I got to thinking the other day about this while driving down the highway. What to use as natural barbed wire? If you live where you can grow blackberry or rasberry bushes, how about putting them around the edges of your property? If grown thick enough, like a hedge, I think they would work like a fnece. Most people on foot avoid going thru the stuff even to pick berries so you could use them to control access. I know they won't stop vehicles but foot traffic could be "guided" to choke points and selected fire lanes. If we look at the trouble that our troops had in WW2 with hedgerows until they found a way to break thru them it is the same principle i suspect. As a side note you could also harvest the berries for food supplies. Rose bushes might also work in this idea. However the berry bushes would attract birds that might work as a natural alarm system. Just an off the wall idea. Feel free to offer thoughts or suggestions as to whether it would work or not.