Consequences of habitat fragmentation for the pollination of wildflowers in moist upland grasslands of KwaZulu-Natal.

Abstract

Large areas of moist upland grassland in KwaZulu-Natal are severely fragmented due to large scale farming of exotic trees. The aims of this thesis were to determine whether habitat fragmentation of these grasslands has a detrimental effect on plant-pollinator interactions and hence the reproductive output of the wildflower species occurring there and whether the magnitude of this effect can be predicted by breeding and pollination system characteristics. The 24 wildflower species included in this study appear to support a rich and diverse pollinator community, including long-tongued solitary bees, long-tongued flies, hawkmoths and sunbirds. Two thirds of the wildflower species appear relatively specialised in terms of pollination, with six species entirely dependent either on a single species or a specific functional type of pollinator for pollination. The majority of wildflower species (90%) were found to be incapable of autonomous self-pollination and thus dependent on pollinators for fruit and seed set. At least six species are obligately xenogamous. Little evidence was found for pollen limitation in undisturbed moist upland grassland, suggesting that these grasslands are characterised by high levels of pollinator activity. Greater levels of pollen limitation of reproductive output in habitat fragments was evident in two species, suggesting that depressed reproductive output in habitat fragments may be the consequence of a decrease in the quantity and/or quality of pollinator services. Significant detrimental effects of habitat fragmentation on reproductive output were evident in two thirds of the wildflower species, with 94% of the species exhibiting overall declines in seed set per flower from the continuous grassland site to the habitat fragments. The median decline in seed set per flower for the wildflower species was found to be 33.0%. Significantly more species experienced overall declines in reproductive output than would have been expected by chance alone. Only specificity of the pollination interaction in terms of number of pollinator taxa involved was found to be significantly related to percentage change in seed set from continuous to fragment habitats. This effect was diminished when other factors were included in a multiple regression. Results support Bond's (1994) hypothesis that degree of specificity in pollination systems is important in determining extinction risk of a given plant species. Declines in reproductive output of a range of wildflower species in grassland habitat fragments may affect the local persistence of these populations, particularly if recruitment is seed-limited.