The brief review of the structure of marine ecosystems presented in Chapter 4 treated these as static entities which, of course, they are not: this chapter is intended to review the processes by which ecosystems respond to the changing environment of the ocean. The strong contrast in the internal structure of relatively open marine and relatively closed terrestrial ecosystems is paralleled by equally different characteristic variability and uncertainty in the external forcing that is characteristic of the marine and terrestrial habitats.

Agricultural scientists are very well aware of the critical consequences of changing weather patterns and also of regional patterns of soils and sub-soils, because an understanding of these factors is essential for the management of farming and animal husbandry. Although fisheries are also dependent on the pattern of change in ocean conditions that is induced at all time scales by variable weather conditions, and also at all spatial scales by the geometry of land masses, the importance of the resulting patterns of variability – for some reason – was late in entering mainstream fisheries science texts (I have already alluded to John Gulland's 1974 The Management of Marine Fisheries, because it encapsulated the thinking of many fisheries scientists in the days when there was great confidence in the future of their endeavour).