The marine ecosystem provides key resources to terrestrial organisms inhabiting oceanic islands. These subsidies of marine resources have the potential to affect species richness, ecology and productivity, especially on islands with high perimeter-area ratios. I investigated the impact and importance of marine subsidies on mammal diversity and diet on islands of British Columbia’s Central Coast. Insular mammal species richness was significantly correlated with island area and quantity of marine subsidy (wrack). However, mink and river otter island occupancy was unaffected by island-level covariates, whereas small mammals were more likely to occupancy islands closer together. Keen’s mice and food items were subsidized directly (i.e., consumption) and indirectly (i.e., fertilization) by marine resources. Beach-dwelling arthropods composed 33% of mouse diets. Furthermore, mouse and terrestrial arthropod abundances and stable isotope signatures (d13C and d15N) of food items were depleted moving inland from the beach. Finally, reproductive male mice consumed up to twice the marine-derived prey as females. Collectively, this work demonstrates that insular mammalian richness, as mediated by island-level factors, may be complex due to variation within populations and the recipient ecosystem (e.g., prey biomass). / Graduate