Ecosystems are intricately connected by the exchange of organisms, inorganic materials, energy, and information that traverse ecotones, forming a complex network of interactions. These inputs from donor systems, known as spatial subsidies, can profoundly shape habitats by influencing primary productivity, altering community interactions, impacting resilience, and changing species composition in recipient habitats. However, human activities such as global shipping and urbanization can disrupt these subsidies through increased nutrient flow, species introductions, and climate-mediated range shifts. In this dissertation, I investigate how invasive species in donor systems and human-mediated movement of organic material across habitats impact subsidy-dependent communities.Chapter 1 explores the impacts of an invasion-mediated shift in seaweed wrack from native kelp, Macrocystis pyrifera, to invasive Devilweed, Sargassum horneri, on subsidy- dependent communities of rocky shores. The study assesses the species-specific impacts on common detritivores (Pagurus samuelis [Blue banded hermit crab], Pachygrapsus crassipes [Striped shore crab], and Tegula funebralis [Black turban snail]), the historically important but now endangered wrack-consuming giant snail (Haliotis cracherodii, [Black abalone]), an assemblage of these grazers, and common native benthic seaweeds (Centroceras clavulatum, Silvetia compressa, and Ulva spp.). Performance impacts are evaluated through long-term feeding assays using common Black turban snails and Haliotis rufescens (Red abalone), as a proxy for the rare Black abalone. Food preference was determined through feeding choice assays using individual species of these wrack detritivores, and a “community assay” in which an assemblage of these grazers fed on three native benthic seaweeds along with either native kelp or invasive Devilweed wrack are conducted. Performance varied among consumers, with abalone growing better on a kelp diet compared to Devilweed and showing intermediate growth on a mixed diet. Contrary to predictions, Black turban snails grow more on the Devilweed diet over kelp, and those that fed a mixed diet grow similarly to those that ate Devilweed alone. Preference assays reveal that although Black turban snails grow more on Devilweed diets, they prefer kelp, while abalone also display a high preference for kelp. Blue banded hermit crabs prefer Devilweed, and Striped shore crabs show no preference. The assemblage of wrack detritivores shifts consumption away from wrack when kelp is replaced by Devilweed, and increases consumption on S. compressa, an already sensitive, canopy-forming rockweed, suggesting Devilweed has the potential to indirectly impact native benthic seaweeds in subsidy-dependent communities. Chapter 2 aims to understand the spatiotemporal variation in native and invasive wrack deposition on beaches that receive large inputs from adjacent kelp forests, recently invaded by Sargassum horneri. We conducted surveys at seven sites on one of the California Channel Islands at four time points across 2022. Our findings reveal spatiotemporal variation in native and invasive wrack inputs to beaches, with Giant kelp, Macrocystis pyrifera, dominating wrack inputs throughout the year, and S. horneri being relatively rare. Kelp was most abundant on west-northwest facing shores, while S. horneri was even more rarely found on west-facing shores. The peak deposition periods for kelp and S. horneri differed, with kelp deposition peaking in September and S. horneri deposition peaking in March. This chapter highlights the complex spatiotemporal variation in native and invasive wrack inputs, and their potential to shape recipient communities.

In Chapter 3, we delve into the intricate interconnections between ecosystems, focusing on the unique phenomenon of Sargassum wrack deposition along coastlines in the Mexican Caribbean. These massive deposits, resulting from seaweed blooms in the Sargasso Sea and the Great Sargassum Belt, present significant ecological and economic challenges. Our study investigates the ecological implications of Sargassum deposition, employing manipulative field experiments simulating realistic biomass inputs of several cubic meters in both beach and forest ecosystems. Contrary to expectations based on ecological theory, our findings reveal comparable decomposition rates between beach and forest ecosystems, challenging the notion that naïve ecosystems are incapable of processing novel subsidies. We assess the relative contributions of arthropods and microbes to Sargassum decomposition, with microbial communities dominating decomposition in the forest and a combination of microbes and talitrid amphipods driving decomposition on the beach. Furthermore, our study provides insights into the long-term effects of Sargassum deposition on nutrient cycling within these two ecosystems. After 12 months, we found that Sargassum may serve as a nutrient subsidy to native plants in the forest, albeit with slower utilization rates compared to non-native plants such as Bermuda grass found on beaches in the area. Overall, our results highlight the capacity of forest ecosystems to assimilate and utilize foreign organic matter, challenging traditional ecological paradigms and offering new perspectives on ecosystem functioning.