The abundance and the distribution of trophic resources available for consumers influence the productivity and the diversity of natural communities. Nevertheless, assessment of the actual abundance of food items available for individual trophic groups has been constrained by differences in methods and metrics used by various authors. Here we develop an index of food abundance, the framework of which can be adapted for different ecosystems. The relative available food index (RAFI) is computed by considering standard resource conditions of a habitat and the influence of various generalized anthropogenic and natural factors. RAFI was developed using published literature on food abundance and validated by comparison of predictions versus observed trophic resources across various marine sites. RAFI tables here proposed can be applied to a range of marine ecosystems for predictions of the potential abundance of food available for each trophic group, hence permitting exploration of ecological theories by focusing on the deviation from the observed to the expected.

Table S_1Table S_1. Literature used to rank the abundance of trophic resources among ecosystems. The last section (biotope typologies) has been used principally to detect natural or anthropogenic pressures influencing the basic patterns of food availability. Reference numbers indicated in Tables 1 and 6 refer to the papers listed here.Table S_2_ RAFI Evaluation for sitesTable S_2. a) Original spreadsheets used to evaluate RAFI in the ecosystems considered in the manuscript; b) User spreadsheets to be filled with own data for computing the Relative Abundance of Trophic Resources (RAFI) in other ecosystems. Please fill only the white cells: other cells must not be modified. Do not consider empty stations: they all initially result equal to "1.0" due to lack of data until your data will be present