The effect of chronic seaweed subsidies on herbivory: plant-mediated fertilization pathway overshadows lizard-mediated predator pathways

Abstract

Flows of energy and materials link ecosystems worldwide and have important consequences for the structure of ecological communities. While these resource subsidies typically enter recipient food webs through multiple channels, most previous studies focussed on a single pathway of resource input. We used path analysis to evaluate multiple pathways connecting chronic marine resource inputs (in the form of seaweed deposits) and herbivory in a shoreline terrestrial ecosystem. We found statistical support for a fertilization effect (seaweed increased foliar nitrogen content, leading to greater herbivory) and a lizard numerical response effect (seaweed increased lizard densities, leading to reduced herbivory), but not for a lizard diet-shift effect (seaweed increased the proportion of marine-derived prey in lizard diets, but lizard diet was not strongly associated with herbivory). Greater seaweed abundance was associated with greater herbivory, and the fertilization effect was larger than the combined lizard effects. Thus, the bottom-up, plant-mediated effect of fertilization on herbivory overshadowed the top-down effects of lizard predators. These results, from unmanipulated shoreline plots with persistent differences in chronic seaweed deposition, differ from those of a previous experimental study of the short-term effects of a pulse of seaweed deposition: while the increase in herbivory in response to chronic seaweed deposition was due to the fertilization effect, the short-term increase in herbivory in response to a pulse of seaweed deposition was due to the lizard diet-shift effect. This contrast highlights the importance of the temporal pattern of resource inputs in determining the mechanism of community response to resource subsidies.

Pulses of marine subsidies amplify reproductive potential of lizards by increasing individual growth rate

“…Persistently high lizard abundance likely acted to suppress herbivores and prevent plant damage over a longer time scale (a positive indirect effect of seaweed on plants). A comparative study of sites naturally receiving chronic seaweed subsidies documented that this indirect pathway does indeed occur (Piovia‐Scott et al 2013). Thus, delayed reproductive responses by lizards help explain changes in the indirect effects of pulsed subsidies on plant damage over time.…”

“…The amount of seaweed added or removed was based on monitoring data from 29 nearby unmanipulated plots that all naturally supported lizard populations. Average seaweed abundance per plot was 0–2.2 kg m −2 over a three‐year period, with peaks in seaweed abundance approaching 10 kg m −2 (Spiller et al 2010, Piovia‐Scott et al 2013). Thus, our manipulation contrasts two realistic target levels of seaweed abundance – high (subsidized) and low (unsubsidized).…”

Recovery of food webs following natural physical disturbances

“…Because herbivory did not become lower in subsidized plots than in unsubsidized plots during the course of the experiment, there was no evidence of an apparent trophic cascade. Using path analysis on data collected in an observational study, Piovia‐Scott et al . revealed that the bottom‐up fertilization effect was stronger than the top‐down predator‐diet‐shift effect.…”

Stress and subsidy effects of seagrass wrack duration, frequency, and magnitude on salt marsh community structure

“…However, its consequent effects on the abundance and diversity of the associated invertebrate community (Bishop and Kelaher ) and rates of herbivory (Piovia‐Scott et al. ) depend on whether it decreases (e.g., by destroying vegetation or altering plant species composition) or increases (e.g., by providing shade/shelter and/or improving plant nutrient content) the quality of existing habitat and resources for each associated species. Further, the coincident allochthonous input of nutrients and invertebrate prey from wrack deposition is also a potentially valuable resource subsidy to multiple trophic levels on terrestrial islands (Spiller et al.…”

“…), or varying in the predominant mechanism underlying the response (Piovia‐Scott et al. ). For example, the migration of anadromous and semelparous fish like salmon can alter stream ecosystems via multiple pathways, including subsidy effects of marine‐derived nutrient loading and stress effects of nest building and habitat disturbance.…”