Natural systems contain a wide variety of species populations that are assembled into communities, for example, the insect community, the bird community etc. Each community contains a few to many species of each broad category. Species populations interact with one another to form stable, functional communities, through which matter and energy flow. The numbers and proportions of the different species that are found in an area make constitute the area’s ecological diversity or biodiversity. Diversity is composed of two components, species richness, which is simply the number of species present and species evenness, which is the relative proportion of each species. For example, if 100 members of a community are sampled and 20 species are found, the community would have high species richness. However, if 81 of the 100 individuals belonged to the same species, and the remaining species each were represented by only one individual, then the community would exhibit low species evenness because one species dominates the community.
Disturbance or alterations of communities can result in declines in species richness and/or evenness. These types of changes can alter the flow of matter and energy, and change the functioning of a community. For example, if prey species richness declines in an area, the ability of predators to find the food they need may be impacted, potentially causing populations declines in predator species. This in turn may impact the ability of other species to obtain the resources they need. Thus, measuring species diversity and comparing species diversity among habitats can provide a window into the stability and complexity of many ecosystems.
A major concern of ecologists and conservationists is the maintenance of biodiversity. Biodiversity is essentially the variability that exists within populations, communities, and ecosystems. Biodiversity is essential for communities to function properly. Losses of species within communities can be devastating; depending on the role the species plays. Maintaining biodiversity in communities can benefit humans because of the services that communities and ecosystems provide, such as flood and erosion control, and air and water purification. When communities are disrupted, by humans or by episodic or catastrophic natural events (e.g., hurricanes, floods etc.) biodiversity may decline. The most common disruptions caused by humans are habitat alteration (e.g., draining/filling wetlands, housing developments, etc.) and removal or addition of species (e.g., predator removal, intentional or unintentional species introductions like zebra mussel, house sparrow etc.). When disruptions occur, ecological processes degrade and ecosystem structure and function is altered. For example, more than 9 million km2 of grasslands and forests have been altered or destroyed due to agriculture or logging, which reduced the ability of the habitat to hold water, while increasing soil erosion. As a result, much of this area is now a desert. The desertification of these areas and the concomitant loss of biodiversity have resulted in losses of economic as well as aesthetic benefits that were part of the original habitats and will likely never be recovered.
Species diversity is a measurable characteristic that reflects some of the organizational features of a community. Diversity is related to the degree of community stability and complexity. In other words, communities of organisms that exhibit high diversity are typically more stable and complex, whereas lower diversity communities are less stable and complex.
In this lab you will measure insect species diversity in a variety of habitats found on campus (i.e., mowed lawns, wooded areas, tall grass, etc.). To measure diversity you will use the Shannon-Weiner Index (H). This common species diversity index takes into account both species richness (number of species present) and species evenness (relative proportion of each species).
The purpose of this investigation is to determine how insect species diversity varies between different habitats
Pens or pencils
You will work in groups of 2-4, depending on the number of nets and students. The procedure is the same in each habitat. Take photos of your field site for your report.
- Create a hypothesis. Of the two habitats, which will provide the greater species richness and evenness?
- Describe the two habitats you are going to sample. Be detailed and include the type of vegetation, its approximate height and/or density, weather, temperature, humidity, recent precipitation, soil features, and any other characteristics that you think are important.
- Sweep the vegetation 10 times using a wide, swinging arc that brings the net into contact with the vegetation.
- Categorize and count the insects collected; record on your datasheet.
- Remain in the same habitat but move a little away from your first sampling site and repeat steps 3-4. Finally, move a little away to a new sampling site in the same habitat and repeat steps 3-4 a third time.
- Repeat Steps 3-5 three times in the second habitat.
Lab – Species Diversity – Habitat 1
Observer: Date: Time:
Trial 1: Trial 2: Trial 3:
Lab – Species Diversity– Habitat 2
Observer: Date: Time:
Trial 1: Trial 2: Trial 3:
After Collecting Your Data, Calculate Species Diversity
Calculate the species diversity for each habitat based on the total number of insects collected at each habitat, using the Shannon-Weiner Diversity Index (H). The formula is
H = ∑Pi |lnPi|
Pi=the number of individuals of species i present, as a proportion of the total
number of individuals in the sample (i.e. 84 black ants / 256 total insects =
15% = 0.1500). All Pi values for a given sample should add up to 1.0000.
|lnPi|= the absolute value of the natural log (ln) of species i
Work through the following example before using Excel to calculate the species diversity (H) of your own samples.
Sample Calculation for Shannon-Weiner Diversity Index (H)
Assume a sample has 256 individuals comprised of 5 species and record the number of each of the species. Then, calculate the proportion of each species in the sample (Pi).
Table 1. Sample data with Shannon-Wiener Diversity Index calculation.
|ln(Pi)| is the absolute value of the natural log of that proportion value for each species and the final column is the multiplication of the natural log value and the proportion.
What does it mean? When using the natural log, the H-value ranges from 0 to ~4.6, but is rarely greater than 4. Typical H values are generally between 1.5-3.5 in most studies of ecological systems. H-values close to 0 indicate low ecological diversity (low species richness and low species evenness) while H-values approaching the higher end indicate high ecological diversity (high species richness and high species evenness).
Species Diversity Lab Report: Guidance and Grading Rubric
This section describes the required elements of the Species Diversity lab report, how to address each element, and how each element will be graded. Your lab report should contain the following sections:
- Study Site
- Materials and Methods
- Reference (or, References)
Table 2 lists the required elements of your lab report and the point values of each one, based on the amount of work required to complete it. For example, the Introduction has fewer points associated with it than the Results section so expect to spend more time on the Results section. In addition, the overall appearance and organization of the report, as well as your spelling, punctuation, and grammar, will be graded.
Table 2. Point values of elements in the Species Diversity lab report, totalling 30 points.
|Study Site Materials and Methods||3 pts 3 pts|
|Spelling, punctuation, grammar||2 pts|
In the introduction to the lab report you provide background information about the topic, and then at the end, explicitly state the rationale for the study (purpose of the study or hypothesis tested). For the Species Diversity lab, address these questions as part of the background information:
- Why is it important to study species diversity?
- What can species richness and species evenness tell us about the environmental conditions in an area?
Use this section to describe the location(s) you sampled in your experiment. Environmental science, unlike chemistry or physics, does not occur in isolation. The type/characteristics of the environment and the type of weather conditions at the time of sampling can affect the results of any experiment. Describe each habitat in prose (full sentences in paragraphs, not a bulleted or numbered list); be detailed and include the types of vegetation, its approximate height and/or density, weather, temperature, humidity, recent precipitation, soil features, and any other characteristics that you think are important. Include at least one photo of the study site.
Materials and Methods
In this section, describe the equipment used and the methods by which you collected the data. Give a step-by-step account of how you applied the method in the field. Present your description in prose (full sentences in paragraphs) not in a bulleted or numbered list. Be detailed, write in past tense (because the experiment is already completed), and avoid the use of first person tense (“I did this,” “we sampled there,” etc.). You might say instead, “researchers sampled by…” (active voice, the strongest and thus the best) or “insects were collected by…” (passive voice, acceptable).
The Results section is one of the most important sections of the report, and is therefore worth the most points. In this section, present the data you collected as well as results of your calculations. Refer to Table 1 and your two data sheets for details on the data and calculations you should report. Present these data and calculations in tables (similar to Table 1 of this document), one table for each of the two habitats in which you collected samples. Provide Shannon-Weiner Index calculations for each of the two habitats.
Though you should present tables or graphs in the Results section, you should also include prose (explanatory text) in this section to guide the reader through the results and to highlight interesting or important points. That is, use text to verbally describe the results as displayed in the tables or graphs, and also include text which helps the reader understand the “take home message” of the graph or table. What species was most abundant? The least? Which habitat had the highest number of species found? Why did you use the Shannon-Weiner Index? Was the value calculated by the index high or low?
The Discussion section of your lab report should cover all the questions posed below.
- Was there a difference in insect diversity between the habitats? List some differences in habitat characteristics that might be responsible for the differences in insect diversity.
- Were some insects more abundant (or only found) in one habitat compared to the other? Why or why not?
- How might differences in insect diversity affect other organisms (e.g., individual role insects plays in an ecosystem, predators, prey etc.) in the various habitats?
- How might the season, or time of year of sampling, affect species diversity in the two habitats?
Do not write your Discussion section as a numbered list of questions with answers. Instead, work your answers into prose that is logical and concise, that addresses the questions, but also takes broader ideas into consideration. Use the questions posed above to guide you, but don’t feel that they are the only questions that need be answered. Think of the Discussion section as the ‘Why’ part of your report: Why did you find the results that you did? What are possible influencing factors that contributed to your results?
Science is not conducted in a vacuum. All investigators depend on information provided by others and build on that. List at least one reference (source of information) you relied on to complete your investigation and report. You might include the Lab Manual itself because you relied on it for the sampling methodology used in this lab, and calculation of’? the Shannon Weiner diversity index (see the Powerpoint for this lab which shows how to cite it). Your textbook might be a good source of background on the importance of diversity in communities. You might have consulted another source for information about the climate of the area when you described the study site.
Overall Appearance and Organization
Use the formatting conventions detailed in Table 3 when writing your paper. When writing scientifically it is important to organize your paper into sections with headers, and that you make your writing concise. Clarity in writing and organization are a goal of scientists (because they have to communicate their findings to others), and it should be your goal in this report as well. Scientists confine their writing to “just the facts” and strive to write in an informative, objective manner. Flowery writing, excessive adjectives, opinions etc. have no place in scientific writing. There are no extra points for filling the space with fluff. This is not an exercise in creative writing, but in communicating clearly and logically. It is also an exercise in critically thinking about an objective, and the data collected to address that objective.
Table 3. Formatting conventions for Species Diversity lab report.
|Margins||1” on all sides|
|Font||12 pt, Times New Roman or Arial|
|Spacing||Double spacing except in Tables, Table captions and Figure captions|
|Paragraph||Indent first line 0.5”|
|Species names||Capitalize each Genus name, species name is all lowercase; both names are italicized (or both are underlined)|
|Major Section Headings (i.e., Introduction, Methods etc.)||All capital letters, left justified, bold|
|Figures and Tables||Number each Figure and Table (e.g., Figure 1) and give each its own caption|