Historical perspectives suggest that evolutionary change is largely dictated by environmental changes in nature. More recent evidence suggests that ecological structure and function may also respond to evolutionary changes in populations leading to potential feedbacks on the evolving organisms. As part of the NSF Frontiers in Integrative Biological Research (FIBR) Guppy Evolution Project, we asked how do ecosystem processes differ in the early stages of a long-term species introduction and light enhancement in neotropical streams? To address this question, we estimated leaf decomposition of 2 riparian plant species (Cecropia and Pachystachys spp.) and algal accrual rates on tiles in 2 reaches of 2 headwater streams in Trinidad. Sets of leaf packs and tiles were placed in 3 pools of reaches with the native killifish, Rivulus hartii and newly introduced predation-adapted guppies (R+G) and in reaches containing only Rivulus (R) and with and without canopy thinning (light and dark). We incubated tiles and bag-less leaf packs from 3 to 28 days shortly following introduction of guppies. We regressed tile chlorophyll a and remaining leaf mass against incubation time to estimate rates and compared rates within and among treatments.

Results/Conclusions

Cecropia sp. decay rates were significantly faster in the dark stream (0.19 +/-0.012 SE day^-1) than the light stream (0.13 +/-0.013 SE day^-1, 1-way ANOVA, p=0.05), suggesting higher heterotropic activity in the darker environments. Both leaf types decayed slightly, but not significantly, faster in R reaches than R+G reaches (Cecropia sp.: 0.018 and 0.013 day^-1, 1-way ANOVA, p=0.051; Pachystachys sp.: 0.27 and 0.23 day^-1, 1-way ANOVA, p=0.059, respectively). Algal accrual rates tended to be faster in the R reaches (0.15 +/- 0.01 SE day^-1) than the R+G reaches (0.13 +/- 0.02 SE day^-1), and only slightly faster in the light (0.14 +/- 0.01 SE day^-1) relative to the dark stream (0.13 +/-0.01 SE day^-1). These results suggest predation-adapted guppies may initially suppress ecological processes relative to Rivulus-only reaches and that increased light may slightly enhance primary productivity but suppress decomposition. Preliminary evidence from guppy diets suggest that predation-adapted fish consume more invertebrates than non-adapted fish, so disruption of grazer-biofilm or shedder-leaf interactions by current guppy feeding behavior might explain some of the differences observed in algal accrual and decomposition rates. Ongoing research will continue to assess changes in ecosystem processes as guppies evolve to their new low-predation environments.