Consolidate duplicate rows

Question

I have a data frame where one column is species\' names, and the second column is abundance values. Due to the sampling procedure, some species appear more than once (i.e.,

Answer 1

> tapply(df$y, df$x, sum)
sp1 sp2 sp3 sp4 
  2   9   7   3 

if it has to be a data.frame Ben's answer works great. or you can coerce the tapply output.

out <- tapply(df$y, df$x, sum)
>     data.frame(x=names(out), y=out, row.names=NULL)
    x y
1 sp1 2
2 sp2 9
3 sp3 7
4 sp4 3