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The differences between temperate and tropical seasonality suggest that ecological models developed for temperate streams do not necessarily apply to tropical streams. While detritus is the base of the temperate zone stream food webs, we expected a seasonal alternation in the energy source in tropical streams (i.e., increase in the autochthonous energy source in the dry season, and an increase in the allochthonous energy source in the rainy season). We sought to evaluate the annual dynamics of the benthic macroinvertebrate functional feeding group (FFG) communities, the availability of their food resources, and the use of FFG ratios as surrogates for stream ecosystem traits in two first-order, tropical streams in Chiapas, Mexico. We found an increase in the biomass of herbivore-FFGs correlated to their autochthonous food resources in the dry season. The detritivore-FFGs biomass did not correlate to the abundance of the allochthonous food resources. Stream ecosystem traits were similar between seasons and were associated with a dominance of scrapers, shredders, and predators throughout the year. We concluded that the temperate classical ecological stream models that suggest allochthonous resources almost entirely support food webs do not directly extrapolate to tropical stream ecosystems. Instead, tropical stream food webs significantly rely on autochthonous food resources. [ABSTRACT FROM AUTHOR]