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Research Interests
This needs to be updated--hopefully soon. 1/8/05

Here are some descriptions of research that I am currently working on.

Process Based CO2 Modeling in a Desert Ecosystem (with D. R. Bowling)

This was work I had completed this past summer (2003) in the University of Utah Biology Department Bowling Lab . The focus of that group is ecosystem ecology, and my task in particular was to develop a model for belowground carbon dioxide cycling in a desert grassland field site to the south of Canyonlands National Park.  CO2 sensors installed at 5 and 15 cm for the two dominant plant species showed a marked increase in CO2 levels during rain events.  The intuitive question is why?  What causes these concentrations to increase?  In particular, are the increases driven by physical or biological processes?  This research was an attempt to discover this.

What we found out was that when it rains, the CO2 concentrations increase because the soil fills with water, effectively trapping the CO2 molecules from diffusing out.  As the soil dries, then diffusion returns as normal.  However soil microbes and root respiration also contribute CO2, as the rain acts as a biological switch for activity for both.  As a result, using a process based model that incorporates biology can increase the accuracy of models than just assuming source is of a particular form (such as constant with increasing depth).  What is unknown is the effect of plant roots on the absorbtion of water-which might dry the soil quicker than what the model would presume.

This work was presented at the fall meeting of the American Geophysical Union.  Link to the poster.

Obnoxious Adolescents and Aggressive Adults: Consequences of Age Structure in Biodiversity Models

This was independent work that I hade done for a class in Tropical Rain Forest Ecology, specifically looking at current models of biodiversity and asking the question: "Does incorporating age structure affect total biodiversity between two species?"

There are two ways to look at biodiversity, and they are different sides of the coin.  One approach is the Null Model advocated by Stephen P. Hubbel. [1]  In such a model birth and death are all likely equal processes, with interactions betweeen competing species all equal.  The advantage to such an approach is that one can generate a "fundamental biodiversity number," allowing one to compare biodiversity from habitat to habitat.  The disadvantage to such an approach is that it is mathematically dense and a discrete probabilistic model (ugh!)

On the other hand, one can use the principles of Competitive Exclusion advocated initially by Lotka, and subsequentially by May, Horn and McArthur. [7]  In a sense, what this does is focus the "lens of biodiversity" to interactions between competing species. As a result, biodiversity is maintained by tradeoffs between species (such as one as a better colonizer whereas the other is a better competitor for resources).  The disadvantage to such an approach is that it is hard to generalize to a range of species over a habitat.

The compelling need to work on such models is that tropical rainforests are vanishing rapidly as we speak.  Estimates (dated of course), place rain forests occupying less than 8 million square kilometers-a 70% reduction in area!  Thus understanding fragmented dynamics is important to understand and maintain the biodiversity we have.  Most models do not look at age structure, which should be important because species age and mature and the nature of their interactions changes.

This model asked what would happen if two species were competing for resource as juveniles, with one species (call it alpha), is a better competitor over the other species (call it beta).  As adults, there is no competition.  To picture this, imagine the scenario for two tree species-while they are still saplings, they compete for space or light (both examples of resources) and one can displace the other while young.  Yet once they mature, each tree is established, so competition is not a factor.  I was able to show that the range of coexistence is considerably lessened under such an approach (see the following graphs).


There are two immediate consequences one can gain from this.  First: biodiversity is much harder to maintain then previously thought.  Second: with an age structure approach, the amount of available resource left is considerably greater, making the habitat more susceptible to invasion or competition from other species.  In the case of trees, it is invasive species overtaking a habitat after clear-cutting.

Sources Cited
   
[1] Hubbell, S. P. 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton: Princeton University Press.

[2] Terborgh, J. 1992. Diversity and the Tropical Rain Forest. New York: Scientific American Library.

[3] Connell, J.H. Diversity in Tropical Rain Forests and Coral Reefs. Science. 199:1302-1310.

[4] Sheil, D. and D. Burslem. Disturbing Hypotheses in Tropical Forests. 2003. TRENDS in Ecolocy and Evolution. 18:18-26.

[5] Molino, J. and D. Sabatier. Tree Diversity in Tropical Rain Forests: A Validation of the Intermediate Disturbance Hypothesis. 2001. Science. 294:

[6] Chave, J. H. C. Muller-Landau, S. A. Levin. Comparing Classical Community Models: Theoretical Consequences for Patterns of Diversity. 2002. The American Naturalist. 159(1):1-23.

[7] Tilman, D. Competition and Biodiversity in Spatially Structured Habitats. 1994. Ecology. 75:2-16.

[8] Roberts, M.R. and F. S. Gilliam. Patterns and Mechanisms of Forest Diversity in Forested Ecosystems: Implications for Forest Management. 1995. Ecological Applications. 5(4):969-977.

[9] Edelstein-Keshet, L. 1988. Mathematical Models in Biology. New York: Random House.

[10] Armstrong, R. A. and R. McGehee. Competitive Exclusion. 1980. The American Naturalist.  115(2):151-169.

[11] Adler, F.R. and J. Mosquera. Is Space Necessary? Interference Competition and Limits to Biodiversity. 2000. Ecology. 81(11):3226-3232.