Reproductive Synchrony of a Tropical Shrub: Experimental Studies on Effects of Pollinators and Seed Predators in Hybanthus Prunifolius (Violaceae)

Abstract

This study compares reproductive success of individual plants flowering and fruiting in an out of synchrony with the population. I test the hypotheses that reproductive synchrony enhances a plant's ability to: (1) attract pollinators, and (2) avoid seed predators. The prediction is that an individual in synchrony with its population has higher fruit and ovule set after pollination and has more seeds at dispersal time than an individual out of synchrony. The test species is Hybanthus prunifolius (Schult.) Schulze (Violaceae), a shrub on Barro Colorado Island, Panama. It normally flowers in response to a heavy rain that interrupts a drought in the dry season. An individual produces most of its flowers on a single day. The flowers are self—compatible, but require animal vectors to effect any pollination. To test hypotheses (1) and (2) I used water sprinklers to induce individual plants to flower prior to the normal triggering rain, thereby creating a highly asynchronous population which flowered before the natural population. Peak flowering day for individuals in the experimental asynchronous population spanned 35 d; the natural population spanned 35 d; the natural population spanned 4 d. Measurements included pollination success (ovule and fruit set) due primarily to the social bee Melipona interrupta, occupation of fruits by microlepidopteran larvae (Cosmoptericidae) and dipteran larvae (Lonchaeidae, Silba sp.), and final reproductive output (number of mature fruits and seeds). Individuals in all size categories in the natural synchronous population matured a greater number of seeds than individuals in the asynchronous population. Mean number of mature seeds per individual was 658 in the synchronous population; it was 62 mature seeds per individual in the asynchronous population. The large difference in such output occurred primarily because individuals in the synchronous population had greater pollination success than individuals in the asynchronous population (86% vs. 58% fruit set; 78% vs. 40% ovule set). In contrast, fruit infestation by microlepidopteran larvae was greater among individuals in the asynchronous population than in the synchronous population (11% vs. 5%). The combined effects of pollinators and seed predators were thus additive and produced intense selection against temporally isolated individuals. Similarly low seed output was occasionally observed for the few nonexperimental plants that flowered in response to a light rain during the dry season. Such individuals had lower levels of pollination and mature fruit production than plants that flowered when all others in the entire forest were in flower. The effect of space was similar to that of time. During the natural synchronous flowering period, individuals in sites of low spatial attracted fewer pollinators and incurred more predation than individuals in sites with high density. An evolutionary interpretation of these results is that stabilizing by both pollinators and seed predators maintains the present low variance in the timing of flowering and fruiting within Hybanthus prunifolius populations. Such effects may have been caused in the past in forging the origin of reproductive synchrony in plant populations.