Minor parties seem to be often built around the appeal of a charismatic founder - Hanson and One Nation, Chipp and the Democrats. Of course, the Democrats survived as a Senate presence for nearly twenty years after Chipp's departure but the dream of a third force in Australian politics was never realised. The rise and fall of One Nation was all over in a brief few years.
Despite the current balance of power position held by the Greens in the Senate, the party's performance has been disappointing in recent state elections in Victoria, NSW and Queensland. The Greens are unlikely to hold the balance of power in the Senate after July 2014 if the current poll results are carried through until next year's election.
Bob Brown is considered to be politically conservative by many in the Greens. I think this conservative image (small town background, bland suits, medical doctor) has greatly helped the Greens extend their reach beyond the far left. Brown also has a legacy of credibility from the anti-dam campaigns of the 1970s and 1980s.
I think it will be much harder for Milne and Bandt to maintain that appeal. For one thing, Milne's language and tone is far more agressive.
Another thing that happens when the charismatic founder departs is that the ideological and personality conflicts in the party come to the fore. The Greens are an agglomeration of state-based organisations, with a strong emphasis on local organisation and local issues. Without the cohesive influence of Brown, it will be a challenge for the Greens to maintain a unified image.
It's going to be a tough period for the Greens - and a fascinating one.