Marine Ecology Field Lab - Biology 160L Spring 2000


Dune Plant Communities - Determining the Distribution, Abundance and Dispersion of Organisms Based on Cover


Field ecologists are usually unable to measure the entire population of organisms or variables being studied. For example, if we wanted to know the average length of Batillaria attramentaria in the Elkhorn Slough we would NOT measure all of the snails in the population. Instead, ecologists usually depend upon sampling techniques to reduce the number of measurements that must be taken. The basic idea of all sampling techniques is to measure some of the individuals in a population but to be able to make inferences about all the individuals in the population of interest. Sampling techniques vary with the nature of the question being asked and the population sampled, but desirable characteristics of all sampling techniques are that they are:


1) Unbiased; the sample should be representative of the population as a whole. For instance, in sampling our snails we would not want to use a technique that did not sample small snails.


2) Repeatable.

3) Logistically feasible in the field.


In this lab we will use two different sampling techniques to: (1) estimate the percent cover of plants and substrates in two different coastal dune communities, and (2) determine if sampling methodology affects estimates of populations.


Analysis of Vegetation


There are several methods used to obtain quantitative information about the composition and structure of plant communities. Some methods are more appropriate for certain vegetation types than others. In this lab we will sample the dune communities at Elkhorn Slough and Franklin Pt (north of Ano Nuevo). Two techniques that can be used with this type of vegetation are the quadrat sampling method and the line intercept of line intercept method. Each group of students will use both methods along the same transect and compare the results between the two methods. Transect lines (50m+) will be set up perpendicular to shore. In consultation with the instructors groups will decide their placement along the shore. Each transect line will start on the beach in front of the dune and will run across the dune crest into the back dune. With these data the class will be able to compare the vegetation found from the foredune - through the mid - to the back dune from multiple transects.




Transect placement - first the groups must decide two things; 1) where to initiate the transects and, 2) how long the transects need to be to reach from the beachfront to the back dune.


A. Quadrat Method


This method of sampling with quadrats or plots of a standard size is a widely applicable technique. It may be used in all major types of small vegetation. The size, shape, number and arrangement of quadrats will vary depending on the type of community and the type of information desired.


Each group of students will set up a transect line through the dune starting at 0 m at the beach front. Square 0.25 m2 quadrats will be used for this analysis. After deciding how long the transect must be to reach across the dune, divide that value by 20 to get the number of sections we will sample (e.g., if the distance is 100 m then 100/20= 5, every 5 m we will place a quadrat). Quadrats are positioned along the transect line at intervals in the middle of the section. (Therefore in our example, the quadrat between 0-5 m will be placed at 2.5 m). Each quadrat is divided into 16 squares. Identify the individual plants within the quadrat. Then count the number of small squares in which the species occurs (this value will range from 0-16). The species needs only to OCCUR in the square to be counted in that square. If, for example species A occurs in all 16 squares and species B also occurs in all 16 squares then both A and B would have abundance values of 16 for that quad. Do not include a plant whose rooted base lies outside the quadrat.


B. Line-Intercept Method

In this method we will estimate coverage of species over the entire transect by writing down all of the plant species and substrates along the transect, starting at 0. You will write the Distance from - to (in meters to the nearest cm) and the species/substrate. Eg. 2.0-2.50 - sand, 2.50 m-2.58 m - beach rocket (Cakile), 2.58-2.70- American dune grass (Elymus). We will provide or create codes for these species, rather than writing the entire scientific or common name.

Determining % cover


A. Quadrat Method


1)       For each quadrat determine the total number of species occurrences (e.g., 16 for species A, 16 for species B, 10 for species C and 8 for Sand = 50 occurrences)

2)       Determine the relative percent cover of the species by dividing the species occurrence by the total occurrence and multiplying by 100 (e.g., for species A: (16/50)*100= 32%).

3)       Calculate average percent cover by species by averaging across all four transects for each quadrat location (e.g., for 2.5m). You should also calculate a standard error.


B. Line -Intercept Sampling Method


1)       Prior to sampling divide your transect into 20 equal parts. Determine percent cover for each of these sections (which will be comparable to the 20 quadrat locations).

2)       For calculating the % cover imagine that each section is 5 m in length. A 5 m transect can be thought of as a line with the area divided into 500 one centimeter segments. Therefore, to estimate the percentage cover along the transect using the line-intercept sampling method, sum together the intercept lengths (in centimeters) of each species from the entire section and divide by the number of centimeters in the section (e.g., 500). This will give the fraction of the section occupied by each species, and multiplying by 100 will convert this number into a percentage.

3)       Calculate average percent cover by species by averaging across all four transects for each sections location (eg. For 0-5 m). You should also calculate a standard error.




Graphs - you will generate graphs for each method to show the species' distribution through the dune system (fore dunes to back dunes). The graphs can have all species combined or separate graphs for separate species.


A. Quadrat Method


1)       Plot the average percent cover (Y axis) as a function of the distance along the transect (X axis). Each value will have the average from all four transects +SE by distance (e.g., 2.5, 7.5... or whatever division the groups have decided on). Do this for all species separately and perhaps combined to examine patterns of zonation.

2)       Calculate the correlations among species to evaluate possible interactions among species. As an example, is the relationship between the cover of species A correlated (+ or - ) with that of species B? We can help with this analysis.


B. Line-Intercept Method


1)       Plot the average percent cover for each distance subdivision along the transect. Each value will have the average from all four transects and the associated standard errors. .


C. Comparison of Methods


To compare the two methods to determine if they sampled species differently, plot quadrat averages (X value) vs. transect averages (Y value) for each species. As an example if you had the following data set for species A. In the graph below the 45 degree line is the line on which all points would lie if the methods sampled identically.




Quad distance

Transect distance

Cover of A from Quads

Cover of A from transects