The influence of salmon-derived nutrients in coastal plant communities

Ecosystem connectivity, facilitated by resource subsidies and organismal movement, is a significant driver of ecological processes. Pacific salmon (Oncorhynchus spp.) deposit marine-derived nutrient subsidies during spawning events, which can have significant ecological effects on terrestrial species. In this thesis, I study the effects of marine and salmon-derived nutrients on the ecology of coastal plants on the central coast of British Columbia, Canada. In Chapter 2, I investigate the effects of marine subsidies on terrestrial plant growth and reproduction. I conducted a large-scale field experiment involving the addition of pink salmon (O. gorbuscha) carcasses and rockweed (Fucus distichus) to a wildflower meadow. I found that salmon carcass deposition had species and context-dependent effects on estuary plants, with observed increases in foliar nitrogen-15, leaf area, floral display size, and seed set. This suggests that marine nutrients can affect terrestrial plant growth and reproduction. In Chapter 3, I test for the effects of nutrient subsides on plant-pollinator mutualisms by examining floral availability and visits by beneficial insects. Using the field experiment outlined in Chapter 2, I found that salmon carcass deposition has a direct positive effect on floral availability, and an indirect positive effect on the floral visits by insects to those flowers. This work is among the first evidence describing the effects of marine subsidies on plant-pollinator mutualisms. In Chapter 4, I investigate the effects of variation in salmon spawning density in 14 watersheds on the leaf traits of riparian plant species. I found that nutrients from spawning salmon affected the morphology and physiology of these plants, with stronger effects observed in nitrophilic plant species. These results included higher foliar nitrogen-15, larger leaf area, and—in one species—an increase in leaf mass per area on streams with higher spawning density. These findings lend support to a mechanism by which certain plant species are more common on productive salmon streams. Taken together, this work demonstrates that Pacific salmon can have significant impacts on terrestrial ecosystems. These findings highlight the importance of considering the connectivity between ecosystems and the role of marine nutrient subsidies in driving terrestrial ecological processes.