What is Biodiversity? A Comparison of Spider Communities?

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What is Biodiversity? A Comparison of Spider Communities?

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What is Biodiversity? A Comparison of Spider Communities1 Lab Objectives: • To explore through classification of life forms the concept of biological diversity as it occurs at various taxonomic levels. • Learn about the various factors that influence reserve design and reserve selection decisions. Background Spiders are a highly species rich group of invertebrates that exploit a wide variety of niches in virtually all the earth’s biomes. Some species of spiders build elaborate webs that passively trap their prey whereas others are active predators that ambush or pursue their prey. Spiders represent useful indicators of environmental change and community level diversity because they are taxonomically diverse, with species inhabiting a variety of ecological niches, and they are easy to catch. This exercise focuses on classifying and analyzing spider communities to explore the concept of biological diversity and experience its application to decision-making in biological conservation. In Task A, you will gain experience in classifying organisms by sorting a hypothetical collection of spiders from a forest patch and determining if the spider collection accurately represents the overall diversity of spiders present in the forest patch. In Task B, you will further sort spider collections from four other forest patches in the same region and contrast spider communities in terms of their species richness, species diversity, and community similarity. Then, in Task C, you will apply this information to make decisions about the priority that should be given to protecting each forest patch in order to conserve the regional pool of spider diversity. Once you have worked through these concepts and analyses you will have a much enhanced familiarity with the subtleties of what “biological diversity” really means. The families of spiders named in this exercise are real, and the illustrations are of real species that occur in central Africa (illustrations used are from Berland, 1955). However, the combination of species and families collected at the different sites, as well as the geographic arrangement of the sites, is entirely hypothetical and might not represent the species assemblages one would find in the wild. 1 Adapted from NCEP Activity developed by James P. Gibbs, Ian J. Harrison & Jennifer Griffiths Task A: Sorting and Classifying a Spider Collection and Assessing its Comprehensiveness 1. Obtain a paper copy of the spider collection for forest patch “1” and cut out each square. The spiders were captured by a biologist traveling along transects through the patch, stroking a random series of 100 tree branches. All spiders that were dislodged and fell onto an outstretched sheet were collected and preserved in alcohol. They have since been spread out on a tray for you to examine. 2. The next task is for you to sort and identify the spiders. To do this you have to identify all the specimens in the collection. To classify the spiders, look for external characters that all members of a particular group of spiders have in common but that are not shared by other groups of spiders. For example, look for characteristics such as leg length, hairiness, relative size of body segments, or abdomen patterning and abdomen shape. Look for groups of morphologically indistinguishable spiders, and describe briefly the set of characters unique to each group. These operational taxonomic units that you define will be considered separate species. To assist you in classifying these organisms, a diagram of key external morphological characters of spiders is provided (Figure 1). Note that most spider identification depends on close examination of spider genitalia. For this exercise, however, we will be examining gross external characteristics of morphologically dissimilar species. Figure 1. Basic external characteristics of spiders useful for identifying individuals to species. 3. Assign each species a working name, preferably something descriptive. For example, you might call a particular species “spotted abdomen, very hairy” or “short legs, spiky abdomen.” Just remember that the more useful names will be those that signify to you something unique about the species. Fill out the table below listing each species, its distinguishing characteristics, the name you have applied to it, and the number of occurrences of the species in the collection. (Note: it is ok if you don’t fill up the table with this task.) Species # 1 2 3 4 5 6 7 Distinguishing Characteristics Name Number in the Collection Species # 8 9 10 11 12 13 14 15 Distinguishing Characteristics Name Number in the Collection Species # 16 17 18 19 20 21 22 Distinguishing Characteristics Name Number in the Collection 4. Construct a “collector’s curve” to determine whether this collection adequately represents the true diversity of spiders in the forest patch at the time of collection. Were most of the species present sampled or were many likely missed? This is always an important question to ask to ensure that the sample was adequate and hence can be legitimately contrasted among sites in order to, for example, assign areas as low versus high diversity sites. To do this you will perform a simple but informative analysis that is standard practice for conservation biologists who do biodiversity surveys. This analysis involves constructing a so-called “collector’s curve” (Colwell and Coddington, 1994). These plot the cumulative number of species observed (y-axis) against the cumulative number of individuals classified (x-axis). The collector’s curve is an increasing function with a slope that will decrease as more individuals are classified and as fewer species remain to be identified (Figure 2). If sampling stops while the collector’s curve is still rapidly increasing, sampling is incomplete and many species likely remain undetected. Alternatively, if the slope of the collector’s curve reaches zero (flattens out), sampling is likely more than adequate as few to no new species remain undetected. Figure 2. An example of a collector’s curve. Cumulative sample size represents the number of individuals classified. The cumulative number of taxa sampled refers to the number of new species detected. To construct the collector’s curve for this spider collection, begin by choosing a specimen within the collection at random. This will be your first data point, such that X = 1 and Y = 1, because after examining the first individual you have also identified one new species! Next randomly select another specimen and record whether it is a member of a new species or not. In this next step, X = 2, but Y may remain as 1 if the next individual is not of a new species or it may change to 2 if the individual represents a new species different from individual 1. Repeat this process until you have proceeded through all 50 specimens and construct the collector’s curve from the data obtained (just plot Y versus X). # o f s p i d e r t a x a # of samples a. Does the curve flatten out? _______ b. If so, after how many individual spiders have been collected? ________ c. What can you conclude from the shape of your collector’s curve as to whether the sample of spiders is an adequate characterization of spider diversity at the site? Task B: Contrasting spider diversity among sites to provide a basis for prioritizing conservation efforts In this part of the exercise you are provided with spider collections from 4 additional forest patches. The forest patches have resulted from fragmentation of a once much larger, continuous forest; a stylized map of the fragmented forest patches, showing their size and proximity to each other, is included at the end of this task. You will use the spider diversity information to prioritize efforts for the five different forest patches (including the data from the first patch which you have already classified). Your instructor will provide you will the additional forest patch collections. 1. Like before, tally how many individuals belonging to each species occur in each site’s spider collection (use your classification of spiders completed for Site 1 during Task A. 2. You can then analyze these data to generate different measures of community characteristics to help you to decide how to prioritize protection of the forest patches. In task C, you need to rank the patches in terms of where protection efforts should be applied, and you need to provide a rationale for your ranking (see last page). You will find it most useful to base your decisions on three community characteristics: The steps below outline how to determine these three community characteristics: a. Species Richness = Species richness is simply the tally of different spider species that were collected in a forest patch. Record this in the final summary data table located at the end of this task. b. Species Diversity = this is a measure of the variety of species in an area; it takes species richness and then factors the relative number of individuals present in from each species. For example, an area with 10 species and 100 individuals of each would have a higher “species diversity” value than an area with 10 species but 990 individuals on one species and 1 individual of each of the remaining 9 species. To calculate species diversity, we will use a standard index called Simpson Reciprocal Index, 1/D where D is calculated as follows: D = ∑pi2 where pi = the relative abundance of the ith species on an island. For example, assume you had a sample of three species with five, six and two individuals respectively. Using this scenario, here is how to calculation species diversity for a particular site: 1. For each species, divide the number of individuals by 13 (the total number of individuals of all species). This gives you your three pi values. Round to the nearest hundredth. 2. Square all the pi values. (ex. 0.382 = 0.14, 0.462 = 0.21, 0.152 = 0.02) 3. Add up all the squared pi values. (ex. 0.14+0.21+0.02 = 0.37) 4. Calculate the inverse of the sum of the squared pi values (the value from #3). This is your species diversity value (D) (ex. 1/0.37 = 2.70) The higher the value, the greater the diversity. The maximum value is the number of species in the sample, which occurs when all species contain an equal number of individuals. Because this index not only reflects the number of species present but also the relative distribution of individuals among species within a community, it can reflect how balanced communities are in terms of how individuals are distributed across species. As a result, two communities may have precisely the same number of species, and hence species richness, but substantially different diversity measures if individuals in one community are skewed toward a few of the species whereas individuals are distributed more evenly in the other community. Use the following tables to calculate the species diversity of the five forest patches. Site 1 Species Total # Relative Abundance (pi) Site 2 Relative Abundance squared (pi)2 Total # Relative Abundance (pi) Relative Abundance squared (pi)2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Totals: (pi)2 Total = Site 1 Species Diversity = 1/(pi)2 (pi)2 Total = Site 2 Species Diversity = 1/(pi)2 Site 3 Species Total # Relative Abundance (pi) Site 4 Relative Abundance squared (pi)2 Total # Relative Abundan ce (pi) Relative Abundance squared (pi)2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Totals: (pi)2 Total = Site 3 Species Diversity = 1/(pi)2 (pi)2 Total = Site 4 Species Diversity = 1/(pi)2 Site 5 Species Total # Relative Abundance (pi) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Totals: (pi)2 Total = Site 5 Species Diversity = 1/(pi)2 Relative Abundance squared (pi)2 c. Community Similarity = determines the percentage of species that two areas have in common. Areas with very distinctive, unique communities will stand out as having lower community similarity values. This can be another important perspective in ranking sites based on how different the communities are from one another. We will use the simplest available measure of community similarity, that is, the Jaccard coefficient of community similarity, to contrast community distinctiveness between all possible pairs of sites: CCJ = c/S where c is the number of species common to both communities and S is the total number of species present in the two communities. For example, if one site contains only 2 species and the other site 2 species, one of which is held in common by both sites, the total number of species present is 3 and the number shared is 1, so 1/3 = 33%. This index ranges from 0 (when no species are found in common between communities) to 1 (when all species are found in both communities). Using the data from your previous tables, calculate this index to compare each pair of sites separately, that is, compare Site 1 with Site 2, Site 1 with Site 3, …, Site 4 with Site 5 for 10 total comparisons: Site Comparison 1 vs. 2 1 vs. 3 1 vs. 4 1 vs. 5 2 vs. 3 2 vs. 4 2 vs. 5 3 vs. 4 3 vs. 5 4 vs. 5 c S CCJ = c/S Percent Community Similarity (=CCJ x 100%) It is useful to determine the average similarity of one community to all the others. Take all the CCJ values that include Site 1 and average them; then take all the CCJ values that include Site 2 and average them etc. and record in the final data summary table below. Final Data Summary Table: Site 1 Site 2 Site 3 Site 4 Site 5 Species Richness Species Diversity Average Coefficient of Community Similarity Task C. Reserve Design Recommendation 1. Once you have made these calculations of diversity (species richness, Simpson’s Reciprocal Index, and Jaccard coefficient of community similarity) you can tackle the primary question of the exercise: How should you rank these sites for protection and why? Making an informed decision requires reconciling your analysis with concepts of biological diversity as it pertains to diversity and distinctiveness. Explain your reasoning. 2. While your reserve decision can be based principally on your collected data, there are many other considerations that need to go into reserve design. Read the following information and then you will be asked to rerank the five forest patches in terms of best candidates for reserves. Guidelines for Reserve Design: • Large blocks of habitat containing a large population of a target species are superior to small blocks of habitat containing small populations. • Blocks of habitat close together are better than blocks far apart. • Habitat in contiguous blocks is better than fragmented habitat because it minimizes edge effects. • Adjacent land-uses that may impact the preserve (edge-effects) should be considered in reserve design • Interconnected blocks of habitat are better than isolated blocks and dispersing individuals travel more easily through habitat resembling that preferred by the species of concern. • Blocks of habitat that are roadless and inaccessible habitat blocks – especially important for sensitive species. 3. Considering the actual map of the five possible reserve sites and using the reserve design principles introduced above, explain how and why you would change your initial ranking of the five forest sites as spider reserve areas. 4. What additional information would you like to have to make a better decision about how to rank these sites? Literature cited Benstead, J.P., P.H. de Rham, J.-L. Gattolliat, F.-M. Gibon, P.V. Loiselle, M. Sartori, J.S. Sparks, and M.L.J. Stiassny. 2003. Conserving Madagascar’s freshwater biodiversity. BioScience 53 (11): 1101-1111. Berland, L. 1955. Les arachnides de l’Afrique noire française. Dakar, IFAN, 130 p. Coddington, J. A. and H. W. Levi. 1991. Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics 22:565-592. Stiassny, M.L.J. 1992. Phylogenetic analysis and the role of systematics in the biodiversity crisis. In N. Eldredge (ed.) Systematics, ecology, and the biodiversity crisis, pp.109-120. Columbia University Press, NY. Stiassny, M.L.J. 1997. Systematics and conservation. In G.K. Meffe and R.C. Carroll (eds.) Principles of Conservation Biology, Second Edition, pp. 70-72. Sinauer Associates, Inc., Sunderland, MA. Stiassny, M.L.J. and M. de Pinna. 1994. Basal taxa and the role of cladistic patterns in the evaluation of conservation priorities: a view from freshwater. In P.L. Forey, C.J. Humphries, and R.I. Vane-Wright (eds.), Systematics and Conservation Evaluation, pp. 235-249. Clarendon Press, Oxford.