Lab 7 -- Field Community Ecology
As in any scientific endeavor, the first step in ecology is to identify patterns in nature, the second is to develop ideas (hypotheses) about what causes those patterns, and the third is test those hypotheses by evaluating their assumptions and predictions. In this lab, you will identify patterns in biological communities, learn to develop methods for sampling communities, and analyze experiments that attempt to discern some of reasons why communities differ from each other. During the first week of fieldwork, your lab group will measure spatial variation in living and nonliving environmental features, either in CERA's prairies or in its pond. The second week, you will devise a sampling scheme for accurately measuring those features in experimental settings and begin to collect experimental data. You will collect additional experimental data during the third week. In the fourth week, you will present your results to the rest of the class as a research poster, a format commonly used at scientific meetings. (These posters should have the same general organization as the scientific papers you have been writing. Detailed instructions appear below.)
One of the central challenges in ecology is to determine what factors control the abundances of different kinds of organisms (community structure) and the movement of energy and materials through ecosystems. These factors include the amount of incoming energy, the availability of nutrients, the frequency of disturbance, and/or the nature of the community's feeding relationships (called its trophic structure). During the first week you will determine spatial variation in either the prairie or pond, and in the following weeks you will sample an experiment that directly manipulates one of the factors thought to influence community structure.
One of the biggest challenges of studying ecological communities is dealing with spatial variation. How can one know that a measurement of a physical or biological parameter is an accurate representation of a community when it varies within a relatively small area? One of your tasks during this is exercise is to determine whether you are obtaining representative measurements in descriptive and experimental settings.
After the first week of work, you should prepare and hand in a methods and materials section and a results section describing your preliminary findings. These will be made available to your fellow students.
Perry Pond is a 14 acre artificial lake created at CERA over 25 years ago by the construction of an earthen dam located at its eastern edge. Like many such lakes in Iowa, it receives high nutrient (nitrogen and phosphorus) inputs via runoff from agricultural fields in its watershed. In aquatic biology, highly productive bodies of water are called eutrophic, and are characterized by high densities of both producers and consumers. The food web, or set of feeding connections among species in a community, determines the pathways through which species affect each other's abundances and through which energy and materials move. A recent focus of community ecology has been how the structure of the food web, including the number of trophic levels (see below) may determine the community structure.
In addition to detritivores and decomposers (organisms that consume dead organic matter), the pond contains four main trophic levels: (1) producers (mainly phytoplankton, small algae); (2) primary consumers (zooplankton, including protozoans and small invertebrates); (3) secondary consumers (mainly bluegill sunfish, Lepomis macrochirus) and (4) tertiary consumers (mainly largemouth bass, Micropteris salmoides).
Week 1 -- Spatial variation in physical and biological parameters of Perry Pond. Each research group will perform one of the following set of measurements to determine how nutrient concentrations, physical conditions, and the densities of organisms vary within the basin of Perry Pond.
Group 1 -- Physical conditions
Physical conditions such as temperature, oxygen concentration, and light affect the physiological activity of aquatic organisms. They can also be modified to some extent by the activities of organisms. Thus they are an important link between the physical environment and biological communities.
Group 2 -- Nutrient concentrations
Essential elements such as nitrogen and phosphorus are resources for autotrophic organisms. The primary productivity (rate of biomass or energy accumulation) of a community is often proportional to the availability of these elements (in the form of inorganic ions that autotrophs can use).
Group 3 -- Zooplankton diversity and abundance
Zooplankton are the diverse set of very small animals and animal-like protists that occur in open water. Many of them consume phytoplankton (the small plant-like protists [algae]) that occur in the same environment.
Group 4 -- Phytoplankton diversity and abundance
Phytoplankton are minute plant-like protists (algae) that live in open water. In many aquatic systems, they are the most important autotrophic organisms, accumulating (through photosynthesis) the energy and materials that support the rest of the food web.
Trophic Cascade Experiment
A general problem in the understanding of how community structure is determined is to tease apart the multiple factors that can influence a species abundance. While it seems logical that the abundance of resources, both biotic and abiotic, can influence a species abundance, it is also clear that predation can limit a species density below that at which competition for resources occurs. The fact that species exist in a web of feeding relationships also suggests that effects of resources or predators could ripple through a community, and thus indirectly effect the abundance of other species. This has generated a debate among community ecologists concerning whether community structure is determined more by the effects of resources on the lowest trophic level (primary producers), or by the effects of the highest trophic level (the primary, secondary, or tertiary consumers, depending on the complexity of the food web). These two views have come to be known as "bottom-up" and "top-down" regulation, respectively.
During the last decade, aquatic communities have been the major arena for testing these hypotheses, partially because they are easily contained physically, and resources and predators can be easily manipulated in replicated experiments. Such experiments have been done at different scales using subdivided lakes, giant floating plastic bags, and artificial ponds. The experiment we have set up at CERA manipulates the top trophic level in artificial ponds to determine the effects of its presence on the lower trophic levels -- something ecologists refer to as a "trophic cascade." The data you will take from this experiment will describe the effects of the presence of bluegill sunfish (a zooplanktivore) on biological and physical parameters you measured in the natural lake last week. Researching the results of other such studies will help you formulate your hypotheses and prepare your posters for the final week of lab.
Experimental protocol (ask instructor for exact dates)
March 1998 Twelve 300-gallon plastic cattle tanks were filled with water from Perry Pond.
April 1998 5 bluegills (10-16 cm in length) were added to each of 6 randomly chosen tanks.
Due to the small volume of the tanks, you should not drag the plankton net through each. Rather, sample a small volume of water (1-2 liters), pour it through a plankton net or other straining device and determine whether a sample of this volume is sufficient to estimate density.
Group 1 -- (Weeks 2 and 3) Sample light, temperature and dissolved oxygen in each of the 8 tanks and the lake. Repeat during week 3. Are treatments different? Are tanks diverging from the lake?
Group 2 -- (Weeks 2 and 3) Compare N and P levels of treatments and of the lake. Can the presence of fish alter the availability of nutrients? Are ponds diverging from the lake?
Group 3 -- Compare treatments for density of each zooplankton type (Daphnia, copepods, ostracods, other). Which taxa are affected the most by the presence of fish? If time permits, measure the average size of Daphnia in each of the tanks using the ocular micrometers. The results will indicate whether the two different treatments select for different body sizes in the Daphnia.
Group 4 -- Compare two treatments for species richness (i.e., number of species) of phytoplankton and total chlorophyll content (as a measurement of total phytoplankton density). Does the presence of a zooplanktivore effect the abundance of phytoplankton? the diversity of phytoplankton?
Chlorophyll protocol (spectrophotometric assay) -- You can estimate the total biomass of phytoplankton in your sample by estimating the total chlorophyll content.
1. Filter the sample onto filter paper.
2. Roll the filter and insert it into a test-tube. Add 5 ml of 80% acetone and cork the tube. Allow acetone to soak paper for several hours (can leave for 24+ hours).
3. Back in the 136 lab, pipette the acetone from each sample into a cuvette, trying to leave behind any plankton that have fallen to the bottom.
4. Set the wavelength on the spectrophotometer to 652 nm. With the cuvette cell closed and empty, use the left dial to set the needle to 0% Transmittence. Insert a cuvette with 80% acetone (a "blank") into the well and use the right knob to set the needle at 0 Absorbence (or 100% Transmittance).
5. Take absorbence readings of each sample.
6. Calculate chlorophyll concentration (mg/ml) in the cuvette as:
Where e (extinction coefficient) = 34.5 ml mg-1 cm-1 and l (cell pathlength) = 1 cm.
7. Since you probably filtered more than the 5 ml volume you have in your cuvette, you should calculate the chlorophyll concentration as:
concentration in tank = (concentration in cuvette)*(.005 L/ volume of tank water filtered).
Tallgrass prairie communities
The community type known as tallgrass prairie once occurred over vast expanses of the center of the North America. Today it exists only in tiny remnants and in small restored parcels like those at CERA. The fate of tallgrass prairies followed from the physical conditions associated with them: deep, nutrient-rich soil and warm, wet summer weather -- in other words, conditions with exceptional potential for agriculture. Imagine the prairies of CERA extending to the horizon like a rolling ocean of grass dotted with herds of bison and elk, and you'll get some idea of what this area once was like.
A common feature of the flowering plants of this community is the ability to resprout following shoot damage. This may be an adaptation to periodic grazing and frequent fire (grass burns easily, especially in fall and spring when it is dry). Some research suggests that by creating open space for colonization and changing physical environmental conditions, fire allows the persistence of some species that otherwise would outcompeted to extinction. This is one rationale for setting fires as a management tool in prairie restoration projects. How does fire change the physical environment? How do communities respond to these changes?
Week 1 -- Physical and biological parameters of burned and unburned prairie -- Each group will assess a set of physical conditions or biotic features of a recently burned patch of prairie and of another patch that was not burned this year. Your instructors will tell you which parcels to sample.
Sampling scheme for week 1 -- Taking a truly random sample in an area requires sampling every position with equal probability. That is often impractical, especially for very large areas. You'll do something even simpler (but not quite as likely to be representative): sampling random points within a haphazardly chosen 30 m section.
Choose an arbitrary position along one roadside edge of one of the two assigned prairie patches, and lay one of your 30 m tapes alongside it. Then, use a random number table to choose five random positions along that line (i.e., the first five numbers between 0 and 30 in an arbitrary column or row of the random number table). At that random point, lay out your second meter tape perpendicular to the road, pick a random point along it, and measure there. Then move to the next roadside point and repeat, sampling at five points in each kind of prairie.
Each group will collect one of the following sets of measurements, to assess: (1) spatial variability within each area; and (2) differences between burned and unburned areas.
Group 5 -- Physical conditions
Physical conditions have important effects on the performance of individual organisms and thus on the dynamics of their populations. Temperature, of course, affects a multitude of physiological and biochemical processes. Relative humidity (RH) is the amount of water vapor in air relative to the maximum that air can hold at a given temperature (warmer air holds more water), measured as a percentage. Organisms are essentially saturated with water, so the lower the relative humidity, the faster they lose water to the air. Wind is a source of mechanical stress, and it increases the rate of water loss from organisms (thus it both cools and desiccates them). Light radiation contains energy that heats organisms, and it provides the energy autotrophs use for photosynthesis.
1. Assemble two meter tapes, a meter stick, a random number table, a digital thermometer/hygrometer, an anemometer, and a light meter.
2. Review the operation of your instruments.
3. At each sampling point (see above procedure), take a vertical profile of air temperature, RH, windspeed, and light intensity, measuring each condition at the soil surface, 0.1 m above it, and 1 m above it.
Group 6 -- Soil temperature, water potential, and nutrient concentrations
These factors affect the physiology of organisms that live partly or entirely in soil, such as vascular plants, bacteria, fungi, soil invertebrates, and burrowing vertebrates. Soil water potential is a measure of how tightly water is bound to soil particles. As in aquatic environments, the concentrations of essential elements such as nitrogen and phosphorus affect the potential productivity of terrestrial communities.
1. Assemble two meter tapes, a random number table, a soil thermometer, a soil-moisture probe, a soil sampler, and 10 soil-sample bags.
2. Review the operation of the soil-moisture probe and the collection procedures for the soil test kit.
3. At each sampling point, record soil temperature by inserting the soil thermometer in soil up to a standard depth (you choose a depth). Record soil water potential at a standard depth, and collect a soil sample for later analysis.
Group 7 -- Above-ground plant biomass and necromass per area
These biological features of the environment are ecosystem-level properties, as they measure the amount of material (and energy) tied up in different components of the living environment, measured on a per-area basis. Biomass is currently living material, which in the prairie consists mainly of green plant shoots. Necromass is formerly-living material that is not yet decomposed. In prairies in spring, necromass consists mainly of standing dead plants from last year, plus plant litter (pieces that have fallen) on the soil surface.
1. Assemble two meter tapes, a random number table, one to a few small quadrat frames, a pair of clippers for each group member (do not lose these!), paper bags, and a marking pen.
2. At each sampling point, place the quadrat frame on the soil surface and clip off all plant material at the base. Put dead material (including plant litter and animal carcasses) into one labeled bag (lab day, group names, prairie, and sample number) and put live material into another. Clip the material into small pieces for ease of handling. The samples will be oven-dried back at the introductory biology labs.
3. Back at Grinnell, at least 2 d after your lab period, record the weight of each sample to the nearest 0.1 g.
Group 8 -- Plant density, diversity, and (live) shoot height
These variables characterize responses of the plant community to fire. Plant density is the number of individuals or stems per unit area. In the prairie, use shoots. Many prairie species are clonal, and single genetic individuals usually produce many shoots. Shoot height is simply vertical growth up to this point of the growing season. Plant species diversity is one of the features we might expect to respond to the fire treatments. At this time of the growing season, most species will not yet be flowering (some, of course, won't even have emerged from the ground), so you may not be able to identify the plants to species in making your diversity assessment. What you should do is count the number of visually distinguishable types of plants. Such types are called "morphospecies."
1. Assemble two meter tapes, a meter stick, a small ruler, hand counters, and one to a few small quadrat frames.
2. At each sampling point, count all shoots inside the quadrat frame, and measure the height of the each shoot, taking a mean.
3. At each sampling point, record the number of "morphospecies" (visually distinguishable types) of plants within the quadrat. For any species that are flowering, collect a sample individual and identify it.
Group 9 -- Soil and litter animals
The response of animal communities to prairie fire is less well characterized than the response of plant communities. Many of the plants in CERA's prairies have been re-introduced deliberately as part of the restoration process. On the other hand, animals have been left to "fend for themselves," colonizing on their own. For these reasons, data on animal diversity and abundance in the
prairie-fire experiment should be especially valuable. Soil animals (primarily arthropods such as insects and spiders) are important and easy-to-sample.
Your instructors will give you more detailed procedures for this sampling at lab. Briefly, the procedure involves collecting litter samples, bringing them back to the lab, and place them in an apparatus called a Berlese funnel. This device heats litter from above with a light bulb, encouraging soil animals to move down to where they fall out the spout of the funnel into ethanol, where they await sorting and identification. During week 1, you'll sort through samples collected by your instructors the previous week, and you'll collect litter samples from the experimental plots for analysis during the following weeks.