Linking species and ecosystems: Different
biotic assemblages cause interstream differences in organic matter.
ABSTRACT: Here we test the
hypothesis that differences in macrobiotic assemblages can lead to differences
in the quantity and quality of organic matter in benthic depositional environments
among streams in montane Puerto Rico. We experimentally manipulated biota
over a 30-40 d period in two streams with distinctly different macrobiotic
assemblages: one characterized by high densities of omnivorous shrimps
(Decapoda: Atyidae and Xiphocarididae) and no predaceous fishes, and one
characterized by low densities of shrimps and the presence of predaceous
fishes. To incorporate the natural hydrologic regime and to avoid confounding
artifacts associated with cage enclosures/ exclosures (e.g., high sedimentation),
we used electricity as a mechanism for experimental exclusion, in situ.
In each stream, shrimps and/or fishes were excluded from specific areas
of rock substrata in four pools using electric ëfencesí attached to solar-powered
fence chargers.
In the Sonadora study stream, the unelectrified control treatment was almost exclusively dominated by high densities of omnivorous shrimps which constantly ingested fine particulate material from rock surfaces. Consequently, the control had significantly lower levels of inorganic sediments, organic material, carbon, and nitrogen than the exclusion treatment, as well as less variability in these parameters. Ten-fold more organic material and five-fold more nitrogen accrued in shrimp exclosures (10.6 g AFDM/m2; 0.2 g N/m2) than in controls (1.1 g AFDM/m2; 0.04 g N/m2). By reducing the quantity of fine particulate organic material and associated nitrogen in benthic environments, omnivorous shrimps potentially affect the supply of this important resource to other trophic levels. The small amount of FPOM that remained in control treatments (comprised of sparse algal cells) was of higher quality than that in shrimp exclosures. This is evidenced by the significantly lower carbon to nitrogen (C/N) ratio (an indicator of food quality - with relatively low C/N indicating higher food quality) in the control relative to the shrimp exclosure treatment.
In contrast, the Bisley study stream was characterized by very low numbers of shrimps, and macrobiota had no significant effect on benthic sediments, organic matter, C, N, and C/N. All parameters were highly variable through time, with levels and ranges in variability similar to the shrimp exclusion treatment in the Sonadora.
Our experimental results are consistent with findings of an independent survey of six streams in four different drainages. Four streams that had an abundance of omnivorous shrimps, but lacked predaceous fishes, had extremely low levels of fine benthic organic and inorganic material. In contrast, two streams that had low densities of shrimps and contained predaceous fishes had significantly higher levels. Results show a strong linkage between species and ecosystem characteristics in montane streams of Puerto Rico: interstream differences in the quantity and quality of fine benthic organic matter resources were determined by the nature of the macrobiotic assemblage. Furthermore, patterns in the distribution of shrimp assemblages reflected landscape patterns in the benthic depositional environment among streams.
In the Sonadora study stream, the unelectrified control treatment was almost exclusively dominated by high densities of omnivorous shrimps which constantly ingested fine particulate material from rock surfaces. Consequently, the control had significantly lower levels of inorganic sediments, organic material, carbon, and nitrogen than the exclusion treatment, as well as less variability in these parameters. Ten-fold more organic material and five-fold more nitrogen accrued in shrimp exclosures (10.6 g AFDM/m2; 0.2 g N/m2) than in controls (1.1 g AFDM/m2; 0.04 g N/m2). By reducing the quantity of fine particulate organic material and associated nitrogen in benthic environments, omnivorous shrimps potentially affect the supply of this important resource to other trophic levels. The small amount of FPOM that remained in control treatments (comprised of sparse algal cells) was of higher quality than that in shrimp exclosures. This is evidenced by the significantly lower carbon to nitrogen (C/N) ratio (an indicator of food quality - with relatively low C/N indicating higher food quality) in the control relative to the shrimp exclosure treatment.
In contrast, the Bisley study stream was characterized by very low numbers of shrimps, and macrobiota had no significant effect on benthic sediments, organic matter, C, N, and C/N. All parameters were highly variable through time, with levels and ranges in variability similar to the shrimp exclusion treatment in the Sonadora.
Our experimental results are consistent with findings of an independent survey of six streams in four different drainages. Four streams that had an abundance of omnivorous shrimps, but lacked predaceous fishes, had extremely low levels of fine benthic organic and inorganic material. In contrast, two streams that had low densities of shrimps and contained predaceous fishes had significantly higher levels. Results show a strong linkage between species and ecosystem characteristics in montane streams of Puerto Rico: interstream differences in the quantity and quality of fine benthic organic matter resources were determined by the nature of the macrobiotic assemblage. Furthermore, patterns in the distribution of shrimp assemblages reflected landscape patterns in the benthic depositional environment among streams.
Pringle, C. M., N. H. Hemphill, W. McDowell, A. Bednarek, and J. March. 1999. Linking species and ecosystems: Different biotic assemblages cause interstream differences in organic matter. Ecology 80: 1860-1872.
Back to Literature Page
| Pringle's Home Page | University of Georgia | Odum School of Ecology |