Hi @feixiang1209,
SILVA does not curate species level taxonomy. Moving forward we will, honor their intent, and likely no longer include the organism names as the species labels by default, for reasons outlined here, under the "Species-labels: caveat emptor!" drop menu.
You can certainly do this your self by using the --include-species-labels flag during download. Otherwise to make your plots, just set your rank depth as follows:
qiime taxa collapse --p-level 6 ...
Remember if something ever goes wrong always check out the help text first by adding --help to the command. 