Incidence of extra-floral nectaries and their relationship to growth and survival of lowland tropical rain forest trees
Mutualistic relationships between organisms represent a fascinating eco-evolutionary phenomenon and play an important role in many ecological systems. Tropical forests, which are filled with mutualistic interactions, represent excellent study systems in which to detect the relationship of mutualism to the performance of those organisms involved. A common mutualism in the tropics exists between ants and plants, where ants will feed from nectar provided through structures called extra-floral nectaries (EFN). While ants receive a free meal, those plants that have EFN receive a fitness benefit from reduced herbivory, as ants will act aggressively toward intruders. Despite these observations, little is understood about effects on the long-term growth and survival of those plants that exhibit EFN. To elucidate the ecological significance of this mutualism, we surveyed 928 species of tree in an Ecuadorian lowland tropical rainforest for the presence of EFN. Following this, we used long-term census data to compare tree abundance and performance as a function of EFN presence. Lastly, we used previous EFN surveys from Panama and Malaysia to compare demographic associations across tropical forests. Our question is: are there demographic patterns associated with EFN presence in plants, and are these consistent across tropical forests of disparate biogeographic histories?
There were multiple significant differences in demographic rates between species with and without EFN at all sites. Species with EFN were 12-60% more abundant than those without in Ecuador and Malaysia, while they were almost 300% less abundant in Panama, which represents more seasonal forest. For growth and mortality, species with EFN had both higher growth rates (14.2%) and higher mortality rates (24.5%) than those without EFN across all sites. These effects were dampened after controlling for phylogeny, but still persistent. Lastly, our survey found EFN on 64 species not previously known to have them, which includes 18 new genera and 2 new families. Taken together, these findings suggest that species with EFN grow faster and die younger than those without, exhibiting a significant life-history association with this trait. Furthermore, this association is consistent across all three locations, suggesting that their functional role may be ubiquitous. It may be that EFN-mediated mutualism is a highly successful strategy in pioneer species, for which this life-history strategy is representative, though this idea is not supported by observed trends in abundance. Regardless, these results suggest an impact of this trait on demography, indicating its significance in affecting tropical forest dynamics.