Biogeographic features mediate marine subsidies to island food webs

Abstract

Although marine subsidies often enrich terrestrial ecosystems, their influence is known to be context‐dependent. Additionally, the multitrophic impact of marine subsidies has not been traced through food webs across physically diverse islands. Here, we test predictions about how island characteristics can affect marine enrichment of food web constituents and how nutrients flow through island food webs. To evaluate enrichment and trace marine nutrients across food webs, we used stable isotopes of soil, flora, and fauna (n = 4752 samples) collected from 97 islands in British Columbia, Canada. Island area was the strongest predictor of enrichment across taxa; we found that samples were more 15N‐rich on smaller islands. Enrichment declined with distance from shore but less so on small islands, implying a higher per‐unit‐area subsidy effect. These area and distance‐to‐shore effects were taxon‐specific, and nearly twice as strong in basal food web groups. We also found that increases in δ15N correlated with increases in %N in basal trophic groups, as well as in songbirds, implying biologically relevant uptake of a potentially limiting nutrient. Path analysis demonstrated that subsidies in soil flow through plants and detritivores, and into upper‐level consumers. Our results reveal an interplay between island biogeography and marine subsidies in shaping island food webs through bottom‐up processes.

Scale‐dependent effects of marine subsidies on the island biogeographic patterns of plants

“…Additionally, it remains unclear whether species richness decreases with increases in soil δ 15 N because plant species cannot tolerate disturbances caused by the subsidy source, or if C. nutkaensis is outcompeting other species on a local scale. Finally, despite previously finding higher wrack biomass corresponding to 15 N enrichment in two plant species on the same set of islands (Obrist et al, 2022), we found no evidence of wrack affecting patterns in plant species diversity at the plot scale nor at the island scale. Given the high productivity of kelp forests in the study area (Steneck et al, 2002;Wilmers et al, 2012), we initially thought that wrack would be one of the main contributions of marine inputs to the islands in our study.…”

“…Finally, despite previously finding higher wrack biomass corresponding to 15 N enrichment in two plant species on the same set of islands (Obrist et al, 2022 ), we found no evidence of wrack affecting patterns in plant species diversity at the plot scale nor at the island scale. Given the high productivity of kelp forests in the study area (Steneck et al, 2002 ; Wilmers et al, 2012 ), we initially thought that wrack would be one of the main contributions of marine inputs to the islands in our study.…”

“…By depositing materials containing the heavy isotope of nitrogen, 15 N, marine subsidies often elevate soil δ 15 N in coastal terrestrial ecosystems (Ben‐David, Bowyer, et al, 1998; Ben‐David, Hanley, & Schell, 1998; Feddern et al, 2019). On the studied islands, marine subsidies likely come from many different sources (Obrist et al, 2022), including deposits of feces, urine, and discarded prey items of North American river otters ( Lontra canadensis ; Ben‐David, Bowyer, et al, 1998, C. Ernst, unpublished data ), wind and wave‐deposited seaweed wrack (Wickham et al, 2020), sea spray (Weathers & Likens, 1997), and marine fog (Art et al, 1974). In our island level analysis, we also consider the potential effects of island slope, while in the plot level analysis, we consider soil moisture, plot slope, forest openness, and distance to shore.…”

Equivalent roles of marine subsidies and island characteristics in shaping island bird communities

“…Marine‐derived nitrogen is likely a good indicator of river otter activity on these islands. Their latrines create major onboarding spots for marine‐derived nutrients that can flow in many directions through different processes, impacting island food webs (Ben‐David et al, 1998; Obrist, Hanly, et al, 2022). In addition to creating hotspots of marine flow, river otter behaviour can also impact habitats through smaller, localized disturbances to coastal vegetation and soils.…”

Nutrients from spawning salmon influence leaf area, tissue density, and nitrogen‐15 in riparian plant leaves

“…fundamental ecological theory, such as the intersection between subsidies and Island Biogeography Theory (Anderson & Wait, 2001;Obrist et al, 2022).…”