Predators are typically larger than their prey, and consequently, trophic level (TL) should increase with body size (size). Theoretical studies have used this general relationship to predict, with some success, food web structure and dynamics in mesocosms and simple communities. However, some have argued that a TL-size relationship may not exist for all kinds of communities or evolutionary lineages. Moreover, influential traits, such as gut length, are usually ignored. We explored the relationship between TL and size in fishes, and how this relationship may change according to other traits. Two hypotheses were tested: 1) size is directly related to TL, and 2) the relationship between size and TL is mediated by gut length. Data were compiled for 19 fish species from Texas with at least 100 specimens analyzed in previous diet studies. Gut length was measured and standardized by size (SL). A mixed model was carried out using TL as response variable, size as the main factor, and species as a random factor. The random effect slopes (TL-size slope for each species) were then related to the relative gut length of each species using a generalized additive model (GAM).
Results/Conclusions
The general relationship between fish body size and trophic level was not significant, indicating that the relationship depend on species identity. Micropterus salmoides, Pomoxis annularis and Menidia beryllina had consistent positive relationships, Lagodon rhomboides had a negative relationship, and the remaining species had no clear relationship. The GAM showed a negative polynomial relationship between the random slopes and gut length suggesting that carnivores (short gut) tend to occupy higher TL as they grow, whereas TL of detritivores and herbivores (long guts) is constant or declines during growth. Preliminary results based on Texas fishes indicate that TL cannot be estimated based on body size due to the diversity of feeding strategies and morphological adaptations among and within species. Further analyses involving species from other regions and additional morphological traits are pending.