Five decades of overfishing could disrupt an ancient mutualism between frugivorous fish and plants in Neotropical wetlands

Background/Question/Methods

Globally, overfishing can lead to changes in the structure of overharvested populations by targeting and increasing mortality of the largest individuals, while favoring the survival of individuals under a capture size threshold that reproduce earlier.  For nearly five decades, Neotropical fruit-eating fish have been intensively harvested.  These fishing activities have reduced population sizes of some species by up to 90% and have likely induced changes in population structure.  In Neotropical wetlands, the fruiting phenology of many tree and liana species is synchronized with the annual flood.  Numerous fish species feed on fruit during the flooded season and in the process swallow viable seeds which are later deposited in suitable floodplain habitats. Here we aim to evaluate potential ecological consequences of overfishing for wetland seed dispersal and forest recruitment dynamics.  Using individual-level data from Amazonian and Pantanal wetlands, we tested the hypothesis that seed dispersal effectiveness increases with fish size within and across fish species.  We analyzed dietary data of 659 individuals of 7 species of frugivorous fishes and conducted germination trials with 1660 seeds of 16 tree and liana species.  We modeled the influence of fish size on the probability of seed dispersal, abundance, diversity and size of intact seeds, and germination success of dispersed seeds. 

Results/Conclusions

Our studies reveal that seed dispersal effectiveness increases with fish size within and across fish species.  Between 25 to 75% of fish species per site displayed the expected pattern of increased probability of dispersing intact seeds with bigger body size.  The abundance, diversity and size of dispersed seeds increased with fish size in both wetlands.  For most seed species, passage through a fish digestive tract neither promoted nor inhibited germination, although for some seed species, seed dispersal by larger fish increased the probability of germination.  For the last 70 million years, frugivorous fishes have contributed important seed dispersal services in Neotropical wetlands.  Our results suggest that the ongoing overexploitation of multiple frugivorous fish species will depress the quantity and diversity of seeds dispersed, as well as the quality of seed dispersal in wetland habitats that extend over 15% of the area of South America.