Habitat modification by tropical fishes: Environmental heterogeneity and the variability of interaction strength

I examined variation across habitats and time in the ability of the flannelmouth characin (Prochilodus mariae) to strongly influence patterns of sediment accrual in a tropical Andean stream and the resulting heterogeneity of the streambed. Prochilodus is an often dominant component of the fish biomass in Andean piedmont streams and is highly specialized to derive nutrition from organic-rich sediments. Previous research demonstrated that its selective removal from pools resulted in highly significant increases in sedimentation and concomitant changes in the composition of algal and invertebrate assemblages. I report new results from a field experiment conducted over a 40-d period, in which Prochilodus was selectively removed from shallow pools as well as riffle habitats. A significant treatment x habitat interaction was observed whereby Prochilodus strongly influenced sediment accrual in pools, yet had no detectable effect on Sediments in riffles. These patterns were established almost immediately (i.e., within 48 h) and persisted over the duration of the 40-d experiment. A 2nd experiment was subsequently performed to evaluate whether other epibenthic-feeding fishes compensated for the loss of Prochilodus in shallow riffle habitats. When the remainder of the fish assemblage was excluded from riffles, highly significant increases in sediment accrual were observed, similar to the effects of selective removal of Prochilodus in pools. These results illustrate that benthic fishes are important sediment processors across habitats in Andean piedmont streams; however, the cast of major players is variable. Prochilodus is a strong interactor in pools, yet is a weak interactor in riffles where other species compensate for the functional loss of this species.