Dome patterns in pelagic size spectra reveal strong trophic cascades

Abstract

In ecological communities, especially the pelagic zones of aquatic ecosystems, certain body-size ranges are often over-represented compared to others. Community size spectra, the distributions of community biomass over the logarithmic body-mass axis, tend to exhibit regularly spaced local maxima, called “domes”, separated by steep troughs. Contrasting established theory, we explain these dome patterns as manifestations of top-down trophic cascades along aquatic food chains. Compiling high quality size-spectrum data and comparing these with a size-spectrum model introduced in this study, we test this theory and develop a detailed picture of the mechanisms by which bottom-up and top-down effects interact to generate dome patterns. Results imply that strong top-down trophic cascades are common in freshwater communities, much more than hitherto demonstrated, and may arise in nutrient rich marine systems as well. Transferring insights from the general theory of non-linear pattern formation to domes patterns, we provide new interpretations of past lake-manipulation experiments.

Can biomass distribution across trophic levels predict trophic cascades?

“…If consumer self-regulation is very weak or zero, we find A ↑ , ξ → ∞ and only the top-down driven regime is observed, as in classical models (e.g. Oksanen et al, 1981;Rossberg et al, 2019) that have been used to showcase counter-intuitive dynamical effects in food webs. The relationship between biomass distribution across trophic levels and response to perturbations, given dissipative loss χ (Box 1).…”

“…Although previous studies have discussed whether the static properties of the food chain, such as its biomass pyramid or its size spectra (the distribution of organism body size) can hint at the underlying dynamics (McCauley et al, 2018;Barbier & Loreau, 2019;Rossberg et al, 2019), a systematic understanding of the link between static and dynamical properties of food chains is still lacking. Our contribution to bridge this gap is to provide a general and synthetic description of the relationship between the strength of trophic cascades and the shape of biomass pyramids.…”

“…Studies of static and dynamical properties of food chains have historically developed in isolation, generating different lines of investigation, which have seldom been connected rigorously (but see Jonsson (2017); McCauley et al (2018); Barbier & Loreau (2019); Rossberg et al (2019)). A number of applied studies have proposed to predict an ecosystem's "health" and dynamical response from its more accessible static biomass pyramid or size spectrum (Shin et al, 2005;Cury et al, 2005).…”

Can biomass distribution across trophic levels predict trophic cascades?

“…If consumer self‐regulation is very weak or zero, we find and only the top‐down driven regime is observed, as in classical models (e.g. Oksanen et al ., 1981; Rossberg et al ., 2019) that have been used to display counter‐intuitive dynamical effects in food webs.…”

“…(2018); Barbier and Loreau (2019); Rossberg et al . (2019)). A number of applied studies have proposed to predict an ecosystem's ‘health’ and dynamical response from its more accessible static biomass pyramid or size spectrum (Shin et al ., 2005; Cury et al ., 2005).…”