C.L., Salzer, D.W., Willoughby, J.W.,
Gibbs, J.P. Monitoring Plant and Animal Populations. London:
Blackwell Science Inc.,2001.
Heyer, W.R., Donnelly, M.A., MDiarmind, R.W., Hayek, LC., Foster, M.S. Measuring
and Monitoring Biological Diversity, Standard Methods for Amphibians.
Washington, D.C.: Smithsonian Institution Press, 1994.
Last update: October 30, 2002