Pollination in the Rainforest: Scarce Visitors and Low Effective Pollinators Limit the Fruiting Success of Tropical Orchids

SIMPLE SUMMARY: The ability to distinguish between mere flower visitors and effective pollinators is of crucial importance to understand pollination ecology. Inefficient and scarce pollinators, coupled with unsuitable abiotic conditions, might explain a failure in plant reproductive success. Here, we quantified the levels of fruit set and tested the dependency on pollinators in 10 orchid species occurring in the tropical rainforest. We show that all species were pollinator-dependent with pollination limitation occurring in all the 20 studied populations. In fact, in 87% of our observation period no visitor was seen on the flowers studied. Floral visitors included flies, butterflies, bees, and hummingbirds although only a small number were performing effective pollinations. We conclude that although these orchid species were visited by different groups of visitors, few could be considered as legitimate pollinators, explaining why these tropical orchids present low values of fruit set. ABSTRACT: A single plant might be visited by many flower visitors but not all might act as pollinators. Legitimate pollinators might also differ considerably in their efficiency, limiting pollination success. Unsuitable climatic conditions such as rain also affect pollinator activity. However, in the evergreen rainforest there is no prolonged dry season and flowering occurs usually under rain. Here, we explore the dependence on pollinators and the efficiency of flower visitors for the fruiting success of 10 Andean rainforest orchids. All species were self-compatible but strictly pollinator-dependent. Overall, we found low levels of fruit set in control flowers while experimental geitonogamous and cross-pollinations increased fruit set, revealing extensive pollination limitation in all populations. Seed viability dropped considerably after self and geitonogamous pollinations suggesting the possibility of early-acting inbreeding depression. Even though we monitored flower visitors on an extensive survey, few visitors were seen in these species and even fewer acted as legitimate pollinators. Thus, even though orchid pollination might be extremely diversified, these results show that few visitors are pollinating these species, explaining the low levels of fruit set recorded in the area studied.