I'm examining the growth rate of species en function of ion carbonate. For each specie, I have different measured of ion carbonate, it can be in different temperature or pH.

For each measure, I'm doing graphics x - ion carbonate; y- growth rate. I know that for each measure I can have one of this models :

1. y=ax+b
2. y=ax^2+bx +c
3. y=a*(1-exp(-x/b)) + c

so I have to choose which one to select. I'm doing this with AIC. But AIC is only for nested models, is my 3 model is nested? 1 and 2 are nested I think.

If not, how can I select what is the "best" model for my data?