Species' traits can affect their trophic interactions with other species. It is not clear, however, how these effects scale up from single interactions to shape species' roles within food webs. Previous efforts to relate network structure to species' traits suggest that a relatively small number of traits may be able to explain patterns of interactions within food webs, but so far efforts to identify whether certain traits are related to certain patterns have met with limited success. Here we use a motif framework (based on the simple building blocks of complex networks) to relate seven basic traits of marine organisms (body mass, trophic level, feeding type, feeding mode, feeding environment, metabolic category, and mobility) to their roles in six large, well-resolved empirical food webs. As different traits are likely to shape the roles of basal resources, top predators, and intermediate consumers, we analyzed each group separately. We first determined the motif positions associated with the major axes of variation in the roles of each group. Next, we identified the traits most strongly associated with the frequency of each position. This allowed us to interpret variation in species' roles in light of ecologically important traits.
We find that species' positions in three motifs (apparent competition, direct competition, and three-species food chains) are most strongly associated with the major axes of variation in the roles of basal resources, intermediate consumers, and top predators alike. The traits associated with these positions, however, vary between trophic groups. For basal resources, feeding environment and body mass are most strongly associated with species' roles. For intermediate consumers, a wider variety of traits--- including trophic level, body weight, feeding environment, and feeding type ---are important. For top predators, trophic level is the trait most strongly associated with species roles. Taken together, our results suggest that relatively simple traits can provide a great deal of insight into species' roles within food webs. In particular, traits such as body mass and trophic level are strongly associated with positions in motifs with clear biological meanings (e.g., apparent competition). As these motifs have previously been studied in isolation, our results suggest that these smaller-scale results may also be relevant to large empirical communities. Our results also highlight trophic level and body weight as particularly important in shaping species roles, lending support to food-web models based on these traits.