Breeding systems of invasive alien plants in South Africa: Does Baker's rule apply?

Abstract

According to Baker's Rule, plant species capable of uniparental reproduction are more likely to be successful colonists than are self-incompatible or dioecious species. Controlled pollination experiments carried out on 17 invasive alien plant species in South Africa revealed that 100% were either self-compatible or apomictic, and that 72% of these were capable of autonomous self pollination. The distribution of breeding systems among these invasive aliens is thus strongly skewed towards uniparental reproduction. While all 13 woody species in our survey showed a capacity for uniparental reproduction, this mode of reproduction is very rare among woody plants in general. Thus Baker's rule, which has generally been considered for short-lived herbaceous plants, may also apply to invasive shrubs and trees. The study species exhibited high levels of fruit set (median = 71.5%). Supplemental hand-pollination experiments carried out on five of the species showed that pollen availability did not limit fruit set. Further work is needed to establish the exact role of uniparental reproduction in various stages of the invasion process. In particular, we need to know whether uniparental reproduction provides reproductive assurance at the population establishment stage (as originally envisaged by Baker) or whether it plays a further role in alleviating pollen limitation in small and established populations.