As part of the NSF-Frontiers in Integrative Biological Research (FIBR) Guppy Evolution Project, we asked: what is the role of macroconsumers in structuring benthic communities and driving ecological processes in high gradient Trinidadian streams? We conducted experiments in two stream reaches characterized by 2 different macroconsumer assemblages: (1) an upstream reach containing Hart’s killifish, Rivulus hartii, and Manicou crab, Eudaniella garmani (identified as Non-guppy reach, NG); and (2) a downstream reach additionally containing guppies, Poecilia reticulata (identified as Guppy reach, G). We experimentally excluded macroconsumers from small areas of the stream bottom for 28 days using 25cm X 50 cm electrified copper wire quadrats placed in 5 replicate pools. We measured macroconsumer visitation to paired control quadrats (as macroconsumers were fully excluded from electrified quadrats) in each replicate pool in addition to background variables such as light level and stream velocity. We hypothesized that in both stream reaches, the macroconsumer assemblage would enhance rates of leaf decomposition but decrease algal accrual rates and algal standing crop relative to macroconsumer exclusion treatments. Additionally, we predicted that macroconsumer exclusion would result in greater invertebrate biomass and taxon richness. 

Results/Conclusions

As predicted, in both reaches leaf decomposition was significantly faster in control (G: 0.194 day^-1, NG: 0.182 day^-1) relative to exclusion (G: 0.109 day^-1, NG: 0.083 day^-1) treatments, while peak algal chlorophyll-a (Day 28) was higher in exclusion versus control treatments (G: 54.47 mg chl-a m^-2 vs. 26.03 mg chl-a m^-2, p=0.0024, NG: 52.07 mg chl-a m^-2 vs. 21.27 mg chl-a m^-2, p=0.0646). Mean algal accrual rate was higher in the exclusion versus control treatment (G: 0.091 day^-1 vs. 0.045 day^-1, p=0.0482, NG: 0.101 day^-1 vs. 0.042 day^-1, p=0.0035). In both reaches, invertebrate biomass did not differ significantly between exclusion and control treatments, although differences between control treatments were negatively related to guppy visitation (P = 0.06, R² = 0.74) in the G reach, suggesting that guppies play a central role in top-down control of invertebrate biomass. Interreach differences between control and exclusion treatments (chlorophyll-a accrual and leaf decomposition) were similar, suggesting an insignificant guppy effect (when co-occurring with Rivulus and Eudaniella in the G reach) relative to effects of Rivulus and Eudaniella alone (in the NG reach). Current studies focus on identifying specific roles of individual macroconsumer taxa in top-down control of benthic structure and function.