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
Vol. 79, No. 1 (Jan., 1998), pp. 269-280

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).

Experimental Removal of Insectivores from Rain Forest Canopy: Direct and Indirect Effects

Dial, Roman, and Jonathan Roughgarden. 1995. Experimental Removal of Insectivores from Rain Forest Canopy: Direct and Indirect Effects. Ecology 76:1821–1834

This study considered the effects of insectivorous Anolis lizards on a large, complex food web of arthropods and associated herbivory in a tropical rain forest canopy. We excluded Anolis lizards for 6 mo from 20—30 m high tree crowns in Puerto Rican rain forest. Simultaneous with lizard exclusion, we sampled orb spiders, airborne arthropods, and leaf arthropods in lizard removal crowns and in controls. We also sampled herbivory at the end of the experiment. Lizard removal had strong, statistically significant, positive effects on arthropods >2 mm in length and weak negative effects on arthropods <2 mm. Parameters of arthropod body size distributions differed between removals and controls for leaf arthropods, but not for airborne arthropods. Among arthropod taxa >2 mm, both predatory, i.e., orb spiders and parasitic Hymenoptera, and nonpredatory forms, i.e. Diptera, Coleoptera, Orthoptera, and Blattaria, showed strong significant and positive responses to lizard removal. Large Psocoptera, Homoptera, leaf spiders, and ants did not show significant overall responses to lizard removal. Frequency of herbivore damage on new leaves was positively correlated with abundance of Orthoptera and Blattaria. This damage was significantly greater in lizard removal crowns than in controls, indicating an indirect effect of anoles on plants. The indirect effect of lizards on small arthropods through the predatory anthropod pathway appeared weak. Results of lizard removal shown by this study corroborate other lizard removal studies from more xeric, ground—level habitats with simpler food webs in the West Indies, particularly with respect to orb spiders and herbivory. Taken together with the results of similar experiments performed in trophically less complex systems, this experiment suggests that food web size is less important than body size in determining interaction strength between community members.
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