Frugivoria por aves em duas espécies de Miconia (Melastomataceae) em área de campo rupestre ferruginoso

Abstract

Seed dispersal by frugivorous is a key ecological process in tropical ecosystems. The seed dispersal effectiveness (SDE) is influenced by several factors such as the degree of frugivory, how to catch and handle the fruits and gut treatment by disperser. The passage through frugivorous digestive tract may positively influence seed germination by means of mechanical or chemical scarification and pulp removal. In Neotropical region, birds are the main seed dispersers, and Melastomataceae is one of the dominants and most diversified plant families. Miconia is the Melastome largest genus and its fleshy fruits are especially consumed by birds. The staggered fruiting seasons in some species of the genus was observed in many South America forests, and possibly results from interspecific competition for dispersers. Much work has been done on SDE in temperate and tropical forests, but few were made in open tropical ecosystems where frugivorous experts are virtually absent. In our study, we focus on Miconia ligustroides and Miconia pepericarpa ecology of seed dispersal by birds, applying SDE theoretical framework to an open tropical site and using conspicuous ornitocorics plants in the vegetation as a model. Here, we aim to answer the following questions about the studied plant species: (1) What are the fruiting season and the fruit availability for dispersers? (2) The dispersers bird species are equally effective? (3) How seed passage through birds digestive tube affects seed germination? We conducted the study in two areas (campo rupestre and campo sujo) in Serra do Rola Moça State Park that is located in the Iron Quadrangle, in Minas Gerais central-south region, southeastern Brazil. We marked and followed 15 individuals of each species to define the fruiting season and we perform focal observation on plants during this period. In the campo sujo area the visitation frequency was low and we focused the sampling effort in the campo rupestre area. We recorded bird species visitors, visit time and duration, number of individuals, consumption and number of fruits consumed and how to catch and handle the fruit. We consider the bird species that fed whole fruits and that mask the fruits in their beaks as dispersers. We graphically represented the quantitative component (number of visits a disperser makes x number of seeds removed per visit) in a seed dispersal effectiveness landscape, where the isolines represent all quantitative component combinations that produce the same SDE. To analyze the effect of the seed passage through birds digestive tract, we provided fruits for captive birds often observed in the field and we collected defecated seeds. Next, we performed three experimental treatments: seeds manually collected from fruits, seeds defecated by birds and intact fruits, and we monitored germination in 24 h intervals for 60 days to evaluate germination versus time, the average time to germinate 50 % of the seeds and the effect of treatments on the proportion of germinated seeds. Temporal segragation between M. ligustroides and M. pepericarpa fruiting peaks and their dispersers assemblages overlap offer an indication that staggered fruiting seasons in Miconia sympatric species avoids interspecific competition for dispersers, as previously suggested for forest environments. Mimus saturninus and Schistochlamys ruficapillus were the most effectives M. ligustroides dispersers, followed by Elaenia spp. and Turdus leucomelas. For M. pepericarpa they were Zonotrichia capensis and S. ruficapillus, followed by Elaenia spp. and M. saturninus. Given that the parent plant fitness strongly depends on seed removal rates by dispersers, such birds play a crucial role in these plant species reproductive ecology. The results obtained in the germination experiment showed a very significant positive effect of pulp removal provided by birds on seed germination, but there were no scarification effect, which were expected for generalized dispersal systems. Finally, we highlight that the annual differences in the composition of frugivorous assemblages, the various phases involving the dispersal and the germination experiments in the field are interesting topics for future research in this type of ecosystems.