With increasing biodiversity loss occurring worldwide, there is a need to understand how these losses will affect ecosystem structure and function. Certain species have a greater impact on the structure and function of ecosystems (i.e. foundation species, keystone species, ecosystem engineers) and therefore their importance suggests a need for understanding their dynamics. I focused on foundation species, which are those that provide the primary habitat and define the communities and ecosystems they create (Dayton 1972, Graham et al. 2007). My dissertation explores different causes and the cascading consequences of kelp forest foundation species loss. In chapter 1, I focus on the consequences of kelp forest biodiversity loss. The loss of the structure and energy that kelps provide leads to changes in species interactions, specifically the trophic complexity of food webs, however how these changes manifest is undescribed (Graham 2004). Sampling the stable isotopes of producer and consumer communities when kelp and other macroalgae are present, versus absent, while comparing these to naturally occurring consumer communities with low biodiversity, has revealed how overall biodiversity loss affects the complexity of food webs. I determined that the reductions in the diversity of kelp forest consumer food resources leads to reductions in the diversity of consumer trophic levels. This comes about through the reduction in food diversity for omnivores and herbivores which propagates to consumers at higher trophic levels. These findings suggest that more intact and biodiverse communities support greater food diversity and trophic complexity. In chapter 2, to better understand kelp physiology and spatial variation, I tested for differences in the stable isotopes and elemental concentrations within and among canopy kelp individuals at islands across the Aleutian Archipelago. Variability was high among islands and led into hypotheses for chapter 3 about potential drivers of this variation in kelp, and ultimately kelp forest communities, across space. In chapter 3, I explored connectivity between terrestrial and marine ecosystems. We understand that connectivity of organic matter, nutrients, and materials can be critical to the structure and function of ecosystems. Interruption of connectivity can occur through top down forcing by invasive upper trophic-level predators. I determined if invasive fox predators have altered connectivity between terrestrial and marine ecosystems using kelp forests at islands that spanned a gradient in invasive fox history, and seabird density, across the Aleutian Island Archipelago. I found evidence that invasive foxes had greater impacts on seabird densities the longer they had been present on islands. The reduction in seabirds has caused less seabird guano-derived nutrients to be available and utilized by kelp forest primary producers, primary consumers, and secondary consumers. This is some of the first evidence that invasive foxes have reduced connectivity between seabirds and kelp forest food webs. Restoring connections between terrestrial and marine ecosystems should be considered in future work and management across the Aleutian Island Archipelago. A consequence of losing kelp forests to urchin grazing is the alteration of competitive interactions. In chapter 4, I tested if higher irradiance and increased intensity of herbivory in areas where urchins have removed kelp forests has altered the tradeoffs between primary producer growth and defense. Across the Aleutian Island chain, the widespread decline in sea otters has resulted in reduced predation on green sea urchins, which has led to dramatic increases in urchin populations, the formation of urchin barrens, and ultimately to overgrazing of much of the region’s kelp forests. The differential recovery of kelp forests and the long-term persistence of urchin barrens at certain islands has created a gradient in irradiance and intensity of herbivory that we expect could alter growth and defense in macroalgae. Field and lab results suggested that a common perennial urchin barren macroalga (Codium ritteri) has greater defense than growth within urchin barrens relative to kelp forest individuals. In the laboratory there was little evidence for increased growth under lower light or altered defenses at high light. An experiment testing for spatial variation of urchin grazing rates on C. ritteri revealed that decreasing growth but increasing defense of C. ritteri correlated with higher urchin biomass in the field. Together, these findings suggest that macroalgae occurring within kelp forests may grow faster and be more palatable than macroalgae occurring within urchin barrens. Urchins may be deterred by urchin barren macroalgae causing them to move toward kelp forests where they can more easily consume the less defended macroalgae there, increasing urchin deforestation potential. Together these chapters suggest that the consequences of kelp loss to communities can be (1) through a reduction in community-wide isotopic dietary niche breadth expressed through the loss of omnivore and herbivore dietary niche breadth. (2) Spatial variation in the stable isotopes and elemental concentrations of canopy kelp suggested that differences in nutrient and carbon availability among islands may affect kelp growth and susceptibility to grazing by urchins, ultimately affecting kelp forest persistence. The loss of kelps leads to (3) a reduction in the interaction chain length and propagation of seabird derived nutrients to nearshore marine communities. Lastly, the reduction of kelp and (4) increased intensity of herbivory by urchins within urchin barrens has led to macroalgae with lower growth and increased defenses.