Here are some thoughts I have about the philosophy and practice of disturbance in permaculture. I'd love to hear your thoughts!
A certain thread within the modern environmental movement is explicitly anti-human (for instance, the human extinction project) and a certain anti-human spirit pervades the wider movement; humans are seen, with some justification, as the problem; only land that is untouched by humans is seen as valuable or healthy. Land ceded to habitation or agriculture is seen as, (and often is) a loss for the biosphere. Environmentalists such as E.O. Wilson hope for a future in which humans are corralled within high density cities, letting the rest of the planet run wild, returning itself to health and beauty, not interacted with except by occasional tourism. In this view, the best we can hope for is to avoid causing damage; only the negatives that humans bring are considered.
Native people around the world saw themselves as part of the landscape, stewards or caretakers of it; for the California natives, for instance, "wild" land was a bad thing, the result of neglect by human beings; they saw "the wild" as a thing which needed them, even as they needed it. We've undoubtably stepped over our proper boundaries, but that does not mean that we have no role to play. No matter what philosophy one holds, to me it would seem strange if we were entirely alien and detrimental to the planet from which we came.
Permaculture is the science of designing permanent agriculture for sustainable human livelihood. Because of this, it is human centered. By definition, it escapes the anti-human mentality discussed above. It studies nature to enable the construction of sustainable human habitats.
In this study, Permaculture practitioners have abstracted certain general principles from nature. For instance "every element should perform multiple functions" "every function should be preformed by multiple elements" "use slow and small solutions" "capture and store energy and nutrients" "use biological and renewable resources" "value diversity" "apply self-regulation and respond to feedback" "integrate rather then segregate" "create edges" etc. These principles are widely true of any ecosystem; there are no single purpose elements in nature, and even the internal organs of animals often preform many functions; all ecosystems capture and store energy, as do all animals and plants; almost all ecosystems are staggeringly diverse, including that in our digestive system; edges, in the sense of a high surface area to volume, are found all across nature, from lungs to trees to estuaries; etc.
Permaculture also uses observations of particular patterns in natural systems. These observations show ways in which particular ecosystems or organisms obey the general principles mentioned above. They may be directly useful to our designs, and they help us to understand the principles better, but they do not have the universal relevance of the principles. For instance, coral reefs catch and store energy by using photosynthesis to power the building of fantastic shapes out of calcium carbonate, thus also creating edge and attracting more diversity. Our designs also need to catch and store energy, build edge, and gather diversity, but unless we are farming coral, they are unlikely to do so by using photosynthesis to build shapes out of calcium carbonate. In fact, our use of calcium compounds in cement is an environmental disaster.
Since permaculture is often seen as a subset of the wider environmental movement, it is not surprising that the anti-human mentality mentioned above tends to slip back in at times. Permaculturists tend to focus on observations of "undisturbed" areas, where animal and human disturbance is absent. For instance, when speaking of tilling, irrigation, or fertilization, the forest is often used as an example; we are told that nobody tills the forest, but it grows trees just fine. The massive size of redwoods or sequoias is sometimes used to further such arguments. Similarly, arguments are made that forests are heavily mulched with organic matter, and contain primarily perennial plants, and thus our designs should be un-tilled, heavily mulched with organic matter, and primarily perennial. Further, such techniques as mono-cultures, rows of crop plants, weeding, and confinement of animals sometimes come in for criticism, because "we don't find rows in nature."
I don't want to critique or defend these individual practices at this point; they may be valid in a particular design or detrimental in another; however, I do have a problem with the reasoning behind these ideas. For one thing, we may not be trying to design a forest. For another, no ecosystem on earth is mulched with a foot of wood chips, though this is often billed as mimicking the forest floor.
But a deeper concern is that we are not factoring our human activities into the picture. Humans are animals, even if rational animals. Animals disturb ecosystems in numerous ways in their search for livelihood, generally to the benefit of the ecosystems so disturbed. Undisturbed ecosystems tend to decline in diversity and productivity. If we don't find rows in nature, neither do we find beaver dams, beehives, bird nests, ant mounds, buffalo wallows, or termite nests; not until something builds them, that is! Beavers are a particularly good case. They would starve or be eaten by predators in an 'undisturbed' forest. So to suit their needs, they cut down groves of trees, and dam streams to create ponds and marshes. They dig canals to float tree branches to the dam, and divert streams into new channels, managing and converting huge areas of valley bottom. Some dams can grow to enormous sizes; the largest stretches for half a mile. This modification radically changes the species composition of the area. Pre-existing trees die as they are flooded, creating snags, and wetland plants move in. The swift flowing, relatively deep stream becomes a pond, spreading out to become shallower, warmer and higher in nutrients. Grazing animals are another good example. Their grazing and trampling tends to destroy woody plants and strengthen grass and forbs. The rapid nutrient cycling caused by grazing increases the rooting depth and carbon capture of grasslands, making them more resilient over time. Prairie dog towns once stretched for miles, creating a huge network of tunnels and disturbed ground, in effect "tilling" the prairie.
Fire is a unique case. It is important for many ecosystems, effecting plant structure, nutrient cycles, and species composition. And humans have a special relationship with fire; we are the only animals that start and maintain fires. Native peoples around the world have tended to use fire to modify the landscapes around them; post colonization, these landscapes have often changed, becoming both less useful and less healthy.
Once we realize that being animals, we are certain to cause disturbance, we can focus our observations in different directions; we should focus on how animals produce habitats for themselves. We won't build ourselves wax combs or live in ponds, but we can study beavers and bees and abstract general principles from their work. Even more importantly, we should study how various cultures have interacted with the land over time. And we shouldn't let cultural prejudice limit which cultures we study. European colonialism is rightly seen as a tragedy and a crime, and perhaps because of this, European traditional land use is sometimes ignored; we should study all land use traditions, with the caveat that those which developed with fairly dense populations in temperate climates will have the most applicability to designs in the USA. European coppice woodlands, small grain fields, pastures, laid hedges, thatched or turf roofs are all "unnatural" as are the slash and burn plots, chinampas, and terraces of Central American cultures, or the rice paddies and canals of Japan; but all could theoretically be used in our designs.
Instead of latching onto certain observations, we should be sure our designs follow the basic principles, and further, we should develop certain objective metrics which we can use to determine if our land use is promoting health or decay in the land. What these metrics should be I don't know; but they must be more sophisticated then gauging the amount of "disturbance" or the "naturalness" of the land. In the following paragraph I'll suggest just a few of the principles we can learn from studying animal activity.
The disturbance caused by some animals is intense; beavers probably cause the most intense disturbance of any non-human animal. On the other hand, some animals cause disturbance over a vast scale; the buffalo herds are a good example of this. But no animal causes extensive and intense disturbances, as we do. We shouldn't be afraid to spade up a few hundred square feet for a vegetable garden, for instance, or build a farm pond of a few acres, or set a controlled fire over a field; we should be wary of Hoover Dams, monoculture forests, and thousand acre fields. And we should "respond to feedback;" if our disturbance seems to be degrading the natural world, we should stop. Most of all, we should not disturb the whole face of the planet; we should leave some areas to be disturbed by other animals. To achieve this end, we may have to increase our disturbance of areas that we already use, while still staying within the bounds of the principles.
Do humans actually provide a benefit to the ecosystem as a whole? We definitely aid some particular species. A whole guild of creatures, from corn, tomatoes, and chickens to mice, dandelions, and purslane, are ecologically linked to human beings; garden writer Carol Deppe calls this "the grand alliance." We, even more then other animals, create disturbance, which is then exploited by the alliance. If we don't overdo it, could we add diversity to the overall picture? The meadows created by the natives of what is now California are diverse places, harboring many species that will not survive in the "natural" conifer monocultures now overrunning them. Similarly, the tilled ground and annual crops and weeds of a small farm provide a very different habitat then the forest or prairie which surrounds it; according to the edge principle, this difference may promote an overall increase in diversity. More then most animals, we transport nutrients; thriving stands of trees are now found on shell heaps piled by the native peoples of the Americas. The shells contain calcium and other minerals from the sea and provided a nutrient source in a relatively depleted environment. Could our importation of kelp meal or limestone for our gardens have similar effects? These are complicated questions, and the answers may be long in coming; but if we let an anti-human, anti-disturbance spirit constrain the range of questions asked, we will never know.