Fish introductions and light availability modulate food web fluxes in tropical streams: A stable isotope tracer approach

Background/Question/Methods

Decades of ecological study have identified top-down and bottom-up controls on food web structure in a diverse set of ecosystems, but relatively little is known about freshwater ecosystems in the tropics. We conducted an experiment to investigate how fish introductions and light availability influence energy and material fluxes in Trinidadian stream food webs, where the evolution of guppies (Poecilia reticulata) has been studied for decades and guppy populations can be manipulated in whole-ecosystem experiments.  We introduced guppies to two headwater streams and compared guppy-introduction reaches to no-guppy reference reaches to evaluate the effects of guppy introduction on food webs.  We thinned the canopy of one of the two study streams to increase light availability and compared it to the other guppy introduction stream, which had a natural canopy, to determine how light availability affects food web linkages.  A nitrogen (N) stable isotope tracer approach allowed us to compare food web fluxes of N and determine how they differed in response to light availability and guppy introduction.

Results/Conclusions

Primary production was higher in the canopy-thinned stream compared with the natural canopy stream, which led to increased fluxes of N to invertebrate taxa in grazer, collector-gatherer and filterer functional feeding groups.  Primary production was also higher in guppy introduction reaches, which resulted in increased magnitude of N fluxes to grazers and filter-feeders.  The magnitude of N fluxes to collector-gatherer invertebrates decreased in guppy introduction reaches, possibly due to predation, resource competition, or behavioral responses of invertebrates.  N fluxes to shredder invertebrates, predatory invertebrates and killifish did not differ across light or guppy treatments.  Effect size of guppy and canopy treatments on N flux to consumers revealed that they have similar effects on most taxa, but guppies have a very strong effect on filter feeding invertebrates and canopy had a very strong effect on collector-gatherer invertebrates.  Combined, these results indicate that light availability and fish introduction can both influence food web linkages in detritus-based tropical streams by increasing autotrophic production, while detritus-based links remain relatively stable.