E-mail: jearmstr@ilstu.edu
Presumably, students will have some idea of general community structure, trophic levels, and food chains. Obviously green algae are autotrophic producers, but are probably not responsible for most of the energy in these food chains. Many of the consumers are detritovores, largely obtaining energy from leaf litter & other organic debris. So in this instance, the PS producers are actually outside the community being observed.
Suitable for 1 3-hour laboratory period. Prior preparation necessary, but very low cost.
Depending on your style, you may give more or less information to begin. Students themselves can decide what characters can be used to sort the critters after initial observations. They can also suggest sampling strategies, initial categories, and actually consult their textbook or other resources to find others, e.g., Margulis & Schwartz, Five Kingdoms (2nd ed), Freeman & Co., 1988. Students may also be directed to consider reproduction in this community. What must organisms inhabiting a puddle be capable of doing? This exercise can be pretty free form, and lead to many different topics. This is one approach to Biological Diversity, i.e., as "natural" communities of organisms, in different population densities and different combinations of species, rather than intellectually sterile, monocultures.
Large glass fingerbowls (9-10" diam)
Pond water
Dry grass or hay (not straw)
Bubble stones
Aquarium pump
Plastic tubing
Pipettes
Slides & coverslips
Microscopes
Algal growth medium or liquid fertilizer
Flourescent shop lights/microscope illuminators
With his invention, the microscope, VanLeeuwenhoek discovered previously unknown biological diversity, entire communities of microscopic organisms. This exercise will allow you to examine the structure and species diversity of a community of aquatic microorganisms. To do this you must observe and catalogue, based upon cellular features, whatever organisms are present.
Whenever a biologist discovers a new community, either in a previously unexplored place or unobserved place, what must be done to begin studying these organisms? What assumptions does the biologist make? What are the most important things to do first? Discuss these questions among your work group and suggest answers. Record your conclusions. Your instructor will collect the responses from other groups. Record other answers along with yours. Evaluate the answers. Are some better than others?
In this portion of the exercise, you will examine the diversity within a community of microorganisms. In this case the community will be a culture of wild organisms whose numbers have been enhanced.
A. Obtain a compound microscope. Examine the oculars and objectives making certain that they are clean. If they need cleaning, use some of the cleaning fluid provided and lens paper. NEVER use paper towels, chemwipes, or TP! Locate the iris diaphragm and make certain that it is working smoothly. Reducing the amount of light is an important technique for observing fine details in small organisms.
B. Obtain a slide and cover slip. If the slide needs cleaning, use soap, water, and a paper towel because what could be duller than dirt?
C. Select a location within the culture to sample (surface, edge, center, bottom), and record where within the culture your sample was obtained. Place 1 drop of pond water culture on the slide and gently lower a cover slip into place. The drop should fill the space under the cover slip completely. If more water is needed, just touch the end of a pipette filled with water to the edge until the volume fills by capillary action. If there is too much water, the cover slip will jiggle around and/or the drop will puddle out on the slide. In either case use the corner or edge of a paper towel to absorb the excess water.
D. Place wet mount slide upon the microscope stage and focus using the 10X objective. Find the edge of the cover slip and move to a corner. Using the corner location for orientation, practice scanning the entire area under the cover slip by moving the slide (or stage) forward (or back) and then over a notch and reversing the process. This will allow you to scan the whole volume of water under the slide in a series of back and forth paths, like you would mow a lawn or plow a field. Remember movements appear inverted under the microscope.
E. Return to the corner, switch to a high power (43X) objective and begin your scan. Is the field diameter of the 43X objective bigger or smaller than the 10X objective? Will this require more scans or fewer? Answer this and other questions in your lab report.
F. Data collection - Record all of the organisms you observe in a complete scan of the slide. Keep track of how many different species of organisms you observe, and generally how many are present on the slide. For these purposes you may count organisms as follows, 1-10, many. Use general sketches and notes about details, either cellular or behavioral, that can be used to identify the creatures. Even fast swimmers will vary in size, rate, pattern, and swimming behavior. Discuss how you will decide if you are observing 1 moving organism many times, or several different organisms.
G. Fill in Table 1 recording the cumulative total number of species per sample. Thus you may record 8 different species in your first sample, so enter 8. You may see 6 of the same species in your next sample, but 3 new species, so for sample #2 enter 11. You may work with 1-3 other students and accumulate your discoveries. So with a team of 4, you get 4 samples, but you will need to compare and look at each others' discoveries, so you know you are counting each species only once.
You may stop when you are confident that you have discovered all of the species in the community. But how can you know? To help you answer that question, graph the number of species discovered as a function of the number of samples examined. This will produce a SPECIES DISCOVERY CURVE. What shape will the curve be? How will this curve help you decide when you have sampled enough?
H. What are the likely sources of error in determining how many species are actually present? Are you likely to err by estimating too many or too few species? What were the characteristics you used for distinguishing different species?
I. If most of these organisms reproduce by simple cell division, how would 2 new daughter cells/organisms relate to a mature cell/organism in size? Did you use size to distinguish species of two similar types of cells? So you either have one large and one small species, or the small cells are juveniles that will grow to the mature cell size. Suppose you could accurately measure the cells, predict what data is expected based on each hypothesis?
Just as machines are composed of parts each with specific functions, communities of organisms are composed of different species with specific functions. In communities certain species are more important than others, and certain functions within a ommunity are essential. Now that you have observed the organisms in a community in some detail, you will attempt to categorize them by function.
A. Categorize the species you have observed with respect to the following. These are only general suggestions. You may decide to use more specific or additional categories, which you may simply specify.
a. Motility
i. Motile or not
ii. Free-floating or anchored in place
iii. Organs or organelles used for locomotion
b. Trophic Level
i. Photosynthetic autotroph
ii. Heterotroph - means of obtaining food (absorption,
ingestion, any special organs or organelles, any particular
behaviors, etc.)
1. Herbivore - feeds on plants
2. Carnivore - feeds on animals
3. Omnivore - feeds on many things
4. Detritovore - feeds on (ingests) dead material
5. Decomposer - grows on and feeds on dead material
c. Size
i. Macroscope - can be seen with naked eye
ii. Large microscopic - seen easily at 100 X
(diameters).
iii. Small microscopic - only seen easily at 430 X
(diameters).
iv. Really little - barely visible at 430 X.
d. Predominate location
i. Free swimming or floating
ii. Attached to or found with another organism
iii. Bottom dwellers
B. Using information from your categorization of the organisms, describe the organization of this aquatic community. If the community you have been observing was actually just a small part of a much larger community, how would you have to extrapolate your findings to account for just 1 small fish?
Classification is a means of organizing and showing relationships within biological diversity. In the previous section you organized biological diversity in terms of ecological unction. Now you will consider a different type of organization, one based upon similarities of the organisms themselves.
Using the information you have obtained so far, attempt to group the species you have observed into kingdoms and phyla. For your purposes you may assume that there are only 5 kingdoms: Bacteria and Blue-Green Algae (Prokaryotes), Protists, Fungi, Plants, and Animals. You might try and decide how you would distinguish these 5 kingdoms. Remember, all of the groups that we now recognize represent observations not unlike the ones you have just made and earlier attempts to organize these observations.
Group your organisms into phyla (they do not need to be named) based upon one or more general characterisitics, indicating what it is that members of this group have in common. Then attempt to group the phyla into kingdoms. What Kingdoms are predominately represented in this pond water community? How do you think the diversity of organisms would differ if this was a marine community instead of freshwater?
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Table 1. Species discovery data
CULTURE NAME OR NUMBER:
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SAMPLE NUMBER: 1 2 3 4 5 6 7 8 9 10
# NEW SPECIES:
CUM. TOTAL:
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LOCATIONS SAMPLED:
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KINGDOMS AND PHYLA:
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