omnivores

the role of omnivory in a neotropical stream: separating diurnal and nocturnal effects

The Role of Omnivory in a Neotropical Stream: Separating Diurnal and Nocturnal Effects
Catherine M. Pringle and Toshihide Hamazaki
Ecology
Vol. 79, No. 1 (Jan., 1998), pp. 269-280

Abstract: 
The role of omnivory in structuring communities is potentially great in lowland neotropical streams that are characterized by an abundance of macroconsumers that consume both insects and algae. Here, we separate effects of natural densities of diurnal fishes and nocturnal shrimps in structuring the benthic community of a stream draining Costa Rica's Atlantic slope. We experimentally manipulated the spatial and temporal access of fishes and shrimps to benthic resources, in situ, using electric "fences" powered by solar-powered fence chargers. Both fishes and shrimps significantly reduced inorganic sediment mass, organic ashfree dry mass (AFDM), densities of larval Chironomidae, and total insects: their combined effects were greater than effects of either group alone, and there was no significant interaction. Fishes shifted algal community composition from diatoms to green and blue-green algae and benthic insect communities towards chironomids, while shrimps had no significant effect on community composition. Effects of fishes were generally greater than those of shrimps, and this is due, in part, to higher natural densities and foraging pressures of fishes. Furthermore, shrimps foraged for significantly longer periods of time in the treatment where fishes were excluded than in the combined fish and shrimp access treatment, suggesting that diurnally feeding fishes are strong "interactors," mediating resource availability to nocturnally feeding shrimps. Natural erosion and sediment-mediated effects of macroconsumers (both direct and indirect) also affected algal communities: a manual sediment removal experiment resulted in significant reductions of diatom biovolume and increases in the filament length of green and blue-green algae. Our results show the importance of: (1) assessing macroconsumer effects in a relatively natural depositional environment subject to background erosion and sloughing (i.e., in this case by using electric exclosures); (2) evaluating effects of natural densities of both diurnal and nocturnal macroconsumers through time in the context of these abiotic effects; and (3) distinguishing between the response of different types of algal resources (e.g., diatoms vs. green and blue-green algae), which are differentially affected by sedimentation and erosion. Cage experiments, short-term observations, or one-time sampling of undifferentiated "algae" may artificially overestimate trophic effects and underestimate abiotic effects. We found no evidence of a trophic cascade. Our findings are in agreement with the theoretical prediction that large-sized omnivores have strong direct trophic (feeding) effects, both on smaller primary consumers (insects) and basal resources (algae).

Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico

Covich, Alan P.; Crowl, Todd A.; Heartsill-Scalley, Tamara 2006. Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico.. J. N. Am. Benthol. Soc., 25(1):99-107.

Abstract: 
Extreme events (hurricanes, floods, and droughts) can influence upstream migration of macroinvertebrates and wash out benthic communities, thereby locally altering food webs and species interactions. We sampled palaemonid river shrimp (Macrobrachium spp.), dominant consumers in headwaters of the Luquillo Mountains of northeastern Puerto Rico, to determine their distributions along an elevational gradient (274–456 m asl) during a series of disturbances (Hurricane Hugo in 1989, a drought in 1994, and Hurricane Georges in 1998) that occurred over a 15-y period (19882002).We measured shrimp abundance 3 to 6 times/y in Quebrada Prieta in the Espiritu Santo drainage as part of the Luquillo Long-Term Ecological Research Program. In general, Macrobrachium abundance declined with elevation during most years. The lowest mean abundance of Macrobrachium occurred during the 1994 drought, the driest year in 28 y of record in the Espiritu Santo drainage. Macrobrachium increased in abundance for 6 y following the 1994 drought. In contrast, hurricanes and storm flows had relatively little effect on Macrobrachium abundance.
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