The experimental range extension of guppies (Poecilia reticulata) influences the metabolic activity of tropical streams.

The ecological consequences of biological range extensions reflect the interplay between the functional characteristics of the newly arrived species and their recipient ecosystems. Teasing apart the relative contribution of each component is difficult because most colonization events are studied retrospectively, i.e., after a species became established and its consequences apparent. We conducted a prospective experiment to study the ecosystem consequences of a consumer introduction, using whole-stream metabolism as our integrator of ecosystem activity. In four Trinidadian streams, we extended the range of a native fish, the guppy (Poecilia reticulata), by introducing it over barrier waterfalls that historically excluded it from these upper reaches. To assess the context dependence of these range extensions, we thinned the riparian forest canopy on two of these streams to increase benthic algal biomass and productivity. Guppy's range extension into upper stream reaches significantly impacted stream metabolism but the effects depended upon the specific stream into which they had been introduced. Generally, increases in guppy biomass caused an increase in gross primary production (GPP) and community respiration (CR). The effects guppies had on GPP were similar to those induced by increased light level and were larger in strength than the effects stream stage had on CR. These results, combined with results from prior experiments, contribute to our growing understanding of how consumers impact stream ecosystem function when they expand their range into novel habitats. Further study will reveal whether local adaptation, known to occur rapidly in these guppy populations, modifies the ecological consequences of this species introduction.