Photos from the Field: Plant-Hummingbird Networks in the Cerrado
Maruyama, P. K., Vizentin-Bugoni, J., Oliveira, G. M., Oliveira, P. E. and Dalsgaard, B. (2014), Morphological and Spatio-Temporal Mismatches Shape a Neotropical Savanna Plant-Hummingbird Network. Biotropica, 46: 740–747. doi: 10.1111/btp.12170.
Complex networks of species interactions might be determined by species traits but also by simple chance meetings governed by species abundances. Although the idea that species traits structure mutualistic networks is appealing, most studies have found abundance to be a major structuring mechanism underlying interaction frequencies. With a well-resolved plant–hummingbird interaction network from the Neotropical savanna in Brazil, we asked whether species morphology, phenology, nectar availability and habitat occupancy and/or abundance best predicted the frequency of interactions. For this, we constructed interaction probability matrices and compared them to the observed plant-hummingbird matrix through a likelihood approach. Furthermore, a recently proposed modularity algorithm for weighted bipartite networks was employed to evaluate whether these factors also scale-up to the formation of modules in the network. Interaction frequencies were best predicted by species morphology, phenology and habitat occupancy, while species abundances and nectar availability performed poorly. The plant–hummingbird network was modular, and modules were associated to morphological specialization and habitat occupancy. Our findings highlight the importance of traits as determinants of interaction frequencies and network structure, corroborating the results of a previous study on a plant–hummingbird network from the Brazilian Atlantic Forest. Thus, we propose that traits matter more in tropical plant–hummingbird networks than in less specialized systems. To test the generality of this hypothesis, future research could employ geographic or taxonomic cross-system comparisons contrasting networks with known differences in level of specialization.
Arguably, hummingbirds are the most charismatic group of pollinators, having unique adaptations to feed on nectar provided by flowers. There are many text book examples of tight co-adaptation between flowers and hummingbirds, such as the association between the Sword-billed hummingbird (Ensifera ensifera) and some extremely long-tubed Andean flowers, or the striking sexual dimorphism found in the Lesser Antillean Purple-throated Carib (Eulampis jugularis), which is associated with the energetic production and floral phenotype of two species of Heliconia flowers. As such, hummingbirds are often regarded as very specialized pollinators and many studies have been conducted on their interaction with flowers, especially from the rainforests in the Neotropics. Recently, we have found that plant-hummingbird interactions in an Atlantic Rainforest community are better predicted by species traits than the abundance of the species, a finding much in contrast to what generally is found in the literature on ecological networks (Vizentin-Bugoni et al. 2014; Proc. R. Soc. B 281:20132397). Nevertheless, these findings are consistent with the long-standing natural history knowledge that plant-hummingbird interactions are indeed specialized.
In this study published in Biotropica, we investigate what factors structure the network of species interactions in a plant-hummingbird community from the Brazilian Cerrado, an ecosystem found in the middle portion of South America (Photo 1 and 2). In contrast to neighbouring rainforest ecosystems, such as the Amazon and the Atlantic Rainforest, hummingbirds from Cerrado seem to be rather generalized in their interaction with flowers, relying much on plants showing little phenotypical adaption toward bird-pollinationMaruyama et al. 2013, Naturwissenschaften 100: 1061-1068). Cerrado is also characterized by the patchy distribution of distinct habitats such as gallery forest (Photo 3) interspersed in between the typical savanna (Photo 4), which makes it a good model system to test whether species habitat preferences in addition to species morphology, phenology and abundance determine patterns of species interactions.
We found that species interactions were best predicted by matches in species morphology, phenology and habitat occupancy, while species abundances and nectar availability were poor predictors. Moreover, the plant-hummingbird network from the Cerrado was modular (organized in sub-units), and modules were associated to morphological specialization and habitat occupancy. In open typical savanna habitat, interactions such as the one between the short-billed Swallow-tailed hummingbird (Eupetomena macroura) and short corolla plants as Palicourea rigida predominate (Photo 5). In forest habitats the morphologically more specialized Planalto hermit (Phaethornis pretrei) interacts with a subset of more specialized “ornithophilous” plants.
In this sense, our study highlight the importance of traits as determinant of species interaction frequencies and network structure, which leads us to propose that traits matter more in tropical plant-hummingbird networks than in less specialized systems. We hope that this study will facilitate future studies to test the generality of this hypothesis, by contrasting ecological networks with differences in the level of specialization, e.g. by employing cross-system comparisons, both geographically or taxonomically.
P. K. Maruyama