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. My dissertation research involved two primary approaches: (1) comparative analyses based on descriptive food web characteristics, and (2) experimental manipulations within important food web modules. Methodologies included monthly sampling of fish assemblages using a variety of techniques, large-scale field experiments, and extensive stomach content and stable isotope analyses. Two themes framed results of our research: substantial spatial and temporal variability in food web structure, and how body-size can be used to generalize species-interactions. 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. We also collected data suggesting 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 pattern, effects of commercial netters on the web may be counter-intuitive. Unlike other systems in which food webs are “fished down” (i.e., correlated reductions in fish size and mean food chain length), food chain length may increase, on average, following removal of large-bodied fishes in tropical floodplain rivers.