Prochilodus mariae (Characiformes: Prochilodontidae) is a detritivorou
s fish distributed throughout the Orinoco river basin of South America
. Spectacular migrations of these fishes occur at the end of the rainy
season into the Andean foothills. Prochilodus ingest large quantities
of sediments and may thereby modify habitats in neotropical streams.
The major objectives of this study were (1) to explore experimentally
the importance of Prochilodus in structuring a tropical stream in the
Venezuelan Andean piedmont, and (2) to determine whether there was suf
ficient ecological redundancy in a diverse and abundant assemblage of
epibenthic fishes to compensate for the removal of Prochilodus. Commun
ity structure was compared among three experimental treatments: (1) Pr
ochilodus exclusion, (2) Prochilodus enclosure, and (3) the natural fi
sh assemblage. Selective exclusion of Prochilodus resulted in striking
changes in community structure as measured by patterns of sediment ac
crual and the composition of algal and invertebrate assemblages. Highl
y significant increases in total dry mass and in ash-free dry mass of
sediments accruing on stream-bottom substrates were observed almost im
mediately following the exclusion of Prochilodus. Moreover the composi
tion of algal and invertebrate assemblages was significantly modified
by Prochilodus. Taxa such as diatoms were reduced in number when Proch
ilodus was present; in contrast, Prochilodus appeared to facilitate ni
trogen-fixing cyanobacteria. Total invertebrate densities were greates
t in the Prochilodus removal treatment; however, a variety of response
s to the experimental treatments was observed among different taxa ana
lyzed individually, including density reductions, increases, and no me
asurable effects. This study suggests that the detritivore Prochilodus
is a functionally dominant species in Andean foothill streams via sed
iment-processing activities. Moreover, it provides little evidence to
support the notion that strongly interacting species are limited to si
mple systems with few food web components.