Breea Govenar, Woods Hole Oceanographic Institution,
Typically, animals that live on the bottom of the ocean are small and sparsely distributed in thick sediment, but at mid-ocean ridges, many of the resident animals are large and cluster around cracks in the rocky substrate. Some of these cracks are on the ocean floor; others are along sides of walls or chimney-like structures. Percolating or diffusing from these cracks are heated fluids that have high concentrations of reduced chemicals and heavy metals. Areas where hot, mineral-rich fluids escape the seafloor are known as hydrothermal vents.
Hydrothermal vents are one of the most fascinating and challenging places to study ecology. 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. Extremophiles like these organisms are able to survive under environmentally extreme conditions. The chemicals in the hydrothermal fluid react with the overlying ocean water, and bacteria use the energy from this chemical reaction to make food, in a process called chemosynthesis. 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 reefs, 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 and not all of the species coexist all of the time or in all areas. In the following data activity, we will use a number of methods to examine the species assemblages found at a hydrothermal vent.
Ecologists use statistics or indices to examine patterns in the different assemblages of organisms. A group of organisms that live in a particular area or have a similar lifestyle is often called a community. Some of the indices used by ecologists to characterize communities include species richness (the number of species), species evenness (the number of individuals per species), and species diversity (a combination of the species richness and evenness). The number of species in a community is always considered to be the species richness, however different scientists and mathematicians have used different formulae to calculate species evenness and diversity. The most common indices are Pielou's species evenness and Shannon-Wiener's species diversity. To understand how animal communities respond to the highly variable environment at hydrothermal vents, it is important to first determine the composition and the structure of the different types of communities, including the species richness, evenness and diversity.
(limpets & snails)
An assemblage of organisms can be described as a community. Just as you might describe your community in your hometown, ecologists use statistics to characterize communities and track their changes over time and space.
Data Activity & Discussion
In September 1999, a deep-sea sampling device was used to collect five different assemblages of animals 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, determine species richness, evenness and diversity for each of the five samples.
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.
Formulas for this activity came from:
Stiling, P. 1999. Ecology: theories and applications. (3rd ed.) Upper Saddle River, NJ: Prentice-Hall, Inc.
Govenar, B.W., D.C. Bergquist, I.A. Urcuyo, J.T. Eckner & Fisher C.R. 2002. Three Ridgeia piscesae assemblages from a single Juan de Fuca Ridge sulphide edifice: structurally different and functionally similar. Cahiers de Biologie Marine 43:247-252.
Sarrazin, J., V. Robigou, S.K. Juniper, and J.R. Delaney. 1997. Biological and geological dynamics over four years on a high-temperature sulfide structure at the Juan de Fuca Ridge hydrothermal observatory. Marine Ecology Progress Series 153:5-24.
Sarrazin, J. and S.K. Juniper. 1999. Biological characteristics of a hydrothermal edifice mosaic community. Marine Ecology Progress Series 185:1-19.