Annals of Botany

Flowers on the floor – rodents rewarded and seeds set

Amongst the very diverse range of pollination mechanisms, pollination by rodents has been observed previously in two different South African ecosystems. Kleizen et al. (Rondebosch and Pietermaritzburg, SA, pp. 747–755) now postulate that two Colchicum species, C. scabromarginatum and C. coloratum, components of the Succulent Karoo ecosystem, are also rodent-pollinated. The hypothesis is based on the geophyte growth habit, floral morphology and colouring, copious nectar production and nocturnal odour secretion. In the field, insects were never observed visiting these plants. Hand-pollination experiments showed that C. scabromarginatum is self-infertile while C. coloratum exhibits low self-fertility. Placing vertebrate-excluding cages round the plants resulted in a 97 % reduction in seed set in C. scabromarginatum and 82 % in C. coloratum. By contrast, exclusion of vertebrates from the insect-pollinated C. hantamense affected seed-set only very slightly. Live-trapping for four nights in a population of C. scabromarginatum resulted in the capture of eight individual rodents, all Aethomys namaquensis (namaqua rock mouse). In three nights of live-trapping in a population of C. coloratum, 28 individual rodents were trapped, representing three mouse and one gerbil species. Pollen was present on the snouts and in the faeces of these rodents; for A. namaquensis, only C. scabromarginatum pollen was identified. From the rodents associated with C. coloratum, almost all of the pollen belonged to that species, although one other unidentified type was present in very small amounts. In order to make direct observations of rodent behaviour, individual rodents were released into large glass tanks containing flowers of C. scabromarginatum or C. coloratum. Flowers were visited at around midnight; pollen transfer occurred as the rodents lapped nectar. Flowers of C. hantamense and Oxalis placed in the same tanks were ignored. These data confirm the authors’ hypothesis that these two Colchicum species are rodent-pollinated. However, one population of C. coloratum was also visited by three bird species, which were observed to carry pollen.