title: Combine data.frames summing up values of identical columns in R
body: <p>I have 3 data frames (rows: sites, columns:species name) of species abundances within sites. Row numbers are identical, but column numbers differ as not all species are in all three data frames. I would like to merge them into one data frame with abundances of identical species summed up. For example:</p>
  
  <p>data.frame1</p>
  
  <pre><code>       Sp1  Sp2  Sp3  Sp4
  site1   1    2    3    1
  site2   0    2    0    1
  site3   1    1    1    1
  </code></pre>
  
  <p>data.frame2</p>
  
  <pre><code>       Sp1  Sp2  Sp4
   site1  0    1    2
   site2  1    2    0
   site3  1    1    1
  </code></pre>
  
  <p>data.frame3</p>
  
  <pre><code>       Sp1  Sp2  Sp5  Sp6
   site1  0    1    1    1     
   site2  1    1    1    5
   site3  2    0    0    0
  </code></pre>
  
  <p>What I want to have is something like:</p>
  
  <pre><code>       Sp1  Sp2  Sp3  Sp4  Sp5  Sp6
   site1  1    4    3    3    1    1
   site2  2    5    0    1    1    5
   site3  4    2    1    2    0    0
  </code></pre>
  
  <p>I guess i'd have to work with merge, but so far my attempts have failed to get what I want. </p>
  
  <p>Any help is appreciated.</p>