The breeding system and demography of the Transvaal Sesame-bush, Sesamothamnus lugardii (Pedaliaceae)

Abstract

The aim of this research was to assess the breeding system, pollination relationship and demography of the Transvaal Sesame-bush, Sesamothamnus lugardii N. E. Br. Ex Stapf. (Pedaliaceae). S. lugardii is an arid savanna succulent shrub which can be found throughout Zimbabwe, southern Botswana and in northern South Africa, where it is anecdotally thought to be rare. Two populations of S. lugardii were assessed in Limpopo, South Africa. The floral traits of S. lugardii suggest that it is specialised for pollination by long-tongued hawkmoths. The flowers bloom in the evening, are large, sweetly scented and pale in colour. The corolla tubes are very long (ca. 10 cm) and narrow. Very long-tongued hawkmoths (Agrius convolvuli) were found to be the only visitors capable of accessing the nectar at the base of the S. lugardii corolla tubes while foraging and simultaneously interacting with the reproductive structures, successfully pollinating the flowers. S. lugardii is an obligate outcrosser, dependent on A. convolvuli hawkmoths for sexual reproduction. The high-risk traits of the S. lugardii breeding system, namely pollinator specialisation and obligate out-crossing, could render S. lugardii vulnerable to extinction. Predation of flowers by scarab beetles in the study population at Mapungubwe National Park resulted in low fruit set. At Morongwa Private Safari Lodge florivory was negligible and fruit set was higher. In both study populations, S lugardii was dominant in patches across the landscape, but only a small proportion of the numerous plants observed were seedlings. Little is known about the population dynamics of S. lugardii, but the absence of seedlings suggests a potentially vulnerable demography. The large shrubs were heavily impacted by meso- and mega-herbivores, but are resilient to herbivory, as they are capable of bark recovery and epicormic resprouting. However, the ever-increasing numbers of large herbivores at Mapungubwe could reach a critical threshold beyond which the populations of S. lugardii lose their resilience, putting them at risk of local extirpation. The number of seedlings in the populations at Mapungubwe decreased between an initial study conducted in 2005 and the recent assessment in 2014. This may be due to sporadic recruitment, but could also indicate a demographic bottleneck limiting seedling establishment. The low-risk traits associated with demography, namely resilience through longevity and vigorous resprouting, offset the high-risk traits by buffering the populations from decline during periods of reproductive failure. The environment in the study populations at present favours the continued survival of S. lugardii, despite the slow population turnover and low rate of reproductive success. Should the environmental conditions become less favourable for S. lugardii, then the extinction risk could greatly increase over a short period of time, and could even result in local extirpation.