Towards a compleat theory of ecosystem size spectra

Abstract

The regularity of ecosystem size spectra is one of the most intriguing and relevant phenomena on our planet. Pelagic size spectra generally show a log-linearly downtrending shape, following a power-law distribution. A constant log-linear slope has been reported for many marine pelagic ecosystems, often being approximately b = -1. Conversely, there are variable trophic-level-biomass relationships (trophic pyramids). The contrasting observations of a constant size spectrum and highly variable trophic pyramids may be defined as the constant size spectrum - variable trophic dynamics paradox. Here, a mass-specific predator-prey-efficiency theory of size spectra (PETS) is presented and discussed. A thorough analysis of available data, literature, and models formed the basis for the PETS theoretical framework, where pelagic marine ecosystems are controlled by complex trophic processes such as resource-limitation stress and top-down regulation, thus establishing a discrete maximum carrying capacity spectrum. PETS consists of a series of equations and proposed stabilizing mechanisms. The slope b of the biomass-body mass spectrum can be predicted from predator-prey mass ratio (PPMR) and mass-specific trophic efficiency E (E = dlog10(B) / dTL), such that b = E / log10(PPMR) = -1. The proposed size-specific trophic equilibrium mechanisms stabilize the size spectrum, but not the trophic level - biomass relationship. The complete size spectrum obtained in situ (including living organisms and non-living particles) is discussed. This paper is intended as a plea for the integration of modeling approaches, to understand and integrate data and processes across communities including bacteria, plankton, fish and mammals, considering the effects of non-organismic particles.