i) I find your choice of subject interesting and important, but your research question seems a bit too academic and overcomplicated. I had to read it a couple of times to get an idea. 

ii) So you will contact ‘environmentally concerned individuals’ online, personally, I’d be more inclined to hear from ‘advocates’ though. ‘equal number of neutral, pro- and anti- grazing responses’ - will you find enough experts on this subject, willing to take part?

iii) Your method of data collection seems reasonable. My question is whether your research country/region specific or global?

iv) The aspects you address I find very relevant - ‘carbon sequestration and biodiversity’.

As for  ‘suitable proxies’ - it may not be necessary and you’ll find sufficient information without broadening your scope.

vi) Beside food production it fits the bill regarding ecosystem services as well.

vii) I take it you wish to challenge that pro-gazing ‘vocal group’ by your research.