INTRODUCTION

We define stability (Chapter 1) as the variation in yield over time, i.e. as relating to repeatability and predictability in farming. This is a narrower definition than those used by ecologists who work with natural systems and focus more on changes in community structure than on variations in production. For them, stability also embraces continued existence of genetic information within the community, i.e. persistence of particular species or genes. Similar questions arise with the genetic structure of agricultural populations (Chapter 4) and for persistence of species in pasture polycultures. Persistence of crops within a farming system, however, is largely a human decision and thus more an issue in sustainability. It depends upon whether production can be sustained and whether it is worth sustaining.

An overlap evidently exists between concepts of stability and sustainability. Large yield variations and persistence of crops both affect sustainability but maintenance of soil resources is the key issue because soil degradation has the effect of gradually lowering the potential of a site until farming is no longer a viable proposition. This chapter focuses on several aspects of yield variation and their causes as features of stability. Causes and controls of soil degradation are discussed in later chapters: acidification (Chapter 7), salinization (Chapters 13 and 14), and nutrient exhaustion and erosion (Chapter 12).

The nature of variations in farming

In addition to fluctuations in markets, farming is affected by weather, pests, and diseases as well as by human errors.