Lab Activity
Limestone Forest Transect
Physical and chemical factors or variables can vary spatially as well as temporally in a
particular habitat. Often, these abiotic factors can form an environmental gradient, in which a
set of environmental variables changes predictably along a linear axis. The concentration of
salt, for example, in a river decreases from the rivermouth as you head upstream. Similarly,
relative humidity often decreases with altitude as you climb a mountain. In marine systems,
the component wavelengths of visible light get filtered out as you dive deeper in the water
column along the reef.
A dynamic pattern of species abundance and dominance along an environmental
gradient is called zonation. Such patterns arise because species typically differ from each other
according to their physiological requirements (or, alternately, tolerances) for certain levels of a
factor, such as an essential nutrient. The growth and abundance of a species will depend on
the levels of that factor along the gradient. As the levels of that factor change, the species
composition may change. Take, for example, how the intertidal algal community in Pago Bay
changes as wave energy decreases:
Relative Abundance of Macroalgae in Pago Bay Reef Flat
September, 2008
14
Number of Points
12
10
Outer Reef Flat
Mid-Reef Flat
8
Green
Brown
6
Red
4
2
0
0
5
10
15
20
25
30
35
40
45
50
Distance from Outer Reef Flat Toward Shoreline (m)
Figure 1. Abundance of three major divisions of macroalgae on Pago Bay reef flat. A modified belt transect was
used to estimate algal cover by counting the number of hits of each particular group within a 16-point quadrat.
Algae were sampled at 5 m intervals.
As you can see from the graph, red algae dominate the outer reef flat while green and brown
algae are much more prevalent towards the shore and away from the high wave energy zone.
The identification of environmental gradients and the zones that develop along that
gradient is a common ecological question that can shed light into patterns of resource use
among species. Such information is also important for resource managers who often must
determine the consequences of altering some environmental regime (e.g., streamflow, canopy
cover, etc.).
Once a gradient is identified (e.g., salinity, windspeed, pH, light intensity, etc.) then the
next step is to characterize the community along that gradient. A variety of sampling methods
can be used to estimate the relative abundance and distribution of organisms along a gradient.
For sessile (i.e., attached) organisms, such as macroalgae and plants, one popular technique is
the line transect. This method involves extending a long tape measure, or meter tape, along
the gradient and identifying the plants that occur within regularly spaced intervals. At each
interval, the identity of the tree is recorded, as well as additional information, such as linear
distance covered by each individual plant. It is not uncommon to have multiple counts of trees
at a particular point along the transect because, after all, the forest is a three dimensional
structure!
Instructions
The purpose of the sampling exercise is to characterize the limestone forest behind
George Washington High School in Mangilao. The trail through the forest runs roughly
perpendicular to the cliffline. Before we begin, what sort of physical and/or chemical variables
would you expect to change as you head through the forest towards the cliffline?
______________________________________________________________________________
______________________________________________________________________________
The class will divide into two to three groups. Each group will establish a separate 50 m
transect along the side of the trail and will identify the species that occur above and below the
transect line within 5 m intervals. Specifically, for each interval you will (1) identify and record
all plant species occurring within each interval, (2) estimate the approximate number of
individuals of each species within an interval, and (3) visually estimate the approximate length
of transect line that the plant occupies or overlaps. Because of the three-dimensional nature
of the forest, remember that it is possible to have several species within a 5 m length. We will
analyze the data at a later class.
A.1
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
0–5m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tangan-tangan may have multiple representatives within an
interval
A.2
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
5 – 10 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.3
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
10 – 15 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.4
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
15 – 20 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.5
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
20 – 25 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.6
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
25 – 30 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.7
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
30 – 35 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.8
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
35 – 40 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.9
Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
40 – 45 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
A.10 Date: __________
Group Members: ______________________________________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle forest, cliff
Interval #:
45 – 50 m
Notes:
Species:
Sample
Number*
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
Species:
Intercept Length (l)
1
2
3
4
5
6
7
8
9
10
11
12
13
Totals
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
B. 1 Date: __________
Group Members: ____________________________________________________________
Habitat Location: ________________________
Transect Location : ______________________________________________
Species
(i)
Number of
individuals
encountered
(entire
transect)
Transect Interval (m)
0–5
5–10
10-15
15-20
20-25
25-30
30-35
35-40
40-45
Total
for
45-50 Species
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
B.2 Date: __________
Group Members: ____________________________________________________________
Habitat Location: ________________________
Transect Location : ______________________________________________
Species
(i)
Number of
individuals
encountered
(entire
transect)
Transect Interval (m)
0–5
5–10
10-15
15-20
20-25
25-30
30-35
35-40
40-45
Total
for
45-50 Species
* Sample number indicates a specific individual plant for a given species within an interval. E.g., tanga-tangan may have multiple representatives within an
interval
C.1 Class Summary of Data from Line-Intercept Plant Sampling
Date: ______________
Observers __________________________________________________________________________
Total Transect Length (L): ______ m
Species
(i)
Number
of
Individuals
(ni)
Totals
∑n =
Linear
Density
Index
(IDi)
∑ID=
Relative
Density
(RDi)
∑RD=1.0
*Data collected in these columns are from sheet B.1
Total # of Transect Intervals: _________
Present in
how many
transect
intervals?
(ji)*
Frequency
(fi)
Relative
frequency
(Rfi)
∑f=
∑Rf = 1.0
Intercept
length (li)*
Linear
coverage
index
(ICi)
Relative
coverage Importance
(RCi)
value (IVi)
∑l =
∑IC=
∑RC=1.0
C.2 Class Summary of Data from Line-Intercept Plant Sampling (continued)
Date: ______________
Observers __________________________________________________________________________
Total Transect Length (L): ______ m
Species
(i)
Number
of
Individuals
(ni)
Totals
∑n =
Linear
Density
Index
(IDi)
∑ID=
Relative
Density
(RDi)
∑RD=1.0
*Data collected in these columns are from sheet B.1
Total # of Transect Intervals: _________
Present in
how many
transect
intervals?
(ji)*
Frequency
(fi)
Relative
frequency
(Rfi)
∑f=
∑Rf = 1.0
Intercept
length (li)*
Linear
coverage
index
(ICi)
Relative
coverage Importance
(RCi)
value (IVi)
∑l =
∑IC=
∑RC=1.0
Lab Activity
Transect Data Analysis
By now, you should be familiar with the techniques that are associated with conducting a line
transect. Recall that you will have broken a 50 m transect line into 5 m intervals and then you (1)
identified each individual plant species within that interval and (2) estimated its linear coverage over (or
under) that 5 m segment. Using this data, we can then estimate the number of individuals of a species
across the sampling space (i.e., the density) and the average individual contribution from each species
to the total vegetative cover (i.e., importance). Both density and importance values are useful indices
for assessing the composition of the forest community and the potential influence of species.
Calculation of Intercept length for each plant species (Data Sheets A.1 through A.10)
In order to estimate the density and
importance of each species of plant, we first
need to convert our field observations into
absolute distances, or intercept length (l)
for each individual plant. Sheets A.1
through A.10 are identical to the data
sheets you used to record your raw data in
the field.
Use attached forms to transcribe
your data into actual coverage data. For
example, in the 30 – 35 m interval, you may
have noted that one individual of tangantangan covered the length from 30.2 m to
33.1 m. The actual distance covered would
thus be 33.1 – 30.2 = 2.9 m. Similarly, you
may have observed another specimen of
tangan-tangan that covered 31.3 m to 32.5
m for an intercept length of 1.2 m:
Use the new sheets to convert your field
measurements to linear distance for each species.
Then total the overall distance each species covered
in each interval and fill in that information at the
bottom of each data sheet (A.1 – A.10):
Summary Coverage Data Across the Transect (Data Sheets B.1 through B.4)
Sheet A.7
Now that you have converted your observations to absolute
lengths and calculated the total length each species covered in a
particular interval, you can proceed to collate the coverage data
for each species across the entire transect. In Sheet B.1, list the
different species you found throughout the entire 50 m
transect. Then, write down the total length it occupied for each
interval (i.e., the number at the bottom of each column on
sheets A.1 – A.10). Also, write the total number of individuals
you encountered along the entire transect. Lastly, sum the total
transect interval each species occupied by summing the
individual transect interval lengths:
Sheet B.1
Class Summary of Data from Line-Intercept Plant Sampling (Data Sheets C.1 & C.2)
The final calculations required to estimate density and importance values involve transcribing
the species identity (i), number of individuals encountered (ni) and intercept lengths (li) into the
appropriate columns on data sheet C.1 and, if necessary, C.2.
The remaining columns are based upon knowledge of i, ni, and li. They are:
1) Linear density index (IDi)
This value reflects the average number of individuals observed over the entire transect length.
It is calculated as:
IDi = ni / L
Where ni is the number of individuals encountered of that species and L is the total transect
length.
2) Relative density (RDi)
The relative density of a species is the number of individuals of a species (ni ) divided by the
total number of individuals of observed (∑n ) (inclusive of the species you are calculating RDi
for):
RDi = ni / ∑n
3) Presence in intervals (ji)
This is the number of line intercept intervals containing species i. If, for example, 5 of the 10
intervals in a 50 m transect contained tangan-tangan, ji = 5.
4) Frequency (fi)
The frequency of a species is simply:
ji / k
Where ji = the presence of a species across intervals and k = the total number of transect
intervals. Using the previous example, if ji= 5 and we have 10 intervals for our 50 m transect
line, then ji / k = 5/10 = 0.5.
5) Relative frequency (Rfi)
The relative frequency of a species reflects the proportion of observations that were of a
particular species. Because it is a proportion, all of the individual values of Rfi should sum up to
1.
Rfi = fi / ∑f
Where fi is the frequency of a particular species and ∑f is the sum of all of the individual
frequencies for all species.
6) Linear coverage index (ICi)
Because we have estimates of the length of each transect interval that species covered, we can
calculate the linear coverage index, or (ICi).
ICi = li / L
Where li is the sum of the intercept lengths for species i and L is the total transect length (L = 50
for a 50 m transect while L = 100 for a 100 m transect).
7) Relative coverage (RCi)
As with relative density, relative coverage is the proportion of transect line that a species
occupied relative to the sum of all the lengths from all the other species. Again, the sum of all
RCi..j will equal 1.
RCi = li / ∑ l
where li is the total length of transect that a species occupied while ∑ l is the sum of all the
intercept lengths that all species covered. Alternately, relative coverage can be calculated as:
RCi = ICi / ∑ IC
Where ICi = the individual linear coverage index for species i and ∑ IC = the sum of ICi for all
species.
7) Importance value (IVi)
The importance value is an expression of the relative density, frequency, and coverage of a
particular species. It is expressed as:
IVi = RDi + Rfi + RCi
What does importance mean? It supposedly reflects the theoretical influence of a
species in the community based. However, note that two species may have similar importance
values despite having different values of RDi, Rfi, or RCi. Ergo, caveat emptor!