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Seed dispersal for the unusual inflated berries of Burmeistera (Campanulaceae)

Abstract:

Examining dispersal is critical for understanding the diversity of Andean-centered plant lineages, like Burmeistera (Campanulaceae). One-third of its species present an unusual inflated berry. Unlike the bright colors of non-inflated fruits in the genus, these fruits are typically dull-green; however, the fact that the seeds are loosely held in the placenta and easily removed when touched seems to suggest adaptation to animal dispersal. We studied two inflated-baccate species, Burmeistera glabrata and B. borjensis, with the aim of testing the non-exclusive hypotheses that their seeds are dispersed by (1) small mammals, (2) slugs, or (3) adult flies that develop inside the fruits. In two sites in the Ecuadorian Andes, we performed observations at dusk and dawn to examine the fate of fruits and seeds; recording fruit fall, formation of holes in the fruits, and seed loss from the placenta. We documented fruit visitors with cameras, and surveyed unopened fruits for the presence of insect larvae and seed condition. Finally, we performed an experiment to examine the effect of holes and rain in germination, in order to evaluate if holes are required for seeds to leave the fruits and subsequently germinate. For both species, most fruits fell and decomposed beneath the mother plant. However, we found limited support for small mammal dispersal; videos and observations revealed that mice and squirrels are potential, but rare, seed dispersers. We found no evidence for slug or fly dispersal; fly larvae were common inside fruits, but acted exclusively as seed predators. Crickets often chewed holes in fruits on plants and on the ground. Holes did not have an effect on germination, which was induced only by rain. Hence, the majority of seeds end up under the mother plant, with rare but potentially important events of primary or secondary dispersal by small mammals. The combination of limited dispersal due to gravity and rare events of mammal dispersal may have played a critical role in the rapid diversification of Burmeistera.