The reason I prefer using Livestock Guardian Dogs (
http://lgd.org ) to guard my stock, is they are fearless against anything that wanders into their territory (which is defined by fencing). Livestock guardian Dogs (LGD) actually live among the flock, bond to the flock (sheep, goats, chickens, people) and take this job seriously...even lay their life on the line for their charges.
I prefer to work my dogs in pairs as they work as a team. One dog moves the flock away from the threat while the other dog stands his ground posturing and barking, giving a warning that you are not welcome here. If the predator/threat doesn't move on the dogs will take care of the problem.
Since I have packs of coyotes and dogs as well as bears, mountain lions, bob cats, raccoons, fox, weasles, hawks, and 2 legged predators to name a few, my dogs will only let us (myself, husband and children) in the pastures. However, if you are with me and I bring you into the pasture, don't be surprised if the dogs don't gently try to move you into flock (oh look, Mom brought us another one!) thinking that is where you belong! <smile>
I also have LGD's that have run of the farm. They freely run around the house, chicken coop and rabbitry and garden areas. These dogs are more socialized toward people but only if you say the magic words! <wink> This is because I have children and they have friends. The dogs recognize the words and the childrens smells and let them have full run of the palce. However, if a UPS man comes onto the property...the dogs let me know!
Another reason for using dogs is they do bark! oh and how they bark, when something isn't right. This then alerts me to a problem and I am able to assist. If a doe or ewe is birthing they let me know, if it is raining outside the dogs nestle up to the baby to keep it warm. My stories about my dogs are not the exception, but rather the rule.
While Alpacas/Llamas or Donkeys do not like canines (unless raised with them..but then goats and sheep don't like dogs at first either), they are no match for a pack of dogs, coyotes or wolves. They can't stop a mountain lion or bear. They do not bark to give warning which usually is enough to cause the predator to mosey on down the road.
I feed my dogs in the pasture with the goats and sheep, I pet these dogs, bursh them, touch them but only while in the pasture.
While a llama or donkey may be a quick fix, a dog takes time to train if you get a puppy. I tired jacks back in the 70's and have found that livestock guardian dogs tend to work better for my and my operation.