So happy together? : Pollination and reproductive strategy of two sympatric species of Acanthus L. in Kenya

Kakamega Forest represents the last remnant of equatorial rainforest in Kenya. Hence, it is of great importance as last habitat for species specialized on these forests and their associated ecosystems and, consequently, for East Africa’s biodiversity. The forest has been divided into several fragments due to anthropogenic habitat change, which is still continuing today. Even though large parts of the forest have been designated protected areas, they are still used for illegal hunting, logging, and firewood collection. This thesis focusses the pollination ecology of two species of Acanthus. A. eminens occupies clearings and riversides inside the forest, while A. polystachyus grows in copses and hedgerows of the surrounding farmland, as well as at the forest edge. Through forest fragmentation, A. eminens continues to loose suitable habitats. In addition, the distance of its populations to the congener is reduced, and the relative abundance of the species shifts towards A. polystachyus. As flowering time and floral morphology of the species are highly similar, it seems likely that habitat fragmentation causes changes in pollination and reproduction of the species. In particular, A. eminens may loose pollinators to A. polystachyus, and receive more heterospecific pollen. As A. eminens is a common species of the natural ecosystem, such effects would indicate that habitat fragmentation causes longterm changes in ecosystem processes of Kakamega Forest, threatening its future existence in its natural state. In my thesis, I demonstrate that the species flower in synchrony during winter dry season between October and February. There are no negative effects of distance to or habitat availability of the congener on pollination and reproduction of the species. However, I find plants in drier habitats and flowers opening later during dry season to display higher fruit set. Both species are pollinated by carpenter bees (Xylocopa). There is no indication for a partitioning of pollinators through divergent flower morphology, as the species display highly similar flowers. A. eminens offers greater quantities of both nectar and pollen, while the total amount of floral rewards is greater in the larger individuals and populations of A. polystachyus. Both species likely represent important resources for pollinators in their respective habitats. Differences in pollinator abundance between populations are likely caused by habitat preferences of the pollinators. Even though the species share most of their pollinators, interspecific pollen transfer does not affect their reproduction. This may be explained by the prevalence of geitonogamy, as bees commonly visit several flowers on the same inflorescence, plant, and neighbouring, likely related individuals. Moderate visitation rates with low pollination success indicate that carpenter bees are ineffective pollinators, and that the greatest proportion of pollen is lost between visits. Consequently, most pollen is transferred between close, potentially closely related neighbours, reducing the incidence of heterospecific pollen transfer. These findings indicate that reproduction of either species may be limited by pollen quantity or pollen quality. Hand-pollination experiments show that both species are limited by pollen quantity but not pollen quality, as fruit set nearly doubles when supplementary pollen, regardless of its source, is provided. As there is no difference in seed set in seed viability between treatments, I conclude that pollen quality is generally high. There is convincing evidence that A. eminens and A. polystachyus are adapted to flowering during dry season, which offers favorable conditions for fruit and seed development as well as for seed dispersal and germination. Differences in humidity are strongly linked to reproduction in either species, and these effects are strong enough to mask all putative effects of competition for pollination and heterospecific pollen transfer.