Bending the rules: exploitation of allochthonous resources by a top‐predator modifies size‐abundance scaling in stream food webs

Abstract

Body mass–abundance (M‐N) allometries provide a key measure of community structure, and deviations from scaling predictions could reveal how cross‐ecosystem subsidies alter food webs. For 31 streams across the UK, we tested the hypothesis that linear log‐log M‐N scaling is shallower than that predicted by allometric scaling theory when top predators have access to allochthonous prey. These streams all contained a common and widespread top predator (brown trout) that regularly feeds on terrestrial prey and, as hypothesised, deviations from predicted scaling increased with its dominance of the fish assemblage. Our study identifies a key beneficiary of cross‐ecosystem subsidies at the top of stream food webs and elucidates how these inputs can reshape the size‐structure of these ‘open’ systems.

Mapping Micro-Pollutants and Their Impacts on the Size Structure of Streambed Communities

“…Our findings evidence the strong effect that the presence of EOCs has over the organization of streambed communities, to the point of shielding the effect of other important environmental factors. Temperature, pH, and productivity are considered major drivers in freshwater ecology and determining factors of the size structure and metabolic capacity of streambed communities [26,37,38,41,42]. Even so, both the intercept and size spectra slope of the N-M relationship exhibited higher sensitivity in this work to EOC pollution than to those environmental variables.…”

“…The size spectra slope of the M-N relationship describes the rate of biomass depletion through different levels of the food web in freshwater systems, typically becoming shallower as this rate increases [25,26]. Consequently, the existence of more abundant tolerant invertebrates in our study might imply a better transference of biomass and energy between trophic levels and potentially longer food chains.…”

“…Size bins were determined from the (log10) body mass range for each sampled community and the abundances of organisms were then summed within each size bin [23]. We used a total of six bins to maximize the number of size bins while minimizing the number of empty size bins in the analysis [23,26]. Next, we built a saturated model comparing reference systems with EOC-polluted (two-level factor), and all environmental gradients significantly related with the NMDS ordination.…”

“…When individual organisms are grouped into body mass classes, regardless of their taxonomy, the slope of the N-M relationship, or 'size spectra', provides an integrated measure of the community structure and the energy flow across trophic levels [25]. This is especially true in freshwater ecosystems, where communities are strongly size-structured [26,27] and gape-limited predation predominates [25]. The intercept of the N-M relationship provides a proxy for the community carrying capacity while the slope represents the energy flow and trophic transference efficiency in the system.…”

Temporal shifts in intraspecific and interspecific diet variation: effects of predator body size and identity across seasons in a stream community

“…However, similar temporally-pulsed inputs of terrestrial arthropods are well documented in forested streams and are known to contribute a large portion of available prey for streams predators (Wipfli 1997, Kawaguchi and Nakano 2001, Romero et al 2005, Chan et al 2007). Consumption of cross-ecosystem subsidies by top predators such as trout can reshape the structure and energetic dynamics of stream food webs (Perkins et al 2018). Our results exemplify how specific community members (i.e., thrips and cutthroat trout in our study) can disproportionally contribute to cross-ecosystem fluxes of nutrients and matter from land to water (Polis et al 2004).…”