I always know if there is someone unwelcome around as the girls heads will go up and usually someone will make the classical high pitch screech, I have even seen them gather together in a bunch and start marching towards whatever they find threatening. I know if someone is walking a dog along the lane, as again their heads will go up and they will stand staring until they have gone.
At the moment we seem to have quite a few foxes around, even in the daytime!! Just the other day I was at the top of the field feeding the boys and saw the girls charging up the field chasing what at first I thought was our cat (which I thought was very strange! as they are used to the cat and don't generally find the cat a threat) but as they got closer it turned out to be a fox and he was running for his life!! with the girls in hot pursuit, they only stopped when the fox jumped the fence into the next field. And even then some of the girl were still screeching and hanging around the fence where the fox had disappeared!
So I was interested in reading the following article. http://www.smallholder-agriculture.co.uk/articles/the-truth-behind-alpaca-guarding-abilities-73/
I also think its very interesting that they seem to know who is a threat and who is not, they get accustomed to family dogs and don't find them a threat.
I also remember reading a newspaper article about Prince Charles having four gelded alpacas guarding his lambs. I believe alpacas make excellent field guards and am happy to promote alpacas as field guards and It would be very interesting to hear about other peoples views and experience on this subject.