Macroconsumers (fishes, crustaceans) can exert strong top-down effects on ecosystem properties and processes in lotic systems.  As part of the NSF-FIBR Trinidad Project, we asked: What are the top-down effects of different macroconsumer taxa on benthic structure and function in Trinidadian high-gradient tropical streams?  To tease out taxa-specific effects, we used point-count methods (20 observation hours) to assess diel behavior and estimate density for different macroconsumers in two stream reaches: one containing Hart’s Killifish (Rivulus hartii) and Manicou crab (Eudaniela garmani) (Rivulus reach), and the other containing these two species plus guppies (Poecilia reticulata) (the Guppy reach). These data were used to design four treatments using underwater electric ‘fences’ to exclude macroconsumers from the benthos based on their diel activity patterns: Control (C, no exclusion), Diurnal (D, diurnal exclusion), Nocturnal (N, nocturnal exclusion), and Electric (E, complete exclusion).  These treatments were placed in standardized conditions in a randomized block design within five different pools within each stream reach. Treatments served as proxies for taxa-specific effects on benthic structure and function.  We quantified several ecosystem processes and properties throughout a 28-day experiment: leaf decomposition rate; leaf pack invertebrate communities; algal standing stock and rate of accrual; and benthic invertebrate communities.

Results/Conclusions

In the Rivulus reach, Rivulus demonstrated no diurnal/nocturnal preference while crabs were largely nocturnal. In the Guppy reach, Rivulus were significantly (p < 0.05) more nocturnal than in the Rivulus reach, while guppies demonstrated nearly uniform preference for diurnal activity.  In the Guppy reach, treatments excluding guppies (E, D) had significantly (p<0.05) higher algal standing crop than control (C) treatments, suggesting that guppy foraging has a top-down effect on algae.  We also found marginally significant (p = 0.051) effects of guppy exclusion on algal accrual rate, suggesting that both top-down pressure (guppy foraging) can regulate algal accrual rate.  Decay rates of Cecropia peltata leaf packs in the Guppy reach did not differ significantly between treatments; however, in the Rivulus reach, leaves in the C treatment (24 hr macroconsumer access) decayed at significantly (p < 0.001) slower rates than those in all three of the exclusion treatments (D,N,E).  Slower rates of decomposition in the C treatment were accompanied by significantly (p < 0.05) reduced biomass and abundance of the system's primary invertebrate shredder, Phylloicus hansoni, relative to all exclusion treatments, indicating the existence of a macroconsumer - macroinvertebrate – detritus trophic cascade in streams where guppies are absent.