Effects of vertebrates on plant functional composition in a neotropical forest

Background/Question/Methods

Defaunation alters interactions between plants and their vertebrate consumers and is often associated with decreased plant diversity.  Examining how plant functional trait composition changes with defaunation can help identify the species interactions and mechanisms underlying such diversity loss, and the potential cascading effects of defaunation.  Here we focus on one pantropical driver of defaunation:  hunting.  We tested three hypotheses: (1) Disrupted seed dispersal favors plant species with unhunted dispersal agents and lianas, which are abiotically-dispersed. (2) Reduced seed predation favors larger-seeded species. (3) Reduced herbivory favors species with lower leaf toughness and wood density, and higher leaf nitrogen.  We censused 12,080 seedlings and 35,069 saplings in a terrestrial vertebrate exclosure experiment in Central Panama to examine how vertebrates shape relative abundances of species classified by dispersal mode, growth form, seed mass, leaf toughness, leaf nitrogen and wood density.  We then compared exclosure results with data from a comparison of 38,250 seedlings in nearby hunted and unhunted sites. 

Results/Conclusions

The exclosures exclude terrestrial seed dispersers, seed predators and herbivores but not climbing, arboreal or volant vertebrates.  Hunting reduces large birds, primates, and some climbing species, in addition to the species reduced by the exclosures.  Hunting, but not exclosures, led to changes in the relative abundance of growth forms and dispersal mode classes, driven by lost dispersers. Higher median seed mass was observed in exclosures, but not hunted sites.  This suggests that while lost seed predators may have benefits for large seeded species, this is offset by the negative effects of lost seed dispersers where hunting occurs.  Excluding vertebrate herbivores resulted in lower mean leaf toughness and higher mean leaf nitrogen in the sapling community.  Unexpectedly, community median wood density was significantly lower in hunted sites, but not in exclosures.  Species dispersed by mammals and large birds had significantly higher mean wood density than wind-dispersed species.  Taken together, these results suggest that seed dispersal, not herbivory, may mediate decreases in wood density in hunted forests.  Our results suggest that decreased dispersal, seed predation, and herbivory are important drivers of plant compositional change resulting from defaunation.