What to ignore?

As I have refined my own interests for my PhD research and beyond, I realize that you cannot measure everything - something falls by the wayside and is ignored in every project.  In some ways, this is "good" for other scientists and also promotes collaborations...although the abandoned and unanswered questions are often the ones that were deemed "too difficult" to address by earlier projects.

My reason for rambling about this topic during my morning coffee intake is this: I am reading an article on modeling nitrogen (N) export from New England watersheds (1) for my Ecological Systems Modeling course.  Many scientists have tried to answer the following questions from a terrestrial, aquatic, and/or whole-watershed approach: why does N export vary over time?  What controls N export?  And why has there been an overall decline in N export in the past 40-50 years?

My frustration is this: the terrestrial modeling papers ignore the fact that any N is processed in streams - the vectors of N transport from the watershed.  Because of this, many terrestrial models fail to accurately predict watershed N export, especially in low export years.  Instead of considering that something might be happening in aquatic ecosystems, terrestrial modelers just try different types of models and ranges of parameters over and over again to try to make better predictions.

It turns out that at least 30% of the decline in N export can probably be explained by higher N processing in streams (streams aren't just pipes!).  The increase in N processing (and decrease in N export; figure below) was a function of higher woody debris inputs from an aging and disease-ridden forest as well as higher primary production (noted by an increase in algae and biotic uptake) in the streams flowing through these watersheds (2).

And looking back at the terrestrial modeling paper that ignores stream processing: their model output confidence intervals do not even overlap with the N export data in years of low export (figure below).  I understand that not all models can include everything (something has to be ignored), but they don't even mention that aquatic N processing could explain this problem with their watershed model.

As someone with more of an aquatic bias, I cannot understand why none of the terrestrial modelers thought to look at the non-forest part of the watershed.  But maybe that's because I have been forced to think "beyond the stream" as a result of my interest in aquatic ecosystem ecology.  Aquatic ecologists are often chastised by reviewers if they do not include "enough" non-aquatic citations in their publications. In contrast, terrestrial ecologists are not generally asked to cite aquatic papers (3). 

This frustration is sometimes carried over to my day-to-day life at Wyoming: there aren't many "aquatic" labs in the Zoology department and often labs do not think beyond specific study organisms or systems.

I think this frustration is one of the main reasons why I really appreciate the Program in Ecology for providing me with a group of colleagues who enjoy thinking about "the big picture" - often well beyond their own study systems or favorite organism - keeping me (somewhat) sane in a field dominated by terrestrial studies.

Okay, time to write my "real" response to this terrestrial modeling paper for class this afternoon.  I will try to be kind....

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READING: (1)Hong et al. (2005) Bayesian estimation of input parameters of a nitrogen cycle model applied to a forested reference watershed, Hubbard Brook Watershed Six. Water Resources Research.

(2)Bernhardt et al. (2005) Can't See the Forest for the Stream? In-stream Processing and Terrestrial Nitrogen Exports. BioScience.


(3)Menge et al. (2009) Terrestrial ecologists ignore aquatic literature: Asymmetry in citation breadth in ecological publications and implications for generality and progress in ecology. Journal of Experimental Marine Biology and Ecology.
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