[R] SAR: nonlinear and linear mixed model comparison using mmSAR - package

[Bernd Lenzner]

Hello,
I have a problem trying to compare model-fit between linear and 
non-linear mixed models. I am using the mmSAR-package from Guilhaumon 
(Version 1.0) to fit different models (power, exponential, lomolino, 
weibull etc.) to my data. Besides that I am as well fitting a linear 
model to the data.
My dataset looks like this:

           a        s
1  21.28038 2.944439
2  21.22179 3.091042
3  22.09917 3.526361
4  21.66947 2.197225
5  21.66702 3.713572
6  21.58499 3.044522
7  22.03311 3.465736
8  22.03289 2.397895
9  22.16306 3.610918
.
.
.

with a = area estimate and s = species richness. Both variabels are log 
- transformed to reach normality and variance homogeneity.

Now I want to compare the model fit of the linear model with the fit of 
the non-linear models by using information theory. I use corrected AIC 
values to estimate model fit but when I use the "multiSAR" function from 
the mmSAR package to genereate non-linear model fit I obtain negative 
AICc values:

 >mod.selection <- multiSAR(MODELS,DATA)

 >mod.selection$filtOptimRes

                     p1 p2                       p3                    AICc
power         1.349178e-01      9.999853e-01     0.00000000 -55.64722
expo          -1.201421e+01    4.856363e+00    0.00000000 -56.93415
negexpo     1.063781e+08    1.268287e-09      0.00000000 -55.64728
logist          3.762325e+00    3.364975e-01      6.02467719 -54.79978
ratio          -2.427444e-02     8.295920e-02      -0.01806225 -54.56568
lomolino     3.925678e+00    5.436657e+02     18.23351785 -54.79876
weibull        3.550748e+00   2.352926e-06      4.39606412 -54.81074

on the other hand I get a positive AICc value for the linear model

 >lin <- lm(log(s) ~ log(a))
 >AICc(lin)

 > 181.50


How do I interpret these results? Visually inspecting the regressions 
shows that the exponential regression line is almost identical to the 
linear one so I would expect that the AICc value should be somewhere in 
the same range.

Thanks for the help or suggestions.