Ecol 206, March 2005
Bonine, Herron, Bachi
Greasewood Park Field Trip
* Please bring calculators to this lab.
Trip Goals:
1. Review identification of desert plants.
2. Learn to use one measure of plant diversity (using line transects).
3. Interpret data using Chi-squared test and Shannon-Wiener Diversity Index to understand patterns of plant distribution and biodiversity.
Introduction
We will be visiting a wash within Greasewood Park (near Anklam and Greasewood) in order to conduct experiments on different vegetation types (distribution and diversity) in and around this riparian habitat. When conducting experiments such as these, it is important to begin with thorough observations of the study site. Be sure to take lots of notes in your field notebook.
Environmental biologists still debate the best ways to measure diversity in communities. There must be a balance between getting a complete, detailed picture of the community and speed, ease, cost, and destructiveness of collection. Biologists must also decide whether to measure all species, or choose a subset as representative. We will be focusing on the plant community as representative of the diversity of the habitats we will sample. We will use transects to compare plants in each habitat (in wash, near wash, and far from wash).
Considering the null hypothesis (that plant diversity is uniform across all three habitats), you need to derive you own alternative hypotheses regarding plant distribution and biodiversity. For example, one alternative hypothesis could be that plant diversity is greater near the wash. You will collect data to test the validity of these hypotheses.
Intermediate disturbance hypothesis
Connell (1978) formulated this hypothesis, which states that biodiversity should be greatest when disturbance is at an intermediate level. Disturbance is a short-term reduction of the number of species in an area because of some environmental event. At a large scale, this could be a fire, flood, or human destruction. On a small scale, this could be a tree falling and creating a gap in the forest, or a rock on the coast rolling and changing the environment of the organisms attached to the rock. Connell suggested that at high levels of disturbance organisms are continually being wiped out, and so the diversity remains low. Intermediate levels of disturbance may perturb the system enough to allow early-stage successional species to become more common, but not keep later-stage successional species from being present. At very low levels of disturbance, a few species have taken over at the end of succession through competitive exclusion, and so diversity is also low. Connell tested this hypothesis in tropical rain forests and coral reefs; we will test it in a desert wash.
Reference:
Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science (199): 1302-1310.
Line transects
There are numerous techniques used to collect field data; we will be using line transects. Transects are lines taken across a habitat; where the line crosses a plant, the species is recorded. The transect will start at some stratified random point and then will be drawn out to a specific distance from the origin (e.g. 30 meters) in a random direction.
Instructions
In your field notebook, include a table with the following column headings for each habitat. Also record the background, hypothesis that you are testing, methods, results and discussion.
Transect # Size of transect Species observed # found
As an example, we will do an initial short first transect (in the wash) as a group. Then you will be divided into three groups, each group responsible for doing one transect in each of the three disturbance areas (in wash, near wash, and far from wash). The groups will then share data so that everyone in the class has multiple datasets for each of the three habitats.
Chi-square X2 statistical test:
Once collected, the data will be analyzed using a X2 test to assess whether or not the null hypothesis is rejected.
X2 = (observed value - expected value)2 / expected value
What is your expected value for the number of species in each of the three habitats, given the null hypothesis that there is no difference between them?
Shannon-Wiener index of diversity:
Compare diversity across all three habitats. The Shannon-Wiener Diversity Index measures species richness and species evenness at the same time. If one habitat has 7 species, but one species has 10 individual plants and the others have only 1individual each, whereas the second habitat has 7 species with 10 individual plants in each species, the index will measure the second habitat as more diverse.
Hence two components are combined in the S-W function:
1) Number of species.
2) Equitability or evenness of allotment of individuals among the various species.
H = (n log(n) - sum (Fi log (Fi))) / n
Where n is the total number of all individuals in a habitat, and Fi is the number of individuals in each species. Sum means that these numbers will be summed after calculating the value for each species. The index (H) should be calculated separately for each habitat.
Assignment
Calculate species richness for each disturbance type. Which is richest? Calculate X2 and the Shannon-Wiener Index for each habitat (show work). Which habitat is most diverse according to this index? How does your calculation relate to Connell’s “intermediate disturbance hypothesis”? How else could you explain your calculation of the S-W index? Use the X2 to analyze your data and explain your results (expected value: mean value across habitats). State and discuss your conclusions about your specific hypothesis (was it rejected or supported?). If a conservation organization were preserving one of the habitats, which should they choose based on your measurements? How do you think the plant community would differ if the wash flowed year-round (disturbance were constant)?
Give examples of other habitats / ecosystems with low, intermediate and high disturbance.
Name:______
Field Lab worksheet.
Introduction / Background (end w/ question, hypothesis, predictions):
______Null hypothesis H0: ______Alternative hypothesis H1: ______
Predictions:______
Methods:
Study Site:______
Study Organisms: ______
Field Procedures (inc. sample size): ______
Results (you may include table and/or graph):
______
Discussion/Conclusions (Do you support or reject H0): ______