Understanding the evolutionary context of the role of animals in ecosystem processes is important in light of the potential for rapid adaptive responses driven by anthropogenic perturbations such as land use, over-harvesting, eutrophication and climate change. As part of the NSF-Frontiers in Integrative Biological Research (FIBR) program, we ask how adaptive changes in phenotypes influence nutrient recycling in streams.  We estimate fish excretion in two phenotypes of Trinidadian guppies (Poecilia reticulata) to account for the differences in life history evolution and potential effects on nutrient recycling. Guppies are well suited to address this interdisciplinary question because they inhabit two distinct environments that can be categorized as high (HP) and low predation (LP) and populations from these two environments demonstrate genetic-based differences in life history traits and they demonstrate rapid evolution in many of these traits.  We maintained HP and LP guppies at 2 densities in flow-through field mesocosms for 4 weeks with comparable constant resource and water supply. We measured dissolved nitrogen as NH₄ and phosphorus as soluble reactive phosphorus (SRP) from LP and HP guppies in bags incubated for 20 minutes.

Results/Conclusions

Average N excretion rates (in µmol N-NH₄ g wet mass^-1 hr^-1) were almost 2 fold higher in HP adult females (1.4) and juveniles (3.6) than LP adult females (0.88) and juveniles (1.7) in low density treatments. This trend was opposite for average P excretion (in µmol P-SRP g wet mass^-1 hr^-1) in adult females with higher rates in LP (0.96) than HP (0.65) guppies, but more than 2 fold higher for HP (1.7) than LP (0.51) juveniles in low density treatments. Lower P excretion rates are expected for animals that sequester P for fast growth and high reproduction rates, which was the case for HP adult male and female guppies, but not for HP juveniles. Applying these life history-specific excretion rates and N:P stoichiometry to published LP and HP guppy size spectra data adjusted to equal biomasses suggests that the HP phenotype guppies contribute almost 190% more to N flux than the LP phenotype guppies, but nearly identical P flux. Faster N excretion rates in the high predation guppy phenotype may facilitate primary production or other nutrient cycling processes in N-limited streams. In conclusion, HP and LP guppies have very different excretion rates when housed in a common environment suggesting that differences in life history evolution of guppies may help regulate nutrient recycling in Trinidad streams.