Biology 150.02--Fall 2000
An Introduction to Sampling: Vegetation Analysis of Prairies
To begin our study of prairies, we will undertake the critical first step in any scientific endeavor,
careful description. The goal of descriptive studies is to seek patterns in nature which are then best
tested experimentally (although well-planned descriptive studies can also be used to test hypotheses
when experimental approaches are not feasible). As you will see, descriptive studies require a great
deal of forethought and can be highly quantitative -- patterns are most precisely described
mathematically rather than simply verbally. In our first study, we will compare a number of
reconstructed prairies at CERA and remnant prairies in the local region. Our data might help us
address whether the reconstructed prairies at CERA are representative of "real" prairies, or whether
different management decisions made at CERA have different effects on the biological
communities there. Hopefully, they will raise a number of questions that we can later address
experimentally.
One of the central challenges in ecology is to determine what factors control the abundances of
different kinds of organisms (community structure) and the movement of energy and materials
through ecosystems. These factors include the amount of incoming energy, the availability of
nutrients, the frequency of disturbance, and/or the nature of the community's feeding relationships
(called its trophic structure). One of the first concerns we should address if we want to measure
any aspect of a community is how we deal with spatial variation. How can one know that a
measurement of a physical or biological parameter is an accurate representation of a community
when it varies within a relatively small area?
What is a sample?
A sample is an independent subset of the population that you are studying. When you do a
biological study (experimental or descriptive), you are studying a population of organisms, be they
plants, animals, or fungi. If we were to analyze the prairie vegetation at CERA, the population
would be all of the plants growing in prairies at CERA.
Why sample?
We sample for one simple reason: in most biological studies, it is logistically impossible to
enumerate the entire population of organisms. Imagine having to individually count all of the plants
in even a small plot of prairie. Or trying to count the individuals in a population of mobile animals
such as butterflies. A biologist would only be able to do a single study in his or her lifetime! Not
surprisingly, sampling plays a very important role in biological studies.
The three types of sampling
There are three basic ways to sample a population: haphazard, random, and systematic sampling.
Haphazard sampling is the simplest type of sampling: you just go into the area you are studying
and arbitrarily decide where to take your samples. It is also the least desirable way to sample for
one simple reason: your samples may be biased. For example, you may unconsciously decide to
sample plants in areas where the vegetation is healthier, biasing your results. For logistical reasons,
you sometimes have no choice but to haphazardly sample, but it is never desirable because of the
potential for bias.
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Biology 150.02--Fall 2000
Random sampling is the best type of sampling because it is unbiased (unlike haphazard sampling).
When you randomly choose where to take your samples from, you can be assured that your samples
are an unbiased subset of the population. So if you remember only one thing about sampling, let it
be this: sample randomly whenever possible!
Systematic sampling falls between haphazard and randomly sampling, both in ease of sampling and
desirability. In systematic sampling, you take your samples at some regular interval (say every 2 m)
across the whole population you are sampling. Researchers studying sessile organisms such as
plants or intertidal marine invertebrates commonly use systematic sampling. For these organisms,
running a transect or series of transects across the population and taking samples at fixed intervals is
simply the easiest thing to do. While systematic sampling is exceedingly easy to do, it suffers from
the same problem as haphazard sampling: possible bias. If there is periodic variation in the
population you are studying, then systematic sampling can yield a biased set of samples. For
example, if soil moisture increases and then decreases every 2 m, then systematically sampling
every 2 m could result in a biased set of samples. Fortunately, there is little evidence of periodic
spatial variation in natural systems (although temporal variation in natural systems is often
periodic). Although systematic sampling is unlikely to be biased, random sampling is generally
preferable.
How many samples?
Within reason, the more samples you take, the better, as your populations estimates will be more
precise. Having said that, you should gauge the number of samples you take by how variable
whatever you are sampling is. For example, if you wanted to look at the effect of fire on
populations of prairie plants and insects, you would take more insect than plant samples. Why?
Because insect populations vary more in time and space than plant populations do. To get good
estimates of insect populations, you would simply need more samples. Unfortunately, you often do
not have good information on the natural variation in whatever you are measuring. In these cases,
you will have to do the best you can.
Today’s exercise
You will be divided into research teams, each of which will undertake one of the following sets of
measurements. Please read the descriptions of each of these, however, as you may find other teams'
results useful in interpreting your own. Before you start collecting data on Saturday, you need to
answer, in your field notebook, the questions listed on the "Investigations Planning Form" (p. 10 of
Investigations).
Sampling scheme for this exercise -- Taking a truly random sample in an area requires sampling
every position with equal probability. That is often impractical, especially for very large areas.
You'll do something even simpler (but not quite as likely to be representative): sampling random
points within a haphazardly chosen 30 m section.
Choose an arbitrary position along one edge of one of the prairie, and lay your 30 m tapes alongside
it. Then, use a random number table to choose five random positions along that line (i.e., the first
five numbers between 0 and 30 in an arbitrary column or row of the random number table). Place
flags at those points to mark them. Then at each random point, lay out your meter tape
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Biology 150.02--Fall 2000
perpendicular to the road, pick 2-4 random point along it, and measure there. [See directions below
to for how many points to sample per prairie.] Then move to the next point and repeat.
Group 1 -- Physical conditions
Physical conditions have important effects on the performance of individual organisms and thus on
the dynamics of their populations. Temperature, of course, affects a multitude of physiological and
biochemical processes. Relative humidity (RH) is the amount of water vapor in air relative to the
maximum that air can hold at a given temperature (warmer air holds more water), measured as a
percentage. Organisms are essentially saturated with water, so the lower the relative humidity, the
faster they lose water to the air. Wind is a source of mechanical stress, and it increases the rate of
water loss from organisms (thus it both cools and desiccates them). Light radiation contains energy
that heats organisms, and it provides the energy autotrophs ("self-feeders") use for photosynthesis.
1. Assemble a meter tape, a meter stick, a random number table, a digital thermometer/
hygrometer, an anemometer, and a light meter.
2. Review the operation of your instruments.
3. At each sampling point (see above procedure), take a vertical profile of air temperature,
RH, windspeed, and light intensity, measuring each condition at the soil surface and above
the highest vegetation.
4. Obtain 10 sampling points for each prairie.
Group 2-- Soil conditions
Organisms that live partly or entirely in soil, such as vascular plants, bacteria, fungi, soil
invertebrates, and burrowing vertebrates, are fundamental parts of the movement of energy and
nutrients through the prairie ecosystems. Soil conditions are both a reflection and a determinant of
such important processes as photosynthesis (for plants), herbivory and decomposition. Two of the
most important parameters that effect these processes are availability of water and temperature.
1. Assemble a meter tape, a random number table, a moisture probe, a soil thermometer, a
soil sampler, small paper bags and a marking pen.
2. At each sampling point, record soil temperatures at 2 depths by inserting the soil
thermometer in soil at 1 cm and 4 cm. Use the probe to measure soil moisture at each point.
3. Push the soil-core sampler straight into the soil, twisting clockwise (only!) if you need
to, until you reach a depth of 10 cm (marked on the soil-core sampler).
4. Back at the lab, use a top-loading balance to determine the wet mass of the soil+bag.
After drying for 2 days in the oven, determine the mass again. Finally, determine the mass
of the bag. Don't throw away your soil! You will use a small portion of each sample to
measure soil carbon content back on campus, as described in an additional handout.
5. Obtain samples for 10 points for each prairie.
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Biology 150.02--Fall 2000
Group 3 -- Above-ground plant biomass and necromass per area
These biological features of the environment are ecosystem-level properties, as they measure the
amount of material (and energy) tied up in different components of the living environment,
measured on a per-area basis. Biomass is currently living material, which in the prairie consists
mainly of green plant shoots. Necromass is formerly-living material that is not yet decomposed. At
this time of year, necromass consists of "litter," pieces of dead plants from last year that have fallen
onto the soil surface.
1. Assemble a meter tape, a random number table, two 0.5 by 0.5 meter quadrat frames, a
pair of clippers for each group member (do not lose these!), paper bags, and a marking pen.
2. At each sampling point, place the quadrat frame on the soil surface and clip off all plant
material at the base. Put dead material (including plant litter and animal carcasses) into one
labeled bag (lab day, group names, prairie, and sample number) and put live material into
another. Clip the material into small pieces for ease of handling. The samples will be ovendried back at the CERA lab for 48 hours.
3. After drying, measure the mass of each full bag -- then empty the bag and get the mass of
the bag itself.
4. Obtain samples for 10 points for each prairie.
Group 4 -- Dominance and height of grass
As you know from your reading assignments, different climatic conditions, soil types and
management practices can favor certain species over others. This group will measure the success of
grasses by determining their "% cover" (how much area they take up) and their height, a reflection
of plant vigor.
1. Assemble a meter tape, a random number table, two 0.5 by 0.5 meter quadrat frames and
two meter sticks.
2. At each sampling point, place the quadrat at the soil surface and estimate the percent of
the area taken up by grasses (practice this a couple times before you begin taking data).
3. Measure the height of the tallest grass plant in the quadrat.
4. Obtain samples for 20 points for each prairie.
Groups 5 and 6 -- Composition and density of forbs
These groups will measure the success of non-grass species at each site, measuring both the number
of species and their density (number per unit area). The species diversity can be considered a
measure of the "health" of the community, e.g., native, undisturbed communities are often more
diverse than reconstructed communities. Two groups will each sample multiple transects at each
prairie site; your instructors will monitor your progress and let you know when to stop.
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Biology 150.02--Fall 2000
1. Each group will need a meter tape, a meter stick and an identification manual.
2. You will be sampling "belt transects" by determining a random point on the edge of each
prairie, laying the meter tape perpendicular to this point, and then counting the number of
flowering forb individuals in each species within 0.5 meter on each side of the tape.
3. Your instructors will help you identify the species if necessary.
Groups 7 and 8 -- Responses of individual species
Individual species respond in different ways to local soil and management conditions. Height of the
plant is a measure of plant vigor and number of flowers an estimate of reproductive potential.
Interesting species to monitor would be remnant-dependent species (i.e., those that are rare in
disturbed prairie habitats) and invasive species (i.e., alien species that invade disturbed prairies and
displace native species). Your instructors will help you identify a species to focus upon.
1. Each group will need a meter tape and 2 meter sticks.
2. If the species exists in high density, you can take pick random points as described above,
measuring the plant closest to your random point. If the species is at low density, sample
"belt transects" by determining a random point on the edge of each prairie, laying the meter
tape perpendicular to this point, and then measuring all the individuals within 0.5 meter of
the tape.
3. Measure the height of each plant and, if possible, count the number of flowers or
inflorescences (groups of flowers).
4.You should try to measure at least 20 plants of your focal species per prairie.
RECORD ALL OF YOU DATA IN YOUR FIELD NOTEBOOK!!!
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