Exercise 6 Bivalve Species Richness, Diversity and Size class distribution: First Landing State Park This exercise focuses on community structure of bivalves in a sandy beach habitat. The students will characterize bivalve species abundance and diversity and create a size class distribution to gain an understanding of how species are associated within this habitat. The site for this lab, First Landing State Park, is located at the mouth on the Chesapeake Bay in Virginia Beach; however, any sandy beach habitat can be utilized for this lab exercise. SUGGESTED ELEMENTS FOR AN INTRODUCTORY LECTURE Bivalves make up a large proportion of littoral benthic communities Bivalves have essential roles in soft bottom habitats as sources of food and major components of biogenic sediment ACTIVITIES 1. To survey bivalve species within the beach habitat of First Landing State Park 2. To identify species collected and calculate species richness and abundance 3. To create and graph a size class distribution for collected species 4. To calculate species diversity (evenness) VOCABULARY Biodiversity Evenness Shannon Index Community Species diversity Size class Species richness MATERIALS FOR ALL PROCEDURES Equipment None Supplies Colored pencils Lab notebook Ruler Organisms Specimens collected from the beach habitat SUPPLEMENTAL MATERIALS Abbott, R. (1996). Seashells of North America: A guide to field identification. New York: St. Martin's Press. Rehder, H. (1981). The Audubon Society field guide to North American seashells. New York New York: Knopf Distributed by Random House. Dance, S. (2002). Shells: The photographic recognition guide to seashells of the world. New York: Dorling Kindersley. Abbott, R. (1995). A field guide to shells: Atlantic and Gulf coasts and the West Indies. Boston: Houghton Mifflin.
White, C. (1989). Chesapeake Bay: Nature of the estuary: A field guide. Centreville, MD: Tidewater Publishers. Lippson, A. (2006). Life in the Chesapeake Bay. Baltimore: Johns Hopkins University Press. http://www.mdsg.umd.edu/programs/education/interactive_lessons/biofilm/diverse.htm#3 VENDORS FOR MATERIALS Field guides can be purchased at www.amazon.com.
LAB OBJECTIVES: Bivalve Species Richness, Diversity and Size class distribution First Landing State Park 1. To survey bivalve species within the beach habitat of First Landing State Park 2. To identify species collected and calculate species richness and abundance 3. To create a size class distribution for collected species 4. To calculate species diversity (evenness) In today s lab you will survey the beach habitat of First Landing State Park for bivalve and gastropod species. Specimens will be returned to the laboratory for examination and identification of species. Sketch the details of the shell, measure shell size, and record the number of specimens collected for each species. Record all data in your laboratory notebooks. Use the field guide books available to you for species identification. Additionally, you can use the following website to help you calculate the biodiversity index H: http://www.mdsg.umd.edu/programs/education/interactive_lessons/biofilm/diverse.htm#3 Species Richness Species richness is a measure of the number of species found in a sample, community or taxonomic group. You will calculate species richness by simply identifying and counting the number of species present in your sample. In your lab book record the species, identifying characters (i.e. those you use to make your identification) and the number of specimens collected in each species. Species Diversity Species diversity differs from species richness in that it takes into account both the numbers of species present and the dominance or evenness of species in relation to one another. As a measure of species diversity, we will calculate the Shannon index, H. H = - p i log p i Where (p i ) is the proportion of the total number of individuals in the population that are in species i (number of individuals in species x). The index is increased either by having additional unique species, or by having a greater species evenness. If a community is composed of a few species, or if few species are abundant, the index (species diversity) will be low. Understanding the Difference between Species Richness and Species Diversity Consider the following data from samples of organisms obtained from two different biological communities, A and B. Community A Species # of individuals A 59 B 12 C 11 D 10 E 5 F 3
Total 100 Community B Species # of individuals A 21 B 20 C 19 D 14 E 13 F 13 Total 100 In both samples the same total number of individuals and the same six species were collected. The only difference seen between the two communities is in the distribution of the number of individuals among the six species. When examining species diversity of these two communities, one species, Species A, in community A numerically dominates the other five species. In community B the six species are more evenly represented. Because of this difference, community B would be considered to be more diverse than community A despite both communities having the same total number of individuals and the same number of species. Thus, when measuring species diversity the relative abundance of each species must be taken into account. In Table 1, species diversity is calculated for the two communities using the formula for the Shannon Index. Table 1: Calculation of species diversity using the Shannon index, H Community A Species # of individ. (pl) ln pl (pl) ln pl A 59.59 0.528.311 B 12.12 2.120.254 C 11.11 2.207.243 D 10.10 2.303.230 E 5.05 2.996.150 F 3.03 3.507.105 Total 100 1.00 1.293 Community B Species # of individ. (pl) ln pl (pl) ln pl A 21.21 1.561.328 B 20.20 1.609.322 C 19.19 1.661.316 D 14.14 1.966.275 E 13.13 2.040.265 F 13.13 2.040.265 Total 100 1.00 1.771 What relative abundance would achieve the highest species diversity index? In summary, the species diversity approach is generally a more reliable measure of biodiversity than species richness. While mathematically very easy to calculate, the limitations of the species richness
concept can be seen when applying it to Communities A and B, where it fails to distinguish their quite different community structures. How to Know When to Stop Sampling A practical problem that arises when measuring species richness and species diversity is determining when you have done sufficient collecting to stop sampling, or in other words, to know when you ve gotten just about all the species that matter. A good technique is to generate a cumulative species/sample (or cumulative species/area) curve, which plots the cumulative number of species collected (y-axis) vs. the number of samples collected (x-axis) as shown in Figure 1. This technique assumes that initially you will be collecting new species with each subsequent sample, but after a while you will be collecting the same species you have already collected in previous samples. Thus, you will get a curve that starts to flatten out. Once the curve levels off there is no need to sample further. Size class distribution In addition to examining species diversity and richness you will also calculate the size class distribution for the species you collected from the site. The size class distribution will provide you information on the ages of each species present in the population. This information is useful in determining sufficient recruitment, mortality and success of a population in an area. For each species create a size range from the smallest to the largest specimens. Divide into different size classes based on your range. Create a histogram of size class distribution for each species collected using Excel. Figure 2 illustrates an example of a size class distribution graph.
Figure 2. The size frequency distribution of Arctica islandica populations in the North Sea. Modified from: http://www.colby.edu/biology/bi131/lab/lab09calcbiodivers.pdf