Chesapeake Bay Population Studies

* Population Density
* Relative Density
* Frequency
* Relative Frequency
* Calculate a Diversity Index
* Establish a Correlation of Association between any two species

* Hard copy of the HTML pages
* Calculator
* optional use of a spreadsheet such as Lotus 1-2-3 or Claris Works
* optional quadrants constructed from directions on the HTML page:

- may be constructed from:

• 4 one-quarter (1/4) inch pieces of PVC piping,
  each .5 meters in length
  
• 4 right angle PVC corner pieces
  
• 20 - 40 # fishing line

This virtual field trip is meant as primer for an actual field study of a marine ecosystem. Field studies are an important and integral part of any ecology or biology course of study. In our area near the mouth of the bay, we have an opportunity to study a somewhat varied ecosystem. High-impact, wave influenced sand beaches, mud flats as well as maritime grass meadows can be found along the bay and its numerous tributaries. Each community differs greatly in the amount of wave action, salinity, temperature, dissolved oxygen content and type of substrate. Seasonal and meteorololgic factors will also affect the make up of the biotic community. Yet all communities share the common experience of living submerged in sea water.

In this virtual field trip you will have the opportunity to determine the effect of the abiotic factors on the biotic community ... in a virtual way. It is hoped that once the methods employed in this lab are mastered, learners will feel confident enough to perform the data gathering in the natural environment.

In the actual field trip a transect line would be drawn along the mean lower low water level parallel with the shoreline. The transect line should be at least ten meters long. Place the quadrants at random along the line as to not bias the sample.

The data from each of the quadrants is collected and analyzed to determine density, frequency, diversity and suggestion of association between non-related species.

There is a complex interaction between the abiotic factors found in the bay and the life each of the communities supports. Areas with more stable conditions such as more uniform temperature, salinity, current speed and substrate allow for a greater diversity as more organisms are capable of adapting to the favorable conditions. In these areas we would expect to see a great number of organisms enjoying the favorable conditions. There would be a great diversity of organisms because one organism's peculiar adaptations are no better suited to the environment than that of another. In areas that are considered quite stressful such as areas with great differences in temperature, salinity, oxygen content, composition of substrate, we would expect to see a lower diversity because one organism's adaptations will allow it to fill the niche in the ecosystem.

Generally, a more stable abiotic environment produces a biological community with many species.

There are several ways to represent diversity. The most elementary way is to just list the organisms found there. A more advanced method of determining populations it to calculate density, relative density, frequency, relative frequency, and determine diversity indices. Further examination of the data can be used to determine the possible association or avoidance of two species.

1. Call up the map of the lower bay area
   
2. Select the location you wish to conduct your virtual population study. This area will be used for comparison to the other sites in the bay. The quadrant will display the population.
   
3. Count the number of species and the total number of organisms of each species. Use this data along with the other 9 quadrant samples.
   
4. The data should be placed into the table listing the number of each species.

5. Fill in the data from the quadrant by writing the names of the various species in the first column. After the data has been collected along the transect line for all ten quadrants, the sum of each species should be calculated ( Sigma - the sum of all individuals). The values should be recorded in the last column.

6. Next calculate the population density. Density is equated to a comparison of various areas such as an urban versus a rural environment. The data is useful for comparison only.
   Calculate the population density as follows:
   (you will need to calculate the area sampled:)

   10 quadrants, each .25 meters2	=	2.5 meters2

   
   

   density	=
   no. individuals total area

7. The relative density can now be determined for each of the species in the sample. This is useful for comparison to all the other species in the sample area. A relative density can be calculated for each of the species in the sample by using the following formula:

   Relative Density of a Species

   =

   density of a species total densities of all species

   x 100

   The relative density of a species is expressed as a percentage of the total density. You may find that a particular type of barnacle makes up 22% of the total population, or a type of crab only makes up 3 % of the total population density.

8. Next, determine the frequency of the species by calculating the sum of the quadrants in which a particular species occurs and factoring the number of quadrants sampled. The frequency is useful for predicting the number of occurrences a species will occur in any one quadrant.

   Frequency

   =

   of a species sampled total number of quadrants sampled

9. The relative frequency of a species can be calculated by factoring the frequency of a particular species and comparing it to the frequency of all species. The relative frequency of a species will be expressed also as a percent.

   Relative Frequency

   =

   frequency of a species frequencies of all species

   x 100

10. The importance value of an organism in the sample will express the statistical importance of each of the species. It can be calculated by the following:

    Importance Value

    =

    relative density + relative frequency

11. Diversity is the statistical value that expresses the number of species present and the number of individuals present in each of the species. A sample area containing 36 animals may consist of four individuals of nine different species. That would be statistically different from an similar sample of 36 animals with 18 individuals of two different species. The former would indicate a more stable abiotic environment. The use of a diversity index such as E. H. Simposon's diversity index will take into account these differences and will give a value useful for comparing the diversity of different sample sites.

    Diversity

    =

    N2 1(n1 + n2 + nj)2

    OR

    Diversity

    =

    N (N-1) n (n-1)

    :

    Where

    N = number of different organism n1 = sum of individuals in species 1 n2 = sum of individuals in species 2 nj = sum of individual in species j j = number of species present

12. The following data table should be completed to allow comparison of values for each organism. The data can be used to predict what factors could cause a difference in density, frequency, or importance value.

NOTE: Develop a diversity index for each of the three sample sites in the bay.