Ecosystem ecology: size-based constraints on the pyramids of life. Trends in Ecology and Evolution

Biomass distribution and energy flow in ecosystems are traditionally described with trophic pyramids, and increasingly with size spectra, particularly in aquatic ecosystems. Here, we show that these methods are equivalent and interchangeable representations of the same information. Although pyramids are visually intuitive, explicitly linking them to size spectra connects pyramids to metabolic and size-based theory, and illuminates size-based constraints on pyramid shape. We show that bottom-heavy pyramids should predominate in the real world, whereas top-heavy pyramids indicate overestimation of predator abundance or energy subsidies. Making the link to ecological pyramids establishes size spectra as a central concept in ecosystem ecology, and provides a powerful framework both for understanding baseline expectations of community structure and for evaluating future scenarios under climate change and exploitation. Ecological pyramids and size spectra: size-centric views of community structure Understanding the processes that structure communities (see Glossary) in ecosystems is a fundamental goal in ecology. Elton laid the conceptual foundation for our understanding of these processes with two key observations: (i) interactions among organisms strongly shape the structure and function of communities; and (ii) the nature of these interactions is governed by both the identities and the sizes of the organisms involved These first ecological pyramids were 'pyramids of numbers', where the 'layers' represented 'bins' of body size, and the width of the layers represented the abundance of all organisms within each size class. The pyramid representation of communities quickly took hold in ecology and pyramids were re-expressed in terms of biomass [2], production, and eventually trophic level (Hutchinson, unpublished, The shape of ecological pyramids qualitatively conveys rich information about the underlying ecological processes Review Glossary Community: the biotic component of an ecosystem; organisms inhabiting a given geographic area and sharing a common resource base. Ecological pyramids: graphs of relative abundance or biomass among body-size classes or trophic levels in ecological communities 0169-5347/$ -see front matter