Is there a function in R that will allow me to estimate the sample  
size required from count data (poisson data?), given the known  
variance and desired margin of error and confidence interval?

My specific data set will be based on a survey of hikers that will be  
asked about the number of animals of species 'x' they observed during  
a given period. I need to know the number of hikers to interview. ie,  
I would like to calculate the mean number of species 'x' +/- margin of  
error with 95% confidence.

This is a simple exercise for normally distributed continuous data,  
but I'm running into roadblocks for count data.

Sincerely,
Shawn Morrison
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