For me, the most interesting part of Fairlie's book is his suggestions for integrating livestock into permaculture systems.  The idea is that if your animals have jobs other than just meat production, they are no longer an inefficient use of land.

The most obvious non-edible use of livestock is collecting the animals' manure to fertilize the garden.  However, Fairlie points out that even there, fertilizing your garden with animal manure is about 10% to 43% less land-efficient per calorie than setting aside a third of the growing area to produce legume and grain compost crops.  (The different percentages depend on what type of animal you're raising to make the manure, and both factor in the value of the meat and/or dairy produced by the animals as well as by the crops.)

While you lose some land-efficiency by depending on animals to concentrate organic matter and nitrogen, traditional societies have long known that livestock are worth the effort in varied landscapes.  Grazers on non-arable land will eat up nitrogen (atmospherically deposited or fixed by legumes) and bring it home when they come to the stable at night, then their manure can be used to fertilize the main growing area.  In other words, animals are acting as self-powered nitrogen-accumulators, a bit like you might plant dynamic accumulators in the garden to bring up minerals from the subsoil that your garden plants can't usually reach.

Meanwhile, livestock can be a good buffer against famine, especially in the absence of refrigeration.  You can fatten a pig during a time of plenty, then kill it and eat the meat when the days get short and not much is growing.  On a larger scale, Fairlie considers eating meat to be a buffer against large-scale problems --- if we're used to the inefficiency of feeding grains to pigs and eating the pigs, when times get tough, we can just eat the grains and have a bit of wiggle room to change what we're doing before we really go hungry.

In general, though, the biggest bonus of animals if you're trying to save energy is that they move by themselves.  If we used our livestock to transport people and heavy objects, to mow between orchard trees, to clear new land, to turn compost, and to control insects in our gardens, they would probably look a lot more efficient than they do now.  On the other hand, as enticing as it is to consider returning to a previous era when a rugged dairy cow might be expected to give milk and also to haul our supplies through the mud from the parking area, energy is just so cheap at the moment that mechanized transportation seems to make more sense.

Perhaps the real lesson homesteaders should take away from Fairlie's book is that we need to ensure we're using the right default livestock before branching out.  Are all of your food scraps being consumed by an animal?  If not, try adding a few chickens or a pig.  Do you have a lot of rough land and an absence of fertility in your garden?  Maybe you need to graze ruminants and fold them in a barn or fallow field at night to get that manure. 

Rather than planning ahead for an apocalyptic vision where we don't have enough land to feed our population, I think it's more useful to make changes in our lives now to increase sustainability and also bliss.  How would you like to tweak your homestead to make its livestock more effective?