I like what manuelinor says about mistletoe as a keystone species, linking to science about it. In that paper, David Watson and Matthew Herring conclude that keystone status is likely in ‘other parasitic plants with high turnover of enriched leaves… most apparent in low productivity systems.’
I wonder how Watson and Herring’s conclusion might apply to the distribution of our own white-berried mistletoe (Viscum album) in the British Isles. On the site I’ve just linked to, Jonathan Briggs says that V. album is abundant in the South West Midlands of England but rarer in North and East England, Wales, Scotland and Ireland. I grew up in the West Midlands, on which Tolkein is said to have modelled his lush Hobbitland – the land doesn’t seem at all ‘low productivity’ there.
[Edit] Jonathan Briggs is another of my fellow bloggers here. He writes Jonathan’s Mistletoe Diary. Today he gives interesting responses to the Watson and Herring paper, and to reviews of it, by asking ‘Is mistletoe good, bad, or somewhere in between?’