Positive density-dependence of floral resources on plant-hummingbird interactions

Bryan G. Rojas, Pedro J. Bergamo, Catherine H. Graham & Isabela Galarda Varassin

How biodiversity is maintained is a question that many scientists are still trying to address. Positive and negative density dependence effects are known to exert different mechanisms that stabilize and maintain diversity in communities. Recent research in plant communities, points to, not just competitive, but facilitative interactions as coexistence process drivers. In pollination networks, conspecific and heterospecific flower abundance can have either facilitative or competitive effects on plant fitness. Furthermore, nectar sugar has also shown positive effects on plant fitness and in the attraction of hummingbird pollinators.

Hummingbirds play an important role as effective pollinators in the Americas thus, testing the effect of conspecific and heterospecific flower and nectar-resource quantity on plant-hummingbird communities, is necessary to have a better understanding of how these communities are perpetuated. We explored the effect of flower density and fine-grained resource availability (nectar sugar content), to test dense-dependent responses on hummingbird visitation in a plant-hummingbird network in the Atlantic forest. To test this, we used GPS plant data to establish plots and quantify conspecific and heterospecific density. We used sugar quantity of nectar and hummingbird visitation rate data, from a southeastern Atlantic forest, Espírito Santo. We performed a GLMM, using Poisson error structure, to quantify the effect of flower abundance and nectar sugar content on hummingbird visitation rate.

Our results suggests that flower density and nectar sugar quantity explain the same variation in terms of hummingbird visitation. We found a positive relation between conspecific flower density and visitation rate, whereas no relation was shown for heterospecific density. At this plot scale, conspecific flower aggregation results to be a stable strategy for plant survival, perhaps because it diminishes heterospecific pollen deposition ensuring fecundation. However, broader scale studies are still needed to better understand the role of these two different components of flower density.