Biodiversity - Week 4

These pages are being generated as part of a senior seminar course directed by Neal Williams at Bryn Mawr College during fall semester, 2007 Among the topics to be considered are

  1. What is biodiversity?
  2. What's happening to biodiversity?
  3. Why is it happening?
  4. Why does it matter?
  5. What should/can be done about it?

This week, a discussion of ways of measuring biodiversity and their significance

  • Kotwicki et al (2005) Latitudinal patterns of meiofauna from sandy littoral beaches. Biodiversity and Conservation 14:461-474.
  • Barcena et al (2004) Latitudinal trends in breeding water bird species richness in Europe and their environmental correlates. Biodiversity and Conservations 13: 1997-2014.
Background notes (Word document)

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Comments

Paul Grobstein's picture

Biodiversity gradients and their signficance

Lots of interesting material with, as Maggie describes, varying degrees of compellingness. What I found mself particularly interested in is the question of what existing diversity gradients have to tell us about the significance of biodiversity itself. Clearly there are, in our biosphere at the moment and, as far as one can tell have always been, locations of higher and lower biodiversity. What this tells us, at a minimum, is that it is naive to think that "natural" corresponds to the highest possible biodiversity or to take "biodiversity" as a monotonic measure of biological well-being.

Existing biodiversity gradients seem to me to hold out the possibility of a more nuanced appreciation of biodiversity, and particularly of the question of whether biodiversity is actually an important functional characteristic of life, one that to some degree at least can be used as a measure of biological "health" or is instead simply a consequence of the tendency of biological systems to explore all possibilities. Perhaps biological diversity, at any given time, is simply a reflection of the combination of resource availability and the amount of spatial/temporal environmental variation, and so will be low when those factors are low and high when those factors are high?

My own inclination is to think that there is more than this to biodiversity, that there is actually some value of diversity to living systems, perhaps characterizable at fixed points in time (in terms of productivity or ... ?) but at least measurable in terms of the ability of systems to respond adaptively to unpredictable change (cf Grobstein, 1989, 2008). Looking forward to reading/talking more along these lines.

 

maggie_simon's picture

Summary of Class Discussion

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.

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