Here are two responses to the question: Should conservation groups focus on cuddly, popular animals? I'm more sympathetic to man who says no. It seems frustrating to spend our time trying to save the Pandas who, even on their best days, seem to not even want to mate. It would be much more just to save the workers of society, those little invertibrates who really display a "I want to LIVE" sort of gusto.
In terms of the under-lying workload of species numbers, the distribution should be more like 1 per cent on vertebrates, 9 per cent on plants, and 90 per cent on invertebrates. Things get worse when we look at the conservation research literature: of some 3,000 papers in the two major journals, 70 per cent deal with vertebrates, 20 per cent with plants, and 10 per cent with invertebrates (and of these, one half were butterflies, a kind of honorary bird).
I imagine that in the end it comes down to what works, what brings in the support, and money.
Campaigns based on attractive mammals and colourful, interesting birds work. It is hard to imagine a successful fundraising campaign for a threatened slug species, never mind the important role that it might play in ecosystem function.