width multicolum not sum of combined cells

Question

I have the following table, but the multicolumn cell width is not the same as the sum of the cells it is combined from.

I assume, this is caused by some margins in each cell, i.e. that the cel width (distance between the two vertical lines) is equal to the set cell width plus some margin - but what is this margin, or what is the easiest way to set the width of the multicolumn cell to be equal to the sum of the widths of the cells it is combined of?

Thanks

\documentclass[10pt]{article}
\usepackage{array}

\begin{document}

\begin{tabular}
    {|>{\raggedright}p{2cm}l|>{\centering}p{0.9cm}|>{\centering}p{0.9cm}|>{\centering}p{0.9cm}|}
    \hline 
     &  & {March} & {April} & {May}\tabularnewline
    \hline 
    \multicolumn{2}{|c|}{{Species}} & \multicolumn{3}{>{\centering}p{2.7cm}|}{{This is the problem cell, as it has not the width as the ones combined}}\tabularnewline
    \hline 
\end{tabular}

\end{document}

results in the following table:

Answer 1

There is 2*\tabcolsep between each column, and in addition (if using array package) \arrayrulewidth for each vertical rule.

If you load the calc package you could use

  p{2.7cm+4\tabcolsep+2\arrayrulewidth}