R help - May 2009 - Apologies: example of data needing zeros-no NA values

Many thanks for your responses.  I realize I made the first-time-poster's
error of not giving an example of the data.  There actually are no NA values, so
I'm not simply replacing values in the data frame. So, the number of species
(and therefore abundance values) listed in each treatment differs, even though
they all have the same number of plots sampled.
 
Sample_ID	 Spec_num	 Species_ID	 Abundance	 Treatment	
1	 1	   SOLCAN	 0.05	 1	
2	 1	   SOLCAN	 1.5	 1	
3	 1	   SOLCAN	 2.1	 2	
4	 1	   SOLCAN	 3.2	 2	
5	 2	   ERISTR	 2.4	 1	
6	 2	   ERISTR	 3.1	 1	
7	 2	   ERISTR	 5.6	 1	
8	 2	   ERISTR	 6.3	 1	
9	 2	   ERISTR	 2	 1	
10	 2	   ERISTR	 3	 2	
11	   2  	ERISTR	 2.1	 2	
 
 
Sorry for the repeat posting.
 
cathy
 
ORIGINAL MESSAGE:   have a dataset that I've read into R as a dataframe.
There are 5 columns: Plot location,species name, a species number code (unique
to each species name), abundance, and treatment. There are 272 plots in each
treatment, but only the plots in which the species was recorded have an
abundance value.  For all species in the dataset, I would like to add zeros to
the abundance column for any plots in which the species was not recorded, so
that each species has 272 rows.  The data are sorted by species and then
abundance, so all of the zeros can presumably just be tacked on to the last
(272-occupied plots) row for each species.