How does abundance scale with body size in coupled size‐structured food webs?

Abstract

Summary 1.Widely observed macro-ecological patterns in log abundance vs. log body mass of organisms can be explained by simple scaling theory based on food (energy) availability across a spectrum of body sizes. The theory predicts that when food availability falls with body size (as in most aquatic food webs where larger predators eat smaller prey), the scaling between log N vs. log m is steeper than when organisms of different sizes compete for a shared unstructured resource (e.g. autotrophs, herbivores and detritivores; hereafter dubbed 'detritivores'). 2. In real communities, the mix of feeding characteristics gives rise to complex food webs. Such complexities make empirical tests of scaling predictions prone to error if: (i) the data are not disaggregated in accordance with the assumptions of the theory being tested, or (ii) the theory does not account for all of the trophic interactions within and across the communities sampled. 3. We disaggregated whole community data collected in the North Sea into predator and detritivore components and report slopes of log abundance vs. log body mass relationships. Observed slopes for fish and epifaunal predator communities (-1·2 to -2·25) were significantly steeper than those for infaunal detritivore communities (-0·56 to -0·87). 4. We present a model describing the dynamics of coupled size spectra, to explain how coupling of predator and detritivore communities affects the scaling of log N vs. log m . The model captures the trophic interactions and recycling of material that occur in many aquatic ecosystems. 5. Our simulations demonstrate that the biological processes underlying growth and mortality in the two distinct size spectra lead to patterns consistent with data. Slopes of log N vs. log m were steeper and growth rates faster for predators compared to detritivores. Size spectra were truncated when primary production was too low for predators and when detritivores experienced predation pressure. 6. The approach also allows us to assess the effects of external sources of mortality (e.g. harvesting).Removal of large predators resulted in steeper predator spectra and increases in their prey (small fish and detritivores). The model predictions are remarkably consistent with observed patterns of exploited ecosystems.

Allometric scaling of population variance with mean body size is predicted from Taylor’s law and density-mass allometry

“…In marine and freshwater food webs where body sizes and species abundances have been reported (12,19), and in the zoological portion of terrestrial food webs, smaller-bodied, more numerous species are generally consumed by larger-bodied, less numerous species. If VMA applied to such webs, population densities of smaller-bodied species should be expected to be more variable spatially or temporally than population densities of largerbodied species.…”

“…DMA is sometimes called the self-thinning law (14). DMA applies to single species, collections of species (15)(16)(17)(18), and foodwebs (19)(20)(21)(22)(23) We called this predicted relation variance-mass allometry (VMA). If b > 0 and v < 0, as is typical, then bv < 0: Increasing mean body mass will be associated with decreasing variance of population density.…”

Benthic biomass size spectra in shelf and deep-sea sediments

“…Consequently, the body-size approach may prove useful in studies of benthic communities in both shallow (Blanchard et al, 2009) and deep-sea environments.…”

“…Other biomass size-based approaches have been used to model fisheries impacts on shelf benthic ecosystems (e.g. Blanchard et al, 2009).…”

Global in scope and regionally rich: an IndiSeas workshop helps shape the future of marine ecosystem indicators

“…This will allow the testing of the sensitivity and specificity of ecological indicators to fishing versus climate, the performance of indicators for decision support, and the identification of reference levels and tipping points of ecosystems submitted to different drivers. Owing to the expertise gathered in IndiSeas II, we will use state-of-the-art ecosystem models including EwE (Christensen and Walters 2004), OSMOSE (Shin and Cury 2004;Travers et al 2009), Atlantis (Fulton et al 2004 and simpler size-based and multispecies models (Blanchard et al 2009Hartvig et al 2011) as test laboratories. Some ecosystems of IndiSeas II will also benefit from ongoing development of ''end-to-end modelling'' (Travers et al 2007;Rose et al 2010;Shin et al 2010c;Barange et al 2011), involving coupling ecosystem models that focus on higher trophic levels with hydrodynamic, biogeochemical and economic models.…”