Pollen limitation and demographic structure in small fragmented populations of Brunsvigia radulosa (Amaryllidaceae)


  • Megan Ward,

  • Steven D. Johnson

M. Ward and S. D. Johnson, School of Botany and Zoology, Univ. of KwaZulu-Natal, Private Bag X01, Scottsville, SA-3209 Pietermaritzburg, South Africa (JohnsonSD@ukzn.ac.za).


The ecological consequences of disruptions in plant-pollinator mutualisms are poorly understood. We examined how seed production and recruitment of juveniles in populations of the spectacular grassland geophyte Brunsvigia radulosa (Amaryllidaceae) correlate with various indices of habitat fragmentation, including habitat fragment area, population size and population isolation. The species was found to be self-incompatible and adapted for pollination primarily by the long-proboscid fly Philoliche aethiopica (Tabanidae). In places where this fly is locally extinct, carpenter bees appear to act as substitute, though less effective, pollinators. Seed production in B. radulosa showed a significant positive relationship with population size, but not with habitat fragment area or spatial isolation of populations when all three indices of habitat fragmentation were included as predictor variables in multiple regression models. Reduced seed production in small populations was attributable to pollen limitation, as supplemental hand pollinations resulted in proportionally greater increases in seed production in these populations. Pollen limitation appears to have demographic consequences; specifically, the proportion of juvenile plants in populations showed significant positive relationships with current levels of seed production per plant and size of populations. Thus the long term persistence of small B. radulosa populations in habitat fragments may be threatened by a pollination deficit.