written by Breea Govenar, Ridge 2000
Do the words "deep sea creature" conjure up images of Jules Verne's giant octopus or even the recently caught colossal squid? In 1977, less than 30 years ago, scientists discovered communities of deep sea organisms that are very different from these giant monsters. While studying the ocean floor near the Galapagos Islands, scientists happened upon hydrothermal vents complete with their own assemblage of organisms living on or around them. One of the unique things about these communities is that food production is not based on energy from the sun, but on energy from within the earth. In an effort to learn more about this unique environment and the organisms inhabitating it, the National Science Foundation established Ridge 2000, a research program that studies the links between the biology and geology of these ecosystems.
Ecology is the study of the interactions between living organisms and their biotic and abiotic environment. The biotic environment includes organisms and their interactions with each other, whereas the abiotic environment is comprised of the physical surroundings of an organism. Ecological studies often begin by exploring the distribution and abundance of species, and then investigating the factors influencing that distribution. A common ecological measurement used to quantify a community is the diversity or variation within a group of different species. To calculate this, scientists count the number of different species ("species richness") and determine how similar the abundances of the different species are ("species evenness").
Hydrothermal vents are one of the most fascinating and challenging places to study ecology. They are located deep at the bottom of the ocean at the mid-oceanic ridges where two oceanic plates are pulling apart. Vents are like underwater volcanos where extremely hot fluid rich with minerals is released from below the seafloor. The variation in temperature and chemistry in this environment makes it uninhabitable to most organisms, however a unique assemblage of animals has been discovered to thrive here. The chemicals in the hydrothermal fluid that percolates through the rock react with the overlying ocean water, and bacteria use the energy from this chemical reaction to make food. The food that the bacteria make is eventually assimilated, or taken up, by all of the species at hydrothermal vents. Among the organisms that live in this extreme environment are tubeworms, polychaete worms, gastropods (limpets & snails), and pycnogonids (sea spiders).
Unlike areas of high species diversity like tropical rainforests and coral reef, hydrothermal vents are typically described as having low species diversity. Although there are many organisms found at these vents (particularly in comparison to the bare rocks on the surrounding young sea floor), the number of different species is low. In the following data activity, we will use a number of methods to calculate the diversity of species found at a hydrothermal vent. Determining the diversity of this community is the first step in understanding how these groups of vent species interact with each other and their environment.
Hydrothermal
Vent Organisms |
|
| Tubeworms | Ridgeia piscesae |
Sessile Polychaetes |
Paralvinella
palmiformis Paralvinella pandorae Paralvinella sulfincola Amphisamytha galapagensis |
| Mobile Polychaetes (worms) |
Lepidonotopodium
piscesae Branchinotogluma sandersi Branchinotogluma grasslei Branchinotogluma hessleri Opisthotrochopodus tunnicliffeae |
| Gastropods (limpets & snails) |
Lepetordilus
fucensis Depressigyra globulus Provanna variabilis |
| Pycnogonids (sea spiders) |
Ammothea verenae |
In
September 1999, five separate collections of species were taken from one sulfide
edifice in the Main Endeavour Field of the Endeavour Segment at the Juan de
Fuca Ridge (NE Pacific Ocean). The samples were brought back to the lab, sorted
by species, identified and counted. All of the different species and the number
of individuals of each species are listed in an Excel table. Using these data,
we will determine species richness, evenness and diversity for each of the five
collections.
Divide the class into five groups and assign each group one of the collections. Using the steps below, have each group calculate species richness, evenness and diversity for their collection. Compare results for the five collections and answer the discussion questions.
Calculating Diversity
- Access the Ridge Excel datasheet. For each collection there is a list of species and the number found. There are also three empty columns, for which we will calculate values that will be used to determine the diversity and species evennness.
- From Column A, count the total the number of species for your collection. This is the Species Richness (S). Enter this value in the yellow cell. (Remember if the number of individuals found for a species is zero, then do not include this species in the total species count.)
- From Column B, total the number of individuals for all species. Enter this value in the yellow cell for Total Number of Individuals (N).
- For Column C, we will calculate Relative Abundance (Pi) for each species and enter the values in the yellow cells. To do this, take the number found for that species (Column B) and divide it by the Total Number of Individuals (N). Repeat for each species.
- In Column D, take the natural logarithm of the Relative Abundance (Column C). Repeat for each species.
- In Column E, multiply the Relative Abundance (Column C) by the natural logarithm of Relative Abundance (Column D). Repeat for each species.
- Add all of the numbers in Column E and multiply by -1. This is the Shannon-Wiener Index of Diversity (H'). Enter this value in the yellow cell.
- Divide the Shannon-Wiener Index of Diversity (H') by the natural logarithm of Species Richness (S). This is the Species Evenness. Enter this value in the yellow cell. Compare your values to the answer sheet.
Formulas for this activity came from:
Discussion Questions
- Which sample had the highest species richness?
- Which sample had the highest species evenness?
- Which sample had the highest species diversity?
- What are the advantages and disadvantages of using species richness, evenness and diversity to describe the structure of different communities?
- Did the sample with the greatest number of species (species richness) also have the highest species diversity? Why not?
- Plot relative abundance of each species in every sample and see if you can determine a relationship between relative abundance and diversity. Are there any other patterns?
- Choose the two collections with the highest diversity, and look at the top five dominant species. Are they the same in each collection? If not, what's different? Can you think of reasons why they would be different?
This type of initial descriptive analysis typically leads to many other questions for investigation. For example, someone may choose to explore why limpets are so much more successful in Collection E by conducting a whole series of physiological and behavioral studies. What type of hypotheses can you suggest for why limpets are so dominant in this collection?
References
Govenar B.W., Bergquist D.C., Urcuyo I.A., Eckner J.T. & Fisher C.R. 2002. Three Ridgeia piscesae assemblages from a single Juan de Fuca Ridge sulphide edifice: structurally different and functionally similar. Cah. Biol. Mar., 43: 247-252.
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