Dear Fernando,

I would suggest the function nls() that fit non-linear model.
You can specify by hand your equation and starting values of your 
parameters (your guessed value)
R include some Self-Starting functions for the model the most used, this 
could be easier for you to work with it. check ?selfStart()
According to wikipedia, Type II functional response is similar to the 
Michaelis--Menten equation, which is implemented in R as self starting 
function:http://stat.ethz.ch/R-manual/R-patched/library/stats/html/SSmicmen.html 

By adpting examples, you could easily fit your model, if the choosen 
equation is suitable.

About statistical comparisons of estimated parameters among curves, I 
have no idea but I found, among other this post:
http://stats.stackexchange.com/questions/26611/how-to-test-the-effect-of-a-grouping-variable-with-a-non-linear-model

Good luck,
Pierre

Le 30/05/2013 07:49, Luis Fernando García Hernández a écrit :
> Dear all,
>
> I am new on the list and on the more complex applications of R, so I ask
> you to excuse me if my mail is too long.
>
> This time I have to do several tasks related tu functional reponse. I want
> to evaluate the relationship between prey density (x) vs prey consumed(y)
> the idea is to fit the data set to the functional reponse equation:
>   Na/TP=a/(1+aThN). I have idea about the value of most of the equation
> parameters, but I am completely lost about how to fit the data points to
> this equation.
>
> On another hand I have to graph and compare if there are significative
> differences on estimated parameters of the curves for lepidopterans and
> ants, the final data should look like the attacher picture.If any of you
> have some idea about how to do this, would be really appreciated.
>
> Below you  can find the data and the pictures.
>
> Thanks in advance
>
>      Prey Density Eaten  Lepidopteran 1 2  Lepidopteran 1 3  Lepidopteran 1 3
> Lepidopteran 3 4  Lepidopteran 3 5  Lepidopteran 3 4  Lepidopteran 5 7
> Lepidopteran 5 5  Lepidopteran 5 6  Lepidopteran 10 8  Lepidopteran 10 10
> Lepidopteran 10 7  Ant 1 3  Ant 1 1  Ant 3 4  Ant 3 2  Ant 5 4  Ant 5 6  Ant
> 10 5  Ant 10 6
>
>
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