5.3: 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 \(\PageIndex{1}\) ). 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 \(\PageIndex{1}\) Basic external characteristics of spiders useful for identifying individuals to species.
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 \(\PageIndex{2}\) ).
Figure \(\PageIndex{2}\)
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 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 \(\PageIndex{3}\) ). 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 \(\PageIndex{3}\) An example of a collectors 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, choose 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 move consistently in any direction to a new specimen and record whether it is a member of a new species. 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 \(X\)). 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 collector's curve as to whether the sample of spiders is an adequate characterization of spider diversity at the site?