BIO112 Laboratory Guide #10   LAKE AND STREAM ECOLOGY
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INTRODUCTION   You will need to familiarize yourself with the following terms and their definitions prior to the start of this week's laboratory:   Habitat - the place where an organism lives Niche - refers to not only the place where an organism lives, but also its function in the community and its specific requirements from its environment (pH, temperature, etc.).  An organism's niche is sometimes referred to as its 'occupation', the sum total of all of its interactions with the world around it. Gradient - any environmental factor which varies continuously with space or distance Distribution - the pattern of relative numbers of organisms at different locations within a habitat   An ecosystem may be characterized by both its biotic (organic) and abiotic (inorganic) components.  The living biotic component is generally referred to as a community.  We will look at three aspects of invertebrate communities: the population density of an individual species, the distribution of that individual species relative to an abiotic gradient, and the community biodiversity (measures of the relative numbers of different types or taxa of organisms in a community).   Population ecology is the study of a single species' interactions with its environment.  An important aspect of population ecology is determining how a population of a given species of organism is distributed within its habitat and what factors determine this distribution.  In the previous lab, we investigated how a variety of abiotic factors such as ambient illumination, dissolved oxygen, temperature, and other factors all varied continuously and systematically with depth, forming environmental gradients.  The niche of an organism is in part defined by the range of values of each abiotic factor within which the organism can survive and reproduce.  Both ecological and evolutionary considerations suggest that any organism should be most prevalent within that part of the habitat corresponding to this range.             Community ecology deals with how populations interact within a multi-species community.  One aspect of this is the makeup of the community.  The species biodiversity of an ecosystem refers to the number of different species present, as well as the relative abundance of each of those species.  The biodiversity of species in an biological community is often used as a measure of the surrounding ecosystem's health and stability; specifically, a high biodiversity is generally thought to indicate a healthy, stable community.  Ecosystems created by people (e.g. cultivated crop fields) tend to have very low diversity.  Such ecosystems are inherently unstable; they tend to be heavily dependent on external inputs of energy and nutrients and are highly vulnerable to attack by specialized pests.  In contrast, natural ecosystems are both more diverse and more stable.  In such ecosystems individuals of a single species tend to be more widely spaced, reducing their vulnerability to pestilence and the spread of disease. Diverse ecosystems are also much less susceptible to disturbances caused by environmental toxins or the elimination of individual species.   After completing this laboratory you should be able to:   1)   define and correctly apply the standard terminology used in describing biological communities and ecosystems   2)   describe what is meant by the "distribution" of a population or organisms and graphically represent that distribution along a simple environmental variable such as water depth   3)   describe and correctly apply the concepts of composition, richness, and heterogeneity as applied to the species diversity of a biological community   4)   calculate and appropriately interpret Simpson's Index as a numerical measure of heterogeneity                         PART I.  POPULATION DENSITY AND DISTRIBUTION Materials   6 light traps placed overnight in Foster Lake at three different depths   placed a t a central table:    6 large finger bowls for holding the contents of the light traps - labeled for depth    glass stir rods    poster guide to microcrustaceans   for each student group:    100 ml beaker    10ml transfer beaker    glass stir rod    hand-held mechanical counter    disposable pipets    dissecting microscope    Procedure   Small aquatic organisms are often attracted to light.  A lighted funnel trap suspended in the water at night is a convenient way to collect a concentrated sample of these organisms.  Foster Lake contains at least three taxa of microcrustaceans: branchiopods (especially Daphnia), copepods (especially Cyclops), and ostracods.  Usually Daphnia are the most numerous and easiest to identify and count.   Your instructor has suspended "light traps" in Foster Lake at three different depths.  The traps were hung last night and retrieved this morning.  Your instructor will explain the design and function of the traps.  The lake water collected in these traps has been transferred to large finger bowls, labeled with the depth at which the trap was suspended, and placed on a central table.  Enter these three depths across the top row of the table below.   1.   Set up your dissecting microscope, turn on the light(s) and center the small watch glass on the stage.  Obtain a 50-100 ml sample of lake water from each of the three finger bowls by the following method:   a. Stir the water in the finger bowl to randomly disperse the organisms in the water. b. Fill the 100 ml beaker somewhat more than half full by dipping it into the finger bowl.  Do this quickly before the suspended organisms have time to settle or redistribute themselves. c. Carry the 100ml beaker to your table. d. Label the 100ml beaker with a piece of tape, marked to indicate the appropriate sampling depth.         You should now have three lake water samples at your desk, each containing in a 100 ml beaker, and each correctly labeled to indicate the lake depth from which that sample was obtained.   2.   For each of your 50-80 ml samples repeat the following steps four times:   a. Stir or swirl the sample in the 100 ml beaker. b. Immediately pour 10 ml of this sample into the 10 ml beaker (fill the 10 ml beaker to the top). c. Empty the 10 ml beaker into the small watch glass on the microscope stage. d. Look through the oculars, focus the microscope, and then move the watch glass to center the 1 cm circle in the field of view. e. Using the hand-held counter, quickly count the number of Daphnia within this circle.  Do NOT count Daphnia outside the circle.  Also, be sure to count ONLY Daphnia, and not other organisms.  Enter this number in the table below. f.  Empty and wipe out the watch glass, then put in back on the microscope stage.   3.  The volume of pond water within which you counted Daphnia for each sample was 0.4 milliliters.  To produce a useable number for the population density of Daphnia, add up your four population counts, then multiply this number by 625.  This will give you an estimate of the density of Daphnia in the light trap at that depth, expressed as # of Daphnia per liter).  Enter these values in the last row of the table below.       Sample # Lake Depth (cm)       1       2       3       4       Sum       Total/L  (Sum x 625)         4.   Discard the remainder of your lake water samples.   5.   For the Worksheet produce a bar graph of population density (in # Daphnia/liter) as a function of lake depth (in meters).  On this graph lake depth (the independent variable) should be on the Y axis and population density (the dependent variable) should be on the X axis.  Note: this is opposite the convention of putting the independent variable on the X axis, but is the accepted practice where the independent variable is inherently vertical (e.g.depth).   PART II.  COMMUNITY DIVERSITY Materials   18 Hester-Dendy samplers placed in Wolf Creek at the back of the Arboretum. class data sheet   for each student group:    pencil or pen    clipboard    sampler data sheets    sampler retrieval containers    bucket    meter stick    stopwatch    small float on a string    lighted magnifier    dissecting microscope    multiple small containers for isolating stream invertebrates    forceps    disposable plastic pipets    pocket calculator     Stream Invertebrate Key    Procedure   The invertebrate population of streams may include fresh-water molluscs such as clams and snails, annelids such as aquatic earthworms and leeches, crustaceans such as crayfish and isopods, arachnids such as mites, and variety of insect larvae and pupae.  These invertebrates constitute a major part of the biological community of the stream.  A reasonable theoretical hypothesis is that the particular distribution of types and relative numbers of these invertebrates will differ between different areas of the stream, based on such factors as water velocity, water depth, and oxygenation.   To assess the biodiversity of an ecosystem, one needs to first collect a sample of the organisms.  One simple way of sampling takes advantage of the fact that many aquatic organisms will rapidly colonize any new surface or substrate introduced into the water.  A standard sampling device for those invertebrates which cling to surfaces is the Hester-Dendy sampler.  This is a set of parallel or circular wooden plates, strung at fixed intervals on a long central spindle, which is held in place in the stream by an anchor.  Over the course of days to weeks a collection of invertebrates will attach themselves, or simply cling to the smooth surfaces of the sampler.  When the sampler is carefully retrieved, these invertebrate residents and visitors may be sorted and identified.   Our samplers each consist of 9 3-inch square hardboard plates, spaced along a 5 ½ inch bolt at approximately ¼ or ½ inch intervals.  They were placed two weeks ago in Wolf Creek.  Half of the samplers were positioned in relatively deep and slow-moving “calm” sections of the creek and the other half of the samplers were positioned in shallow and fast-moving “riffle” areas.  You will start this lab today by retrieving each sampler into a container with water from the creek and carrying these samplers to our lab area for invertebrate identification and counting.  Our operationalized experimental hypothesis for this part of the lab is that the particular distribution of types and relative numbers of invertebrates collected on our samplers will differ between the calm and riffle conditions, reflecting the underlying community differences.       Independent variable:           categorical – “calm” and “riffle” conditions           descriptive - location of sampler           numerical - stream surface flow rate     Dependent variables:           numerical – richness - the number of different taxa                             heterogeneity - calculated value of Simpson’s Index           descriptive – the specific makeup of the sampled community   Sampler Retrieval   Your class will be retrieving 18 samplers for analysis, 9 each from the calm and riffle areas.  You will work in pairs to retrieve the samplers.    1)   Take a small plastic transfer container, a float-on-a-string, a meter stick,  and a bucket and wade into the creek.  Locate an appropriate sampler.  Each sampler may be found by the small float attached to it.    2)   Use the float-on-a-string, meter stick, and stopwatch to measure the depth and the local surface flow rate of the stream.  The instructor will demonstrate how to obtain these measures.  Characterize the sampler site as either "calm" or "riffle", then enter this along with the measured flow rate on a new sampler data sheet.   3)  Fill the transfer container with stream water.  Try to avoid including any debris floating in the water. Rapidly but carefully lift the sampler out of the stream and transfer it to the filled container.   Unclip the rock/brick anchor and float and transfer these to an empty bucket.  Note the number written on the sampler plates and enter this on your data sheet.   4)  Place one or two samplers from in each transfer container, making sure that both samplers come from the same region of the creek (either calm or riffle) and that both samples are odd numbered or both are even-numbered.  Remember to wade back out of the creek when you are done.  Seal each transfer container by clipping the lid in place.    5)  Carry all of the samples in their sealed containers to the lab for analysis.  Also bring along your bucket of bricks, youe data sheets, and your other measurement implements.   Identifying and Tabulating Stream Invertebrates:   1)  Carefully transfer each sampler and some of its surrounding water to a clean observation container.  Find the data sheet corresponding to the sampler ID number (1-38) .  Carefully disassemble the sampler by removing the wingnut then sliding the plates and spacer bolts off of the central bolt and into the water in the tray.  Invertebrates may be clinging to the plates or to the spacer bolts, so keep all of the hardware in the water.   2)  Carefully examine each plate from the sampler.  Use a magnifier and/or dissecting microscope as necessary to locate and examine each organism.  Use the Stream Invertebrate Key to identify each organism to the taxonomic level of Order (insects) or Class (all other invertebrates).  If you are at all uncertain about your classification, consult the instructor or course assistant.  As the lab progresses you will become familiar enough with these organisms to classify them on sight.  Record each new  taxon on your data sheet and keep a running tally of the exact number of organisms of that taxon.    3)   If you wish, you may transfer individual  tallied organisms to one of the fingerbowls for additional observation.  Once you are sure that you have counted each organism on an individual plate, transfer it and all of its organisms to the “waste” bucket.     4)   Continue until you have examined and discarded all 9 plates from that sampler.  Then examine the remaining hardware (spacers and bolt) and tally any additional organisms, discarding this hardware into the waste bucket when you are finished.  Now carefully examine the water sample that the sampler was in and tally any organisms that you find there.  Dump this water into the wastewater bucket when you are finished, then proceed to the next sampler.   Compiling the Class Data and Calculating Community Measures: 1) When you are finished with all of  the samplers assigned to your two-person team, add your tallies to the master data sheet for the class.  Be sure that you add the numbers to the correct invertebrate taxon and to the correct stream condition – calm or riffle.   2)  Count the number of different taxa (not individuals!) under each condition - calm and riffle.  This number is the richness of the community.  Enter these values in the table below.  Which stream condition showed the higher richness?   3)  Calculate Simpson's Index for each condition, using the procedure detailed below.  This number is a measure of the heterogeneity of the community.  Enter these values in the table below.  Which stream condition showed the higher heterogeneity?  What additional information does a measure of heterogeneity provide beyond that of a simple measure of richness?   4)  Finally, examine the specific community composition for each stream condition.  Which organisms were most common under each condition?  Where any organisms found only under one of the two conditions?   Stream Condition Diversity Measures  Richness  Heterogeneity Calm     Riffle         Simpson's Index An mathematical “index” is a number which varies between 0 (minimal value) and 1 (maximal value).  Simpsons Index is a quantitative measure of the diversity of a collection of discrete types of objects or organisms.  Simpson’s Index always has a value between 0 (no diversity) and 1 (infinite diversity).   The formula for Simpson’s Index (SI) that we will use is:       SI = 1 – (sum(ni2)/N2 )         where ni is the number of organisms of each taxon i           and N is the total number of organisms of all taxa ( = sum(ni ))   When all of the organisms are of the same single taxon, then sum(ni2) = N2, the ratio sum(ni2)/N2 = 1, and SI = 0 , meaning zero diversity.    As the number of distinct taxa becomes large and the organisms become evenly distributed across all of the taxa, then sum(ni2) becomes very small as compared to N2, the ratio sum(ni2)/N2 approaches 0, and SI approaches 1, meaning infinite diversity.   As an example, imagine that the data set for Sample I is:   Taxon A – 8 organisms Taxon B – 6 organisms Taxon C – 4 organisms Taxon D – 2 organisms TOTAL – 20 organisms   SI = 1 – { [ (82) + (62) + (42) + (22)] / (202) }      = 1 – { [64+36+16+4] / 400 }      = 1 – {120/400 }      = 1 - 0.3      = 0.7   Now, imagine that the data set for Sample II is:   Taxon A – 25 organisms Taxon B – 2 organisms Taxon C – 1 organism Taxon D – 1 organisms Taxon E – 1 organisms TOTAL – 30 organisms   SI  = 1 – { [ (252) + (22) + (12) + (12) +  (12) ] / (302) }      = 1 – { [625+4+1+1+1] / 900 }      = 1 – { 632 / 900 }      = 1- 0.7      = 0.3   Even though Sample II has more total organisms and more taxa (a higher "richness"), the organisms are predominantly of the single taxon A.  By comparison, Sample I has organisms more evenly distributed more evenly across several taxa.  So Sample I has the higher heterogeneity, as reflected in its higher Simpson’s Index value.
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