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Nonnative species can have serious negative effects on regeneration and restoration of rare plant taxa, particularly in insular ecosystems. An endangered Hawaiian shrub, Delissea rhytidosperma (Campanulaceae), produces fruits and viable seeds, but no regeneration has been observed in the wild. We used cages and vegetation removal to explore direct and indirect effects of three groups of nonnative species on suspected seed predation of this endangered plant: a mat-forming grass (Oplismenus hirtellus), rats (Rattus spp.), and invertebrates. Substantial seed removal occurred in all treatments. Both rat exclusion and clearing of nonnative vegetation had strong significant negative effects on seed removal. Highest removal rates occurred with rats not excluded and vegetation present, and lowest removal occurred when rats were excluded and vegetation cleared. Without rat exclosures, 100% of seeds were removed within 15 days. Even when protected from rats, most seeds were removed by smaller herbivores, unless ground vegetation was cleared. Vegetation appears to harbor invertebrates that eat seeds, including nonnative slugs. These results revealed that different nonnative species combine to greatly increase rates of seed removal in endangered D. rhytidosperma.