Determination of Nitrogen Effects for ExxonMobil

The following provides a brief discussion of the potential adverse
effects to leafy prairie clover, and eastern prairie fringed orchid from
Nitrogen deposition. Information provided by ExxonMobil raises two
questions: 1) Is 1g/m2/yr an appropriate threshold value above which we
would conclude that a listed species would be adversely affected by a
proposed action, but below which we would determine that the proposed
action is not likely to adversely affect the listed species? 2) If that
threshold value seems too high, what would be a more appropriate value?

ExxonMobil proposes a threshold value of 1 g/m2/yr for Nitrogen
deposition based on WHO air quality guidelines for Europe, which
indicated a critical load value (similar to a no adverse effects level
or NOEL), or between 1 -- 1.5 g/m2/yr. That value was associated with a
decline in sensitive species in a species rich heathland.

ExxonMobil determined that the current background level of Nitrogen
deposition is 0.71g/m2/yr., that Indeck adds a small fraction of
0.01g/m2/yr (based upon Calpuff modeling), and that the project would
add 0.08 g/m2/yr, suggesting that the total Nitrogen deposition after
construction of ExxonMobil will be 0.8g/m2/yr (below ExxonMobil's
proposed threshold value of 1.0 g/m2/yr).

Weiss (1999) found that bay checkerspot butterfly populations had
declined or become extirpated in areas with higher Nitrogen deposition.
Sites that received Nitrogen deposition of 1.0-1.5 became invaded by
introduced grasses (e.g., *Lolium*), which crowded out the butterfly's
host plant. Sites that had deposition in the range of 0.4 to 0.6 g/m2/yr
did not become invaded by introduced grasses. Serpentine soils are
Nitrogen limited. Exxon Mobil indicated that the deposition values
reported by Weiss may be low because wet deposition was not accounted
for. However, our read of the paper indicated that the author did
consider wet deposition, and that it is likely lower than estimated by
ExxonMobil. Even if the wet deposition figure used by ExxonMobil is
correct, and adverse affects are present at 1.1 g/m2/yr, and absent at
0.7g/m2/yr, that would suggest a threshold value below 1 g/m2/yr.

Stevens et al. (2004) found that for every 0.25 g/m2/yr, one could
expect a reduction of a single species. ExxonMobil suggested that this
paper may not be appropriate because it evaluated nitrogen deposition in
acid grasslands. Stevens et al. (2000) show a regression with data
points beginning at 0.5 g/m2/yr, and species diversity declining as
nitrogen deposition increases beyond that point.

Wedin and Tilman (1996) looked at Nitrogen addition and noted that it
was associated with the loss of species diversity, with the greatest
losses occurring between 1 and 5 g/m2/yr. The graph they present shows
losses beginning below an application of 1g/m2/yr. Chris Clark (Graduate
Student in Tilman's lab at Cedar Creek, 2005 pers.comm.) indicated that
the graph showing effects below 1 g/m2/yr was based upon an
extrapolation. Based on their work they suggest that a threshold value
of 0.5g/m2/yr may be appropriate (Clark 2005, pers.comm.).

Suding et al. 2005 examined a number of studies and concluded that
species that are rare in their environment, or nitrogen fixers, or
perennial are more likely to become extirpated than are other species.

Summary:

WHO indicates that 1-1.5g/m2/yr is sufficient to protect sensitive
species in species rich heathlands, Weiss (1999) found that serpentine
grasslands retained native species at deposition rates of 0.4 to 0.6
g/m2/yr, but had become invaded by non native grasses at 1.0 -- 1.5
g/m2/yr, Stevens et al. (2004) indicated that on average, for every
0.25g/m2/yr of Nitrogen deposition a single species would be lost. Wedin
and Tilman noted most losses in species diversity between 1 and 5
g/m2/yr. Clark (2005, pers.comm.) indicated that a threshold value of
0.5g/m2/yr may be appropriate. These studies suggest that losses in
diversity, and increased abundance of weedy species, occurs at Nitrogen
deposition rates below 1 g/m2/yr, likely between 0.5g/m2/yr and
0.7g/m2/yr.

This analysis suggests that there is reason to believe that the current
background level of 0.71 g/m2/yr is close to a threshold value, and may
already be causing losses in species diversity by favoring invasive
species. ExxonMobil's addition of 0.08 g/m2/yr, when added to the
background level, would put Nitrogen deposition at levels where studies
show losses in biodiversity and increased abundance of invasive species.
The presentation in these studies shows a linear effect, suggesting that
ExxonMobil's addition would be noticeable.

Literature Cited:

Stevens, C. J., N.B. Dise, J.O. Mountford, and D.J. Gowing. 2004. Impact
of Nitrogen Deposition on the Species Richness of Grasslands. Science
303: 1876-1879.

Suding, K. N., S.L. Collins, L. Gough, C. Clark, E.E. Cleland, K.L.
Gross, D.G. Milchunas, and S. Pennings. 2005. Functional- and
Abundance-based Mechanisms Explain Diversity Loss due to N
Fertilization. Proceedings National Academy of Sciences.
102(12):4387-4392.

Wedin, D.A. and D. Tilman. 1996. Influence of Nitrogen Loading and
Species Composition of the Carbon Balance of Grasslands. Science 274:
1720-1723.

Weiss, S.B. 1999. Cars, Cows, and Checkerspot Butterflies: Nitrogen
Deposition and Management of Nutrient-Poor Grasslands for a Threatened
Species. Conservation Biology 13(6):1476-1486.

World Health Organization. 2000. Air Quality Guidelines for Europe,
second edition. WHO Regional Publications, European Series No. 91.