Cinaruco River Food Web

The Cinaruco River, located in the Venezuelan “llanos”, is a floodplain system characterized by very high species diversity (e.g. >280 species of fish). The food web is extremely complex, rendering obsolete any simple approaches to elucidating food web structure and function. Work to this point (Albrey Arrington, Perry Institute for Marine Science; Kirk Winemiller, Texas A&M University) has involved two major approaches: (1) comparative analyses based on descriptive food web characteristics, and (2) experimental manipulations within important food web modules. Methodologies include monthly sampling of fish assemblages using a variety of techniques, large-scale field experiments, and extensive stomach content and stable isotope analyses. Two themes unite results of our work to this point: substantial spatial and temporal variability in food web structure, and how body-size can be used to generalize species-interactions across this complexity. Spatial variability occurs at various scales, from among small fish assemblages on seemingly homogeneous sand banks, to differences among landscape scale units (e.g. between lagoons and main river channel). Seasonal variability is apparent in predation patterns, with relative prey availability and body size driving a pattern of decreasing prey sizes with falling water levels. Body size is also related to functional outcomes of species interactions, for example, a size-based response of prey fishes to large-bodied piscivore exclusion. This pattern has been further substantiated at the landscape-scale, as differences in assemblage structure among netted and un-netted lagoons are largely size-based. Trophic position of fish and body size have not found to be related, likely due to the diversity of prey available to consumers.

Much work is currently on-going. For example, we are continuing an evaluation of the effects of commercial netters on food web structure. Netters only target some floodplain lagoons, providing a large-scale natural comparison in which we can directly document netter impacts. We are also attempting to scale-up results from mechanism-based experiments to the landscape level. For example, I have demonstrated that peacock bass respond behaviorally to presence of piranha by moving into shallower habitats. This may create a synergistic effect in which piranha make peacock bass more susceptible to the commercial netters. Other large-scale manipulations are planned, including removing the migratory detritivore, Semaprochilodus kneri, from entire floodplain lagoons. We also are continuing to examine the underlying reasons why this river supports an extremely “compressed” food web, i.e. all predators seem to be feeding at the lowest trophic level possible. Due to this property, effects of commercial netters on the web may be counter-intuitive. Unlike other systems in which food webs are “fished down” with correlated reductions in fish size and mean system food chain length, food chain length may increase, on average, following removal of large-bodied fishes in the Cinaruco River.

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