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General Ecology Lab (BIO 160) Spring 2009 Dr. Jim Baxter MEASURING BIODIVERSITY (REVISED) If you take a walk outdoors, say along the American River or in the foothills of the Sierra Nevada, you’ll encounter a tremendous variety of species – plant, animal, fungal, and microbial. Even at this very local scale and in a brief walk, you’re likely to come across dozens if not hundreds of different species. Each of these species has evolved to succeed in its particular niche and is carrying out important ecological functions that affect many other species and even ourselves. On a much larger scale, Earth’s ecosystems support an amazing diversity of species. To date, scientists have identified and named approximately 1.4 million species worldwide – and we’re still discovering new species every day. It is estimated that the total number of species on planet Earth may be as much as 10 million! Over the years, ecologists have discovered that the number of species in any particular community can vary from very few species to many hundreds. For example, tropical rainforests are considered to harbor some of the highest numbers of species per unit area on the Earth whereas tidal marshes have relatively few species. These differences among communities pose interesting questions. For example, why do some communities have more species than others? What factors influence how many species a community has? How can so many species coexist? What are the functional roles of the different species in an ecological community? Which species are vital for maintaining certain ecosystem functions? The central importance of these and other questions have prompted ecologists to study the concept of biological diversity or biodiversity for short. Biodiversity describes the sum total variation of life forms across all levels of organization – from genes to ecosystems. Species abundance Because biodiversity is a broad and complex concept, a variety of measures have been created to measure it empirically. Most commonly, biodiversity is measured at the level of the species. The simplest measure of species diversity is species richness, which is a count of the number of species in a given area. Generally, species richness is determined for taxonomic communities or functional groups, such as the plant community (taxonomic) or zooplankton (functional) community. Another measure of species diversity is species evenness, which is a measure of Species 1 Species 2 how equitable the species in a community are in their Species 3 abundance. For example, a community with high Species 4 Species 5 evenness would be one in which all species are more or less of equal abundance, whereas one with low evenness would be one in which the community has one or a few dominant species and many rare ones. To illustrate this concept, four imaginary communities with the same richness (5 species) are shown in Figure 1; A B C D species evenness decreases from left (Community A) to Community right (Community D). Figure 1. Abundance of species in four imaginary communities (A - D) containing five species each. Although there are some quantitative measures of evenness, an informative graphical approach to describing evenness is to plot a rank-abundance curve. In this approach, species are plotted in sequence from the most to the least abundant along the horizontal (x) axis, with their abundances typically displayed in a log10 format on the y-axis. The advantage of a rankabundance curve is that both species richness and evenness are displayed together in a single graph and any differences in these measures among communities can be quickly compared. General Ecology Lab (BIO 160) Spring 2009 Sea star present Sea star absent Log Abundance For example, imagine two rocky intertidal communities along the coast of California in which a keystone species (i.e., sea star) is removed from one community but not the other. A rank-abundance curve of these two communities provides two basic pieces of information about the species diversity of these two communities (Figure 2). First, species richness of the community with sea stars present is 30 and with sea stars absent is 17. Second, as indicated by its steeper negative slope, the community with sea stars absent has lower species evenness than the community with sea stars present. Dr. Jim Baxter 0 5 10 15 20 25 30 35 Species rank Figure 2. Rank-abundance curves for two rocky intertidal communities in which sea stars are either present or absent. The intertidal community with sea stars present has both higher richness and evenness than the community with sea stars absent. Although these two measures are commonly used, ecologists have developed several diversity indexes that combine both richness and evenness together into a single index of diversity. These species diversity indexes are often used when comparing the diversity of one community to another and rely on abundance or frequency data of species in a community. Two commonly used diversity indexes are: Simpson’s diversity index and Shannon’s diversity index. Simpson’s diversity index (D) is based on the probability that two individuals chosen randomly from the same community belong to the same species. The index is calculated as follows: D= 1 s ∑p 2 i i =1 where pi is the proportion of individuals of the ith species to the total number of individuals in the community: ni/N (ni = the number of individuals of species i; N = the total number of individuals of all species) and s is the total number of species in the community. Simpson’s index is increased by having additional unique species (increasing species richness) and/or by having greater species evenness; it ranges from 1 to s. Shannon’s diversity index (H’) also combines richness and evenness into a single index of species diversity and is a measure of the likelihood that the next individual in the sample will be the same species as the previous sample. The index is calculated as follows: s H ' = −∑ pi ln pi i =1 where pi is the proportion of individuals of the ith species to the total number of individuals in the community: ni/N (ni = the number of individuals of species i; N = the total number of individuals of all species); s is the total number of species in the community, and ln is the natural log. Shannon’s index is also increased by having additional unique species (increasing species richness) and/or by having greater species evenness. The index can also be scaled so it ranges from 1 to s by taking the inverse natural log: eH’. Lab Exercise In this lab, we will compare the species diversity of two insect communities that occur on different plant species along the American River. We will collect our insect samples in the field using sweep nets and then preserve them for counting. To compare the diversities of our insect General Ecology Lab (BIO 160) Spring 2009 Dr. Jim Baxter communities residing on our two plant species, we will compare them using the measures of species diversity described above. Orders of insect herbivores we’re likely to find... Hemiptera – aphids, leafhoppers, cicadas Coleoptera – beetles (ladybugs, etc.) Diptera – flies (house flies, midge flies, crane flies, etc.) Neuroptera – lacewings, antlions Hymenoptera – ants, bees, wasps, sawflies Orthoptera – grasshoppers, crickets, katydids Counting the critters Since we want to count the abundance of each insect Order, it’s best to do the following: 1. First scan your sample and separate individual insects into similar groups 2. Use the Key to Insect Orders to key your insects taxonomically into Orders 3. Count the number of individuals in each Order in both of your samples. Data Analysis Our objective is to compare the diversity of insect communities living on the two plant species. To accomplish this, we will compare the two insect communities using the following quantitative statistics/approaches: species richness, species evenness, Simpson’s index, Shannon’s index, relative abundance, and rank abundance. Steps: 1. Develop a testable hypothesis about the insect diversity or composition of your two communities. Do you expect them to be the same or different? 2. Enter the number of individuals of each insect Order into the Excel spreadsheet provided. Note: your ‘counts’ will be transferred to the appropriate sheets in your Excel file to calculate and plot your diversity measures. 3. Plot (bar graph) the abundance of each species in your two insect communities. 4. Calculate and plot (bar graph) the means (± SE) for species richness, Simpson’s index (D), and Shannon’s index (H’ & eH’) for each community. 5. Compare the species diversity of your two communities by conducting a t-test on your species richness and Simpson’s index data. 6. Plot (scatter plot) a rank-abundance curve for each insect community. ASSIGNMENT (10 pts; Due next week): Interpreting Your Results The questions below require you to complete the data analysis section above. To receive full credit, you must include the results above (graphical and statistical) to support your answers. 1. Which insect order/s was/were dominant on each plant? Which were rare? 2. Did your two insect communities differ in the relative abundance of species? If so, how? 3. Did your two insect communities differ significantly in species diversity? If so, how? Note: Be sure to address both richness and evenness and to include any statistical and/or graphical results. 4. Why do you think you got the results you did? Was your hypothesis supported? Be sure to provide an ecological interpretation of your results.