SUCCESSFUL SURF-RIDING ON SIZE SPECTRA - THE SECRET OF SURVIVAL IN THE SEA

All ecosystems require constituent species to survive against a backcl oth of biotic and abiotic scenery. How this scenery shapes the life-hi story strategies of the players and how they in turn shape the scenery are important themes of the play of life. Species surviving in temper ate and Arctic shelf seas do so against a scenery dominated by seasona l changes in the size-spectrum of other players. Successful survival i n such an environment requires species to ride the big wave of annual productivity as it rolls through the extended size spectrum from phyto plankton to large fish. This wave flattens and broadens as it moves to wards higher sizes. We speculate that in a seasonal shelf seas environ ment the wave shape is such that the Sheldon-Sutcliffe spectrum of equ al biomass per log size interval is approximately true as an annual av erage although it may not be true at any particular moment in the year . Such spectra are structured by biomass being moved up the size spect rum mainly by predation processes, with growth of individuals being a second order process. However, the problem for an individual is to gro w up through a size spectrum from its size at birth to its size at rep roduction. Hence species need to find survival paths through the fluct uating scenery. This scenery is composed of the biomass of the prey, t hat of animals of a similar size, and larger predators. The paths foll owed dictate the life-history strategies of the species. This central problem for sea dwellers in temperate and Arctic shelf seas generates a broad similarity in the choice of life-history strategy for many key players over quite wide geographic areas of the globe. In these seas, strategies of high fecundity, high mortality and high growth rate are particularly common while strategies of low fecundity and parental ca re are rare for much of the size range. These seas also seem to favour longer trophic chains than terrestrial systems and either several gen erations per year or multiannual life cycles rather than annual cycles .