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.
Date: __________
Group Members:______________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle
forest, cliff
Interval #:
0–5m
Notes:
Sample
Number*
Species:
Species:
Species:
Species:
Species:
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
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
Date: __________
Group Members:______________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle
forest, cliff
Interval #:
5 – 10 m
Notes:
Sample
Number*
Species:
Species:
Species:
Species:
Species:
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
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
Date: __________
Group Members:______________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle
forest, cliff
Interval #:
10 – 15 m
Notes:
Sample
Number*
Species:
Species:
Species:
Species:
Species:
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
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
Date: __________
Group Members:______________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle
forest, cliff
Interval #:
15 – 20 m
Notes:
Sample
Number*
Species:
Species:
Species:
Species:
Species:
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
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
Date: __________
Group Members:______________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle
forest, cliff
Interval #:
20 – 25 m
Notes:
Sample
Number*
Species:
Species:
Species:
Species:
Species:
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
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
Date: __________
Group Members:______________________________________
Habitat Location: Limestone forest
Transect Location (circle one): outer forest, middle
forest, cliff
Interval #:
25 – 30 m
Notes:
Sample
Number*
Species:
Species:
Species:
Species:
Species:
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
Intercept Length
(l)
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