Trait-mediated indirect effects of waterbirds on piscivorous fishes in experimental tropical floodplain food webs

Background/Question/Methods

Piscivorous fishes are often considered top predators in freshwater ecosystems, but piscivorous waterbirds frequently play a key role in aquatic ecosystems and often occur near or at the top of the food web. Piscivorous waterbirds may act in two ways in aquatic food webs: direct consumption and trait-mediated indirect effects (TMIE) such as behavioral changes and anti-predator strategies of prey populations. For fish populations, previously demonstrated TMIE of waterbirds include changes in foraging behavior, habitat selection and activity. Examples of TMIE of predators in food webs are frequent in the literature, but there is some debate on their nature and relative importance in relation to direct consumptive effects or in more complex food webs. The purpose of this study was to test for magnitudes of TMIE of piscivorous waterbirds on consumption rates of pairs of piscivorous fishes (one native and one non-native) with similar or divergent ecomorphologies. Data from a long-term ecological research program were used to establish realistic stocking densities of four prey species in the presence of the piscivorous fishes and waterbirds. These experiments allow us to quantify the TMIE of waterbirds on predation rates of separate piscivorous fishes with different degrees of ecomorphological overlap.

Results/Conclusions

The frequency of waterbirds visiting experimental ponds was correlated with fish density. We identified five species of piscivorous waterbirds, with Vanellus chilensis and Ardea cocoi the most abundant. Non-native piscivores had significantly higher consumption rates than native predators, independent of the presence of waterbirds. TMIE of waterbirds included significantly reduced prey consumption and change in prey preference by the native piscivores, For example, Hoplias aff. malabaricus consumed Aphyocharax sp., Gymnotus sp. and Astyanax altiparane in the absence of waterbirds, but preferentially consumed Gymnotus sp. and reduced consumption of A. altiparanae and Monkhausia forestii in the presence of waterbirds. For both non-native piscivores (Hoplerithrynus unitaneatus (Characiformes) and Astronotus crassipinnis (Perciformes)), prey-specific consumption rates were primarily determined by prey species relative abundances. Therefore, the functional identity (zoogeographical origin) of the fish piscivores appeared to mediate TMIE effects of waterbirds, with significant consequences for prey assemblages.