what did you get running the various options on your sample file?
start with the online help
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/An_overview_of_the_Overlay_toolset/000...
How do you merge 2 identical polygons and combine / concatenate attributes
use a relate after a dissolve (before dissolve, attribute table is exported for the relate later).
I may not be understanding the example clearly but the Dissolve aggregates my separate polygons (if done after a Union) or simply does nothing when applied to my initial sample file and run on the Species field
"ABC" & "-" & [FID] + 1