Lecture 08 and 09 Species Diversity & Communities

Lecture 08 and 09 Species Diversity & Communities -...

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1 Biology 5C, Spring 2007 April 18 and 20 - Walton LECTURES 8 and 9: Species Diversity and Ecological Communities Ecological communities do not all contain the same number of species; yet, some very distinct trends are evident when we compare the number of species found in communities of different biogeographic regions of the Earth. For example, biogeographers and ecologists noted that animals and plants were more abundant and varied in the tropics than in other parts of the world. The regularity of these patterns for a number of very different taxonomic groups suggests that the patterns are the result of some set of basic underlying principles rather than accidents of history. Today, we want to consider how some of these trends in species diversity might be explained. Today's topic is also relevant to the lab exercise that you will carry out this week!! What does species diversity mean and how is it measured? The simplest measure of species diversity is a count of the number of species . This is the oldest measure of species diversity: species richness . i.e., 4 species: 0.25,0.25,0.25,0.25 vs. 0.8,0.05,0.05,0.1. Of course, not all species are equally abundant. One problem with species richness is that it treats rare species and common species as equals. Heterogeneity is also important when we consider the distribution of species in different habitats. Evenness : a measure of the relative abundance of species in communities . When species are more equally abundant, we say that the species exhibit a more even distribution. Let's look at an example: Community 1 : Species Abundance Rank Rel. abun. (p i ) p i ln(p i ) A 25 2.5 0.25 -0.3465 B 25 2.4 0.25 -0.3465 C 25 2.5 0.25 -0.3465 D 25 2.5 0.25 -0.3465 totals ( Σ ) 100 1.0 -1.3860 Community 2 : Species Abundance Rank Rel. abun. (p i ) p i ln(p i ) A 80 1 0.8 -0.179 B 5 3.5 0.05 -0.150 C 5 3.5 0.05 -0.150 D 10 2 0.1 -0.230 totals ( Σ ) 100 1.0 -0.709 Community 1 greater has a more even distribution than does the Community 2. We can compare the evenness of the two communities by plotting the relative abundance of the species in each community: ranked species abundance curves . Community 1 . Community 2
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2 Biology 5C, Spring 2007 April 18 and 20 - Walton Relative . Abundance A B C D A B C D Rank (ranks are equal: species plotted w/o overlap) Rank (ranks of B = C) Both richness and evenness should be included in a measure of species diversity. A mathematical index that includes both concepts is the Shannon Index of diversity. S H = - Σ p i ln(p i ) , where S is the total # of species i=1 H increases with greater richness and evenness. Is bounded by 0 and ln(S): 0 < H < ln(S). For a perfectly even community, H = ln(S). A way to express evenness in a community is to factor out species richness: J = H/ln(S) = H/H max J varies between 0 and 1. For Community 1, J = H/ln(4) = 1.386/1.386 = 1. For Community 2, J = H/ln(4) = 0.709/1.386 =0.511 * For a given species richness, a community with a more equitable distribution of species will have a greater species diversity than will a community with a comparatively inequitable distribution of species (e.g., the community composition is dominated by one species).
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