Factors contributing to variation in seed production among remnant populations of the endangered daisy Gerbera aurantiaca

摘要

The species-rich mist-belt grasslands of southern Africa have been severely reduced in extent as a result of commercial afforestation, thus confining many endemic plants and animals to small populations in habitat fragments. We investigated the influence of various environmental factors on seed production and seedling recruitment in remnant populations of the endangered grassland herb Gerbera aurantiaca (Asteraceae). Experiments with color traps showed that Eriesthis beetles, which appear to be the primary pollinators of G. aurantiaca, were most abundant in the two largest extant populations. Seeds are produced in a very small proportion (typically <10%) of the ca 80 female florets in a capitulum. The mean number of seeds produced in undamaged capitula was found to be significantly lower in small than in large populations. Pollen limitation was evident from a significant overall increase in seed set after supplemental pollination in three populations over two seasons. The proportion of capitula containing seed predators did not differ marked among populations or years, but lepidopteran larvae, which destroy all of the seeds in a capitulum, were most abundant in the two largest populations in 2003. The presence of juvenile plants varied markedly among populations, but this could not be linked firmly to estimates of seed production. Clonal growth is likely to contribute to the persistence of small isolated populations of G. aurantiaca, even when seed production is severely compromised by pollen limitation and predation.