  A list $\Lambda =\{\lambda _{1},\ldots ,\lambda _{n}\}$ of complex numbers
(repeats allowed) is said to be \textit{realizable} if it is the spectrum of an
entrywise nonnegative matrix $A$. $\Lambda $ is \textit{diagonalizably
realizable} if the realizing matrix $A$ is diagonalizable. $\Lambda $ is said
to be \textit{universally realizable} if it is \textit{\ realizable} for each
possible Jordan canonical form allowed by $\Lambda .$ Here, we study the
connection between diagonalizable realizability and universal realizability of
spectra. In particular, we establish \textit{\ indices of realizability} for
diagonalizable and universal realizability. We also define the merge of two
spectra and we prove a result that allow us to easily decide, in many cases,
about the universal realizability of spectra.
