Natural systems as models for the design of sustainable systems of land use

Natural ecosystems, whose components are the results of natural selection, are sustainable; most are productive, responsive to pests, and retentive of nutrients. Thus, they are appropriate models on which to base the design of new systems of land use. Abiotic and biotic stressors are related non-linearly; the nadir of total stress being mid-way along a gradient of environmental harshness. Superimposing the stress functions on Holdridge's life zone chart yields four broad categories of environments for agriculture: climates where annual rainfall is similar to potential evapotranspiration, plus three other categories that are either too cold, too arid, or too wet. Extremely cold lands have no potential for agriculture. Lands that are arid or infertile can be used successfully, although the cost of compensating for environmental limitations increases exponentially with increasing abiotic stress. Grazing animals (which act as trophic buffers between people and environment) have proven successful in dry and infertile environments. The humid tropical lowlands epitomise environments of low abiotic stress but overwhelming biotic intricacy. Here it pays to imitate natural systems rather than struggle to impose simplicity on ecosystems that are inherently complex. The keys to success are to (i) channel productivity into outputs of nutritional and economic importance, (ii) maintain adequate diversity to compensate for losses in a system simple enough to be horticulturally manageable, (iii) manage plants and herbivores to facilitate associational resistance and not associational susceptibility, and (iv) use perennial plants to maintain soil fertility, guard against erosion, and make full use of resources.