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عنکبوتها – A comparison of spider
عنکبوتها – A comparison of spider

 

مقایسه ای از عنکبوتها – A comparison of
spider

Procedures

    Spiders are a highly species rich group of
invertebrates that

    exploit a wide variety of niches in
virtually all the earths

    biomes


. Some species of spiders build elaborate webs that    passively trap their prey whereas others are active predators    that ambush or pursue their prey. Given spiders taxonomic    diversity as well as the variety of ecological niches breadth    along with the ease of catching them, spiders can represent    useful, fairly easily measured indicators of environmental    change and community level diversity.         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. The exercise can be undertaken in three parts,    depending on your interest level.           Level (1):  You will gain experience in        classifying organisms by sorting a hypothetical collection        of spiders from a forest patch and determining if the        spider collection is adequate to accurately represent the        overall diversity of spiders present in the forest patch.              Level (2):  If you wish to explore further, you        can sort spider collections made at four other forest        patches in the same region and contrast spider communities        in terms of their species richness, species diversity, and        community similarity. 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.              Level (3):  If you wish to explore the concepts        of biodiversity yet further, you will next take into        account the evolutionary relationships among the families        of spiders collected. This phylogenetic perspective will        augment your decision making about priorities for patch        protection by accounting for evolutionary distinctiveness        in addition to diversity and distinctiveness at the        community level.          Once you have worked through these concepts and analyses you    will have a much enhanced familiarity with the subtleties of    what biological diversity is.          Level 1:  Sorting and Classifying a Spider Collection and    Assessing its Comprehensiveness    Obtain a paper copy of the spider collection for forest    patch “1.” The spiders were captured by a biologist    traveling along transects through the patch and striking a    random series of 100 tree branches. All spiders dislodged    that fell onto an outstretched sheet were collected and    preserved in alcohol. They have since been spread out on a    tray for you to examine. The spider collection is    hypothetical but the species pictured are actual spiders    that occur in central Africa (illustrations used are from    Berland 1955).         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, leg length, hairiness, relative size    of body segments, or abdomen patterning and abdomen shape    all might be useful characters. 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 beetles 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 spec.   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. Construct a table listing each species, its    distinguishing characteristics, the name you have applied to    it, and the number of occurrences of the species in the    collection (Figure 2).  Last, ask 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 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 collectors      curve (Colwell and Coddington      ۱۹۹۴). These plot the cumulative number of species    observed (y-axis) against the cumulative number of    individuals classified (x-axis). The collectors 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 3). If sampling stops    while the collectors curve is still rapidly increasing,    sampling is incomplete and many species likely remain    undetected. Alternatively, if the slope of the collectors    curve reaches zero (flattens out), sampling is likely more    than adequate as few to no new species remain undetected.    To construct the collectors curve for this spider    collection, choose a specimen within the collection at    random. This will be your first data point, such that       and     because after examining the first individual you    have also identified one new species! Next move consistently    in any direction to a new specimen and record whether it is    a member of a new species. In this next step,    , but  may remain as    ۱ 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    collectors curve from the data obtained (just plot     versus    ). Does the curve flatten    out?  If so, after how many individual spiders have been    collected? If not, is the curve still increasing? What can    you conclude from the shape of your collectors curve as to    whether the sample of spiders is an adequate    characterization of spider diversity at the site?             Level 2: 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 other forest patches. The forest patches    have resulted from fragmentation of a once much larger,    continuous forest. 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). Here are the additional    spider collections: (See Figure 4, Figure 5, Figure 6, and Figure 7)

 

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