One of the primary reasons that conventional agriculture is unsustainable is due to the focus on high harvest index, i.e. the proportion of the land cover grown that is harvested. It is normal for the harvest index of conventional agriculture to hover around fifty percent. That means that half of what is grown on the land is removed. Along with the energy fixed by plants in photosynthesis, nutrients fixed by the plants are removed from the system. To support this rate of removal conventional farmers add inorganic nutrients. Annual agriculture combined with these rates of removal result not only in severe soil degradation and erosion but a barren desert like landscape in fall, winter, and most of spring.
So how does one harvest sustainably? To keep harvest quantity constant while reducing harvest index, overall productivity must be increased. The productivity of our permaculture system allows us reduce the percentage harvested while maintaining economic harvest levels. Biodiversity and niche partitioning result in not only greater productivity but also increased system efficiency. Biodiversity also means that we are not harvesting all of the energy or nutrients from the system at one point in time, but instead slowly over the entire season. Harvesting a lesser proportion of the system over a more sustained time course allows us to rely only on the inputs produced by the plants and remain sustainable. A permaculture system also means that our landscape is covered with life year round (benefiting both the eyes and waterways).