FIELD CHARACTERIZATION OF AN ECOSYSTEM
Ecosystem Characterization:
Evaluating and measuring populations and distributions
in a disturbed Oak-Hickory forest association.
Ecosystem Introduction
The Shenandoah Valley lies in the Mid-Atlantic Highlands between the Blue Ridge Mountains
and the Allegheny Mountains. Vegetation in the valley is dominated by dry forest called an oak-hickory
association. On the JMU campus this association is best seen from the JMU Arboretum extending south
along the East side of University Avenue. The forest in this area is relatively young, as most of the area
was cut over and farmed in the early part of the 20th century. During the depression era many of the
farms in the valley were abandoned as a result of both the troubled economy and the switch from horses
to automobiles and trucks. Shenandoah regional farms had supplied small grains to feed the horse
populations in major cities like Washington D.C., Richmond and Baltimore.
Arboretum land was not without disturbance between the 1930’s and the present, though the
forestland was basically unmanaged. Trees came back strongly and today there is a mix of eastern
hardwoods, with oak and hickory the dominant species. There are a few remnant trees of the formerly
dominant American Chestnut in the woods, these having been wiped out by a stem destroying fungus
called chestnut blight. Trails were built though the woods a number of years ago, mainly for the use of
runners and other transient visitors. The arboretum itself was established in the mid-80s, primarily
though the efforts of Dr. Norlyn Bodkin of the biology department. Most of the arboretum’s work is
focussed on the bottomland of the ravine in the northern end of the forestland. There Dr. Bodkin and his
team have planted numerous species, most of which are native to Virginia.
Today the arboretum is under threat from two fronts. The first and most obvious threat comes
from development of student housing and commercial property around the borders of the forest. Uphill
to the south of the managed area of the arboretum is a student apartment complex, built on steep slopes
with only token planning for erosion control and runoff management. At present any significant rainfall
will lead to extensive slumping of the artificially steep slopes of the development and a thick sediment
layer in the forest area of the arboretum.
The second less conspicuous problem is the presence of introduced, alien species, spread by
people, birds, or wind, throughout the understory of the forest and arboretum. These plants include the
spotted-leaf and purple-flowering pulminaria called “Mrs. Moon”, the deep rooted, yellow flowering
garlic mustard, and two types of honeysuckle, one a vine and the other an erect shrub. These invasive
plants crowd out less aggressive native species, ultimately affecting plant and animal diversity in the
woods.
Species populations and distribution in an ecosystem
Every ecosystem consists of landforms, soil, water, and various species of plants, animals, fungi,
protists (single-celled eukaryotes), and bacteria, adapted to the particular climate. In the arboretum
woodland the dominant species are trees, primarily oaks (northern red, black, white and chestnut),
hickory (shagbark, false shagbark, pignut and bitternut), red maple, black gum, and a few others. These
form the canopy, or upper story of the woodland. Shrubs and small trees to 20 feet or so make up the
next layer. Finally there is a relatively thin herbaceous layer on the forest floor growing through a
blanket of leaves in various stages of disintegration. These decaying leaves store many of the nutrients
available to plants, which are released slowly during the year.
Though we identify species on an individual level, ecologists prefer to think of them in terms of
populations. Characterizing an ecosystem often means obtaining a count of the individuals in a given
area of land, providing an idea of its relative abundance, determining its distribution within that
ecosystem, and evaluating its relationship with other species. Some species have a relatively even or
uniform distribution, others are clumped, and still others are random. Because of their stability and size
we will be looking primarily at the population and distribution of plants. A variety of factors can
influence plant distribution including seed spreading method (wind, mammal, bird, etc.), moisture
content of soils, nutrient availability, exposure to light, soil depth, and much more. Understanding plant
distribution gives an idea of soil type, potential animal populations and so on.
Determining “how many are there?” in a given area is a function of the type of species you are
examining and the size of the area. For example, counting all the chestnut oak over 6 inches in diameter
in a 1 hectare woodlot (that’s 100 meters by 100 meters) could be just a matter of counting every
individual in the woodlot. Expanding the area to include all of the 125 acres (about 50 ha) of the JMU
woodland would daunt even experienced census takers. Ecologists have devised a variety of different
methods to make estimates of populations in given areas. Your task, as groups, will include counting
and measuring various species in the woodland of the arboretum.
Sampling techniques:
When ecologists study a new area they often do a visual survey of a site, obtaining a idea of the
general make-up of the ecosystem. This would include a general idea of the range of plants, animals and
other variables. Then they will come up with a list of questions they want to ask of the area, just as we
are doing below (see your tasks). Next they determine which methods, or sampling techniques, best
answer the questions with the degree of precision and accuracy desired. This method is normally
decided upon based on the preliminary site information and not evaluated until data is collected. A
degree of randomness, or unbiased site selection, is essential. It is easy to skew a sampling by
deliberately siting a “random” sample at a point where the studied species is known to be. How you
randomize is part of your method.
There are numerous ways to take samples in a given area. Some examples are provided in
Figure 1 (below). Good sampling methods are geared to yield statistical significance. This means that
another person using the same method, though different placement of sampling sites within the study
area, would come up with estimates within an acceptable range from your own. This implies that any
method you choose is replicable and is well described in your reports. Remember that assessing the
strengths and weaknesses (accuracy and precision) of a chosen method is an important part of any
report. Three possible methods are described below.
1. Transect lines – These are lines of a predetermined length that start at a randomly chosen
point and head in a randomly chosen direction. If you are looking at plants you would count
every plant of the type desired that lies within a certain distance from the line. For example
you could choose a line length of 100 meters, starting at point A, heading SW. You then
could count all the saplings under one inch in diameter within 2 meters of the line on either
side. This would cover an area of 400 m2, or 4% of a hectare, a sizeable sample. Doing two
more transect lines, starting from different points and going in different, randomly chosen
directions would give minimally adequate data to estimate a sapling population in that area of
the woods.
2. Circles on a transect – An adaptation of the above is to again randomly choose a transect line
of predetermined length, but instead of counting with a distance of the line, count all items
that lie within a certain radius of preset points along the line. These circles could be small
for high density objects, or large, for say counting larger diameter trees.
3. A third possibility is to draw a map of an area to be studied, then randomly select sample
sites of predetermined size and number. Smaller sizes can be used to count groundcover
plants like dandelions in a lawn. Larger sizes would be used to count trees, like red oaks in
the arboretum woods. The number of plots or samples counted could be determined after a
preliminary visual survey gives you an idea of distribution. If a field has relatively even
distribution, a relatively small number of samples may be required. Those species with
sparse, clumped, or random distribution may take comparatively more samples.
It is important to stress the idea of multiple sampling and replicability. Limiting factors are not
uniform within an ecosystem. Soil type, nutrient availability, water holding capacity, slope, sunlight
exposure, and many other limiting factors, vary from point to point even within what appears to be a
fairly uniform stand of woods. Therefore randomizing selection of good sized sample plots or transects
determines the quality of the information you collect. You must make certain that your selection of
samples is sufficient to give you a range of data points that provides statistically valid data, and that
readers of that method and data report can replicate your process. Failure to do this in the past has led
some scientists to some embarrassingly public failures, such as the “discovery” of cold fusion in the late
1980’s.
Tasks
We will meet in the arboretum. Keep in mind that this is an outdoor lab and wear appropriate
clothing. Sandals and shorts are not recommended as rocks, thorns and other obstacles can cause
problems. Be prepared for any type of weather – we go rain or shine!
After an introduction, you will break into your lab groups and choose a species to study.
Estimate a distribution pattern from your observations, then choose a sampling method. Randomly select
your starting points or sampling sites, then do a count of the chosen species. Equipment for this effort
will be provided from the mobile environmental lab vehicle stationed in the maintenance parking area.
Your deliverable for the day will be a single page, in-field write-up of your findings. (BRING
WRITING MATERIALS AND WEATHER PROTECTION FOR THEM!) It will include:
 Names of all group members
 Species of plant selected and reason for the choice
 Preliminary assessment of the type of distribution of this species
 Sampling methods selected and the specifics of the design
 Data, including counts from each sample site
 Population estimate of this species on a per hectare basis (1 hectare is 100 X 100 meters)
 Any change in your assessment of distribution.
This page should be given to your TA prior to your group’s departure from the arboretum.
Figure 1: Examples of sampling techniques for estimating populations and distribution in a given area.
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A. Series of counts within a limited area (0.25 m2, 1 m2, 2 m2 etc.) randomly placed within the study
area. These are then added and this figure used to calculate a reasonable estimate of population over
the larger area. The variation in count between the randomly spaced areas helps determine the
nature of the population distribution.
B. A transect line of predetermined length is placed randomly within the study area and a census is
taken of the population within a preset distance of the transect line. This is then used to determine
population and distribution of the target species over the entire area. Multiple lines are commonly
done.
C. Again a transect line is made, but this time measurement is done with a set of circular areas at preset
distances along the transect line. Knowing the area of the circles can give an estimate of population
(from the total) and distribution (from the counts in the circles).