Ancient plant use among the Island Chumash is much less well understood than other aspects of islander lifeways. There is a long history of research on faunal assemblages from Island Chumash sites, whereas comparatively little paleoethnobotanical research has been done. The resulting disparity in faunal vs. floral data combined with field observations of an island landscape ravaged by historical overgrazing, led various researchers to suggest that island plant foods were too marginal to support island populations and that mainland plant foods, subsequently, may have been a major motivating factor behind cross-channel exchange networks and increased sociopolitical complexity seen later in time. Within the context of optimal foraging theory and diet breadth models, I explore the significance of plant foods to the Island Chumash of Santa Cruz Island, using archaeological and paleoethnobotanical data from three sites with bedrock mortars, located in upland and interior areas. These non-coastal sites occur in some of the most productive terrestrial areas on the island, representing a range of time periods and site types: a logistical encampment (Sunburst -- AD 1260-1500); an interior residence (Brodiaea Ridge, 4330 BC-AD 1630); and, a village (Diablo Valdez, 3920 BC-AD 1800). Deep, well-stratified deposits and excellent preservation of domestic features (i.e., roasting pits, hearth clearing pits, structural floor) at Diablo Valdez provide a high-resolution record of archaeological and paleoethnobotanical remains, where I was unable to identify any significant change in plant food subsistence for nearly 6,000 years. Brodiaea corms were the most ubiquitous taxon identified at this site, sometimes occurring in great abundance and associated with large roasting pit features.

Here, I argue that carbohydrate content, rather than caloric value, may be a more appropriate currency for ranking plant foods in island contexts, where abundant marine resources provided ample fats and protein. In this scheme, the ranking of plant foods on the northern Channel Islands, in terms of optimal foraging and island archaeobotanical data (ranked high to low) are: 1) geophytes; 2) kelps and seaweeds; 3) small seeds; 4) fruits, berries, and non-toxic pits; 5) leaves, stems, and stalks; 6) toxic nuts and pits; 7) non-toxic nuts; and, 8) aquatic roots/rhizomes. While there is a preservation bias between these various plant food categories, this general ranking scheme appears to be supported in the island archaeobotanical record.

As Channel Island vegetation communities recover form more than a century of overgrazing, it has become clear that the phenomenally abundant geophyte resources that occur on the islands are significantly larger and denser than their mainland counterparts in the absence of gophers, moles, ground squirrels, deer, and other terrestrial herbivores. The brodiaeas are particularly well represented in archaeobotanical assemblages for the islands, used for at least 10,000 years and harvested in multiple seasons. The diversity and unparalleled abundance of island geophyte resources would have provided easily procurable and substantial carbohydrates for the Island Chumash and their ancestors. Combined with the diverse and abundant edible marine plants and algae surrounding the islands, geophytes and other island plants provided the Island Chumash with ample food, medicine, and raw materials that were more abundant and stable than previously assumed.