Effects of omnivorous shrimp in a montane tropical stream: Sediment removal, disturbance of sessile invertebrates and enhancement of understory algal biomass.
Abstract: Freshwater shrimp dominate the faunal biomass of many headwater
tropical streams: however, their role in community organization is
unclear. Enclosure/exclosure experiments in a montane Puerto Rican
stream examined direct and indirect effects of two dominant taxa of
atyid (Atyidae) shrimp, Atya lanipes Holthuis and Xiphocaris elongata
Guerin-Meneville. Both shrimp taxa caused significant reductions in
sediment cover on rock substrata, reducing sedimentation and enhancing
algal biovolume on clay tiles in cages. When tiles incubated in shrimp
exclosures for 2 wks were placed outside of cages, atyid shrimp removed
100 % of the sediment cover within a 30 min observation period. Atyid
shrimp appear to play an important role in stream recovery after high
discharge events by rapidly removing sediments and detritus deposited
on benthic substrata in pools. We evaluated the mechanism by which A.
lanipes influences algae and benthic insects by comparing patterns of
algal biomass, taxonomic composition, and insect abundance between
shrimp-exclusion and shrimp-presence treatments both with and without
manual sediment removal. The shrimp exclusion treatment without manual
sediment removal had significantly lower algal biomass and greater
sedimentation than all other treatments. The treatment in which shrimp
were excluded but sediment was manually removed, however, accrued
almost the same algal biovolume as the shrimp enclosure treatment,
supporting the hypothesis that sediment removal enhances the biovolume
of understory algal taxa. Algal community composition was similar
between stream bottom bedrock exposed to natural densities of shrimp
and all experimental treatments for both Atya and Xiphocaris: a diatom
community strongly dominated (78-95%) by the adnate taxon, Achnanthes
lanceolata Breb ex. Kutz. Atyid shrimp are important in determining the
distribution and abundance of benthic insects through both direct and
indirect effects. Sessile, retreat-building chironomid larvae
(Chironomidae: Diptera) are negatively affected by both A. lanipes and
X. elongata, through direct removal by foraging activities and/or
indirectly through depression of sediment resources available to larvae
for the construction of retreats. In constrast, the mobile grazer,
Cloeodes maculipes (Baetidae: Ephemeroptera) was not adversely affected
and atyid shrimp have the potential to exert positive indirect effects
on this taxon by facilitating its exploitation of algal resources
and/or through enhancement of understory algal food resources through
sediment removal.
Pringle, C. M., G. A. Blake, A. P. Covich, K. M. Buzby and A. Finley. 1993. Effects of omnivorous shrimp in a montane tropical stream: Sediment removal, disturbance of sessile invertebrates and enhancement of understory algal biomass. Oecologia 93: 1-11.
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