História natural da erva-de-passarinho Psittacanthus robustus (Loranthaceae) em uma área de campo rupestre do sudeste brasileiro : interações com hospedeiras, dispersores, polinizadores e insetos herbívoros / Natural history of the mistletoe Psittacanthus robustus (Loranthaceae) at a rupestrian filed site in southeastern Brazil : interactions with hosts, seed dispersers, pollinators and insect herbivores

Tadeu José de Abreu Guerra

In this study I investigated some aspects of the natural history of the woody mistletoe Psittacanthus robustus Mart. (Loranthaceae) in rocky outcrops in the Serra do Cipó, southeastern Brazil. This species was recorded parasitizing eight host species in Five families, although trees within Vochysiaceae comprise the main hosts. Pattern of distribution was clumped within host trees but also at landscape scale. Prevalence increased with height for the four hosts species evaluated. Dissemination occurs exclusively through seed dispersal and the fruiting period extends from April to September (dry season). The fruits are ornithochorous and the main seed disperser was the plain-crested Elaenia, Elaenia cristata (Tyrannidae), which regurgitated and deposited seeds on perch during bill wiping. This bird perched preferentially on already infected hosts and experimental removal of mistletoes decreased significantly seed deposition on four host species. These results indicate that aggregated distribution on host trees is related to behavior of seed dispersers, which promoted non-random seed deposition on already parasitized trees. In addition, abundance of E. cristata increased with availability of fruiting mistletoes on rocky outcrops, promoting a positive feedback in seed dispersal and deposition. This seems to be the main processes leading to a highly clumped distribution of woody mistletoe at landscape scale. Taller hosts have higher chances to receive seeds, what indicates that increased prevalence on taller trees is related to perch selection by seed dispersers. Psittacanthus robustus bloomed from November to March (rainy season). Flowers presented characteristics typical from ornithophilous plants, including diurnal anthesis, yellow corolla, and secretion of abundant diluted nectar, which is reabsorbed if not utilized by pollinators. I observed eight hummingbird species and two passerine species as flower visitors, but the hummingbirds Eupetomena macroura and Colibri serrirostris (Trochilidae) were the main pollinators. I found no differences in seed set between flowers hand pollinated with own pollen or with pollen from distinct genet. However, these treatments set significantly more seeds than flowers exposed to pollinators, indicating that reproduction is pollen limited. This plant did not produce seed through apomixis and presented a limited capacity to produce seeds through autonomous self-pollination, highlighting the role of hummingbirds on plant reproduction. Finally, I reported data on natural history of the stinkbug Eurystethus microlobatus (Pentatomidae), a phytophagous species associated to P. robustus. This stinkbug presents a gregarious behavior and maternal care. Individuals are likely to rest on roots and shoots where they camouflage within host bark of P. robustus. Moreover, this stinkbug produces honeydew that is utilized as food source by ants, which protect them against their natural enemies throughout the day. The intricate interactions involving hosts plants, frugivorous and nectarivorous birds and insect herbivores suggest that woody mistletoes have an important role in sustaining the biodiversity in rupestrian fields in the Serra do Cipó.

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