Plot Sampling: Density
How do scientists measure the size of plant populations in
a study area? They could count every individual plant, but you could imagine
how long this would take for small plants in a large area - just imagine trying
to count all of the individual blades of grass in a football field! When conducting
plant ecology research on invasive or other species, scientists often select
several smaller sample plots inside a larger study area. The scientists thoroughly
study the smaller areas, and use information from these plots to make generalizations
about the larger study area.
For plants that are large and relatively easy to count, scientists
typically count each stem in a sample plot to determine the density.
Density is the number of plants per area. For information on how to measure
smaller plants, read about Protocol 2 and percent cover in Invasion
Ecology. In this protocol we have included instructions for measuring
density in 1 m2 plots. If this area is too big (for example, because
plants are too dense), or too small (for example, because plants are too sparse),
you may alter it to suit your needs.
Before you make measurements on your plants, you will need
to determine what species you hope to study and in what study areas you will
work. Refer to the Early Detection
Protocol for information on how to select a study area and study species.
Once you have determined your study area and what species
you will be measuring, you need to select sample plots. There are many ways
to select plots, but using an accepted method will help you avoid bias. For
example, imagine a research project on the side of a steep mountain. Getting
to plots at the top of the mountain takes more effort than getting to plots
closer to the bottom, so at the end of a long day of fieldwork, researchers
might - perhaps unknowingly - select more plots near the bottom. To avoid this
and other types of bias, scientists have developed several methods to locate
plots in a large study area. Read about
two methods you may use for locating your sample plots.
- Decide how many plots (also called quadrats) you will
sample. A rule of thumb is either a minimum of ten plots or one 1 m2
plot per 100 m2. The number of people you are working with may affect
how many plots you use.
- Locate the sampling plot using either random
or stratified selection methods.
- Lay out the 1 m2 quadrat
on the ground at the first sampling point you have chosen.
- Count the
number of stems in the plot (i.e., density). Record this number separately for
each species. If the stems are numerous, you can avoid double counting by starting
at one corner of the plot and moving systematically across the plot. You also
can hold or mark stems you have counted, or place a plastic wand or thin stick
between the counted and uncounted stems. For information on measuring percent
cover (as opposed to density), read about Invasion
Ecology. Percent cover measurements are useful in plots containing
a large number of smaller plants.
- If you encounter species you cannot
identify, (a) describe the species (for example, 1 m tall, purple flowers), (b)
collect a sample from outside your plots, (c) try to identify the species using
identification keys or by asking experts, and (d) press the specimen for use as
reference for future surveys.
- Repeat this procedure for all of your plots.
You can use the data from your plot surveys to make generalizations
about density in the larger study area. To do this, you will need to compile
the results for all the plots in a particular area. You can determine the average
density for each of the species you measured for all your plots. After gathering
together all the plot data, calculate the average density for each species
found in your large study area using the formula below.
Average species density = [ (density in plot 1) + (density in plot 2)
+ (density in plot X) ] / total number of plots
For example, consider a series of 5 plot surveys conducted
in a schoolyard. Each plot is 1 m2. Purple loosestrife was found
in 4 of the plots in the following amounts: a) 85 stems, b) 53 stems, c) 64
stems, d) 33 stems.
Average species density = [ 85 + 53 + 64 + 33 + 0 ] / 5 = 47 stems/m2
You can use the average density to compare different species,
to compare the same species from several different sites, and/or to compare
sites where control measures have and have not been implemented. However, you
cannot compare species measured using density with species measured using percent
A Look at Variability
much variability did you find in your results? Was the species density similar
in all of your plots? For example, did some of your plots have very few or
no plants, while other plots had a great number of plants (high variability)?
Do you think your average species density is an accurate representation of
what is growing in the larger study area? If not, would sampling more plots
help? What could be some reasons for any variability you see?
Questions to consider
You may be
interested in designing experiments that will help you answer some of the following
- Did you find any species that were present in all plots
or had very high densities? What are some reaons that would explain this? How
could you test your reasons? What about species that were not present in all plots
or had very low densities?
- How did the density of the invasive species
compare with the other species in the study area? If you go back to the same site
during several seasons, you may be able to monitor a change - if so, what is the
- If you have more than one study area, how do the densities of
a single species compare between the two sites? Are the densities very similar,
or are they very different? What are some reasons that might explain this?
Invasion Ecology, an EI publication,
has additional protocols and suggestions for how you can measure the impact
invasive species have an ecosystems.