Energy Availability in Biological Systems:
Increased productivity in a community can support heightened trophic structure and biomass. Theory predicts that the latter of these can manifest itself as either increases in body size and/or abundance of organisms. Less clear is whether trophic complexity, abundance, or body size distributions respond concordantly from immediate to geologic time scales. My previous work demonstrates that at both regional and global levels, gradients in productivity are a major driver of body size and abundance despite steep gradients in oxygen concentration, temperature, etc. Although an overall reduction in size across clades is observed as food availability becomes more limited, intra- clade and specific trends are more complex than previously thought. Over a decreasing resource gradient, both positive and negative trends occur simultaneously, depending on the initial body size of the organism. Large organisms tend toward smaller sizes because of the high energetic costs of being big. Conversely, small organisms evolve larger because of the associated advantages of increased foraging areas and starvation resistance. Alternatively, some species evolve new strategies that allow them a refuge from food limitation. My work suggests that a variety of macroecological phenomena, e.g. the island rule, abyssal gigantism and miniaturization, etc., may reflect a broader relationship relating body size and productivity, with the potential to reconcile contradictory findings of both negative and positive clines.
Much attention is given to uncovering the mechanisms underlying the species-productivity relationship. Recent work posits that in part these gradients rather than reflecting energy availability arise because of geometric constraints placed on species’ ranges by hard boundaries. My research demonstrates that species-productivity gradients are not an artifact of constrained range placement but rather represent the manifestation of biological processes. For example, invertebrate assemblages over productivity gradients display heightened competition at intermediate levels of productivity. Energy availability may also greatly alter the morphospace, disallowing species with energetically expensive forms. Extreme reductions in food availability can lead to source-sink dynamics over 1000’s of kilometers, with nearby eutrophic regions serving a source of recruits, that greatly impact community structure, body size distributions, and ultimately species diversity. At heightened productivity, increases in biomass and body size are not necessarily coupled with greater diversity.