Sizing up life and death.

D etermining the ‘‘rules’’ that govern the lifespans and birth and death rates of organisms is a central goal of population biology and lies at the heart of understanding a broad range of ecological and evolutionary phenomena (1, 2). Yet, until recently, quantifying these rules and, more importantly, providing a mechanistic explanation for how they operate have eluded biologists. This gap in our knowledge is understandable. Organisms manifest lifespans that range on the order of minutes or hours in the case of bacteria to many hundreds or thousands of years in the case of some tree species. In addition, for organisms like some unicellular algae and plants, which can reproduce asexually, the application of concepts like ‘‘birth’’ and ‘‘death’’ can be problematic or ambiguous. Nevertheless, the work of Marbà et al. (3) in this issue of PNAS provides reason to hope that the life and death dynamics of otherwise very dissimilar organisms abide by predictable and explicable rules. Equally important, their work demonstrates that lifespan and birth/death rates scale with respect to body size across aquatic and terrestrial and unicellular and multicellular plant species in ways that accord remarkably well with those predicted by a generalized theory for the metabolic optimization of life history traits (4, 5). To understand this theory and fully appreciate the significance of the analyses of Marbà et al. (3), we must first consider the observation that many biological traits vary with body size in a manner conveniently described by the allometric equation Y 0M , where Y is the variable of interest, 0 is the normalization ‘‘allometric constant’’ (that can nevertheless vary numerically with the nature of Y, the kind of organism studied, or the environmental conditions attending growth and development), M is total individual body mass, and is the allometric or ‘‘scaling’’ exponent. Second, numerous studies of organisms ranging from bacteria and algae to terrestrial plants and animals have shown that many important functional traits scale as quarter-powers of total body mass. For example, across ecologically and phyletically very different organisms spanning 20 orders of magnitude in body size, is 3/4 for metabolic and growth rates, 3/4 for population densities, and 1/4 for biomass-doubling times or ‘‘birth rates’’ (5–11) (Fig. 1). Yet, despite many attempts to explain why these and other scaling exponents take on the numerical values that they do, no theory has been entirely successful, that is, perhaps not until the one proposed by Geoffrey B. West, James H. Brown, and Brian J. Enquist (4), which has been transfigured to predict the scaling of life history traits based on the perspective of metabolic optimization (5). Specifically, this metabolic theory predicts that lifespan, E, should scale optimally as the 1/4 power of total body mass (i.e., E M1/4) and that birth and death rates, B and D, should scale as the 1/4 power of body mass (i.e., B M 1/4 and D M 1/4), from which it follows that population growth rates are predicted to be size-independent across different kinds of organisms (i.e., B D). Some of these scaling relationships have been observed empirically and validated statistically well before the publication of this theory or the work of Marbà et al. (3–5). For example, across three very different functional plantspecies groups (unicellular algae, herbs– graminoids, and tree species), individual growth in body mass, G, has been shown previously to scale as the 3/4 power of body mass (i.e., G M3/4) (see refs. 12 and 13), from which it follows that the doubling time in body mass will, on av-