Guppy (Poecilia reticulata) populations in streams with different predation regimes exhibit marked differences in their life-history traits. Differences have been attributed to direct effects of predation, but predation may also play a role through its indirect effects on guppy population structure and density and thereby the per capita resource availability. Specifically, resource levels could affect prey selectivity and nutritional gain, potentially leading to changes in life-history traits. As part of the Trinidad NSF-FIBR Project, we asked the question: do diet and gut length co-vary with habitat type and might these attributes have evolved in concert with the life-history? To answer the first part of this question, we examined differences in guppy diet (n=84) among stream types with different predation levels (HP=high; LP=low) and how they relate to gut length (n=161) and benthic invertebrate biomass (n=36) in streams during the dry season. We also compared gut content and gut length from fish collected during the dry and the wet seasons (n=54 gut content, n=46 gut length). Fish and invertebrate samples were collected from two HP and two LP reaches of two different study rivers (Aripo, Guanapo). Wet season gut samples were collected only from the Aripo HP and LP sites.

Results/Conclusions

During the dry season, HP guppies ate significantly (P<0.001) more invertebrates than LP guppies, which instead fed significantly (P<0.001) more on algae and detritus. There was no strong correlation between stream invertebrate biomass and invertebrates in guppy guts (r²=0.089). These findings suggest that diet differences are caused by evolved differences in food selectivity and not by the amount of resources available. When we compared diet across seasons, there was a significant effect of predation (P<0.001) and season (P=0.035), but there was also a significant interaction between predation and season, owing to the fact that diet differences between HP and LP Aripo fish were large in the dry season but minimal in the wet season. Despite seasonal differences in diet, LP guppies maintained longer guts than HP guppies across both the wet and dry seasons. In conclusion, guppies inhabiting sites with different predation pressure have evolved distinct diet preferences during the dry season, which may be linked to the evolution of different life-history traits. During the wet season, diet differences seem to disappear, possibly due to reduction of available food and guppy density.