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Our discussion this week focused on patterns of biodiversity.  The readings dealt with latitudinal trends: the general pattern being that species diversity increases as one moves closer to the equator, with lowest diversity near the poles.  The question was raised as to whether organisms diversified at the equator, or had instead simply converged there.  It was noted that the latitudinal biodiversity gradient has gotten steeper since the Miocene.  Evidence suggests that extinction rates have decreased or speciation rates have increased in the tropics (rather than death rates at the poles increasing).

It was learned that general patterns of diversity fall into three categories: latitudinal, altitudinal, and radial.  Latitudinal trends tend to follow an increase in biodiversity towards the equator and a decrease in biodiversity towards the poles.  There are, however, cases of reverse latitudinal gradients, such as in the pitcher-plant.  In terms of altitudinal patterns, plant and animal diversity tends to decrease for increasing elevations.  The idea behind radial gradients is that diversity can either increase towards a location, or can increase away from (decrease towards) a location.  We explored the idea of radial gradients and struggled to find examples of an increase in diversity away from an area. 

It was observed that there are many exceptions to the general rule of biodiversity trends.  It was therefore asked whether the cases that do not fit are really exceptions, or perhaps the general rule does not hold across phyla, having only been well-documented among invertebrates.  In the readings, the pattern holds in the case of resident waterbird species and almost holds in the case of sandy-beach meiofauna.  It does not hold for the case of aestival waterbird species.

In the Kotwicki paper, figure 3 shows species composition pie charts in terms of relative abundance percentages.  It was noted that the region where the paper had concluded highest biodiversity (temperate) had the least amount of evenness.  Figure 5, a graph of total number of taxa as a function of latitude, shows high variability, and it was mentioned that trying to fit a line to the data was a poor way of describing the general pattern.  This figure, as well as figure 4 (number of taxa as a function of region, showing alpha, beta, and gamma diversity) brought us to the conclusion that they did not take enough data points for their regions, particularly in the tropics.

In the Barcena paper, it was mentioned that redundancy, being an important result of species richness, is a way of safeguarding against losing an entire niche to disease or other event.  As we examined the variables used in describing factors that may influence the diversity patterns of European waterbirds, the complexity of separating the numerous environmental factors became clear.  Many of the environmental factors depend on more than one variable.  Sometimes the variable’s relationship to biodiversity may be directly related through one factor, but indirectly related through another.  The discussion made it clear that determining patterns of biodiversity is a difficult endeavor.