Seed production in a threatened Aloe is not affected by bird exclusion or population size

Abstract

Small populations of many plant species have been shown to exhibit ecological Allee effects. These effects are expected to be pronounced in plants which are obligate outcrossers and rely on pollinators which forage preferentially in larger populations with greater nectar availability. We examined the breeding and pollination systems, level of pollen limitation and seed production in populations of a threatened ''ornithophilous'' species, Aloe pruinosa. Experimental hand-pollinations showed that A. pruinosa is genetically self-incompatible and thus an obligate outcrosser. Experimental exclusion of birds from inflorescences did not affect seed production, suggesting that insects are effective pollinators. Supplemental hand-pollinations in several populations showed that seed production in A. pruinosa is not pollen limited. Further, there were no significant relationships between population size and any measure of reproductive success in this Aloe species. Small populations of A. pruinosa are thus viable in terms of pollination processes and should be protected from more direct threats, such as habitat alteration.

Notes on the flowering and pollination of the endemic grassland <i>Aloe reitzii</i> var. <i>reitzii</i> (Asphodelaceae)

“…Ratsirarson 1995;Stokes & Yeaton 1995;Wilson et al 2009). While species-specific predictions of pollinators can be made on the basis of pollination syndromes, it is clear that determination of pollinator assemblages requires thorough field study (Fenster et al 2004;Geerts & Pauw 2009;Johnson & Steiner 2000).…”

African aloe ecology: A review

“…Direct observations and experiments on several Aloe species have confirmed the existence of extensive bird pollination systems (Wilson et al, 2009), and the majority of South African Aloe species have floral traits consistent with bird pollination (Botes et al, 2009a;Symes et al, 2009) (Table 2). These traits include redorange tubular perianths, strongly exserted anthers and stamens, and copious supplies of dilute nectar at the base of the flowers (Botes et al, 2009a;Symes et al, 2009).…”

“…However, aloes appear to show extensive variation among species in the contribution that bees make to their pollination (Wilson et al, 2009). Jordan (1996) notes that eight Zimbabwean Aloe species exhibit floral characteristics suggestive of insect pollination such as bell-shaped flowers (e.g.…”

“…comm.). Furthermore, bees also have the potential to clog stigmas with self-pollen, which would prevent further cross-pollination and therefore diminish reproductive output in the generally self-incompatible genus Aloe (Wilson et al, 2009). Overall, there appear to be three roles played by bees in aloe pollination ecology.…”

“…A. marlothii). However, there is still a lack of clarity regarding the general importance of bees in aloe pollination ecology, and further studies are required to elucidate this subject further (Wilson et al, 2009).…”