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University of Cape Town (2004)

Variation in pollination across a fragmented landscape at the Cape of Africa

Pauw, Anton

Titre : Variation in pollination across a fragmented landscape at the Cape of Africa

Auteur : Pauw, Anton

Université de soutenance : University of Cape Town.

Grade : Doctor of Philosophy 2004

In this thesis I assessed the ability of established conservation areas to conserve plant-pollinator interactions. The study area was the species rich, but highly fragmented West Coast Lowlands of the Cape of Africa. I focused on the Nectarinia sunbird pollination system and the oil-collecting Rediviva bee pollination system. Before exploring the consequences of habitat fragmentation, it was necessary to study the natural history of the plant-pOllinator interactions. Among 16 conservation areas, I studied variation in the pOllination of Brunsvigia orientalis (Amaryllidaceae) by Nectarinia sunbirds. Pollen supplementation experiments and censuses of sunbirds showed that the observed failure of seed set could be attributed to the lack of sunbirds. Three factors : 1) the post-fire successional stage of the vegetation, 2) the number of flowering B. orientalis plants, and 3) the size of the conservation area, explained 75 % of variation in seed set. Seed set failed in small conservation areas (22 30 ha), and was reduced in medium-sized conservation areas (811 - 1113 ha) compared with large conservation areas (> 15 000 ha). Among 27 conservation areas, I studied variation in the pollination of an oil secreting orchid, Pterygodium catholicum, by the oil-collecting bee, Rediviva peringueyi. Variables that Significantly influenced the ability of nature reserves to conserve the orchid bee mutualism were : 1) the soil type, 2) the post-fire successional stage of the vegetation and 3) the interaction between reserve size and the nature of the surrounding land-use. A lack of pollinators led to reproductive failure in consecutive years in small conservation areas (4 - 385 ha) in an urban matrix. Correlated patterns of variation in seed capsule production suggested that the results obtained for P. catholicum could be extrapolated to several additional plant species which are also pollinated by R. peringueyi. Are these apparent anthropogenic declines in pollination real ? Declines can only be detected with certainty by comparison with historical rates. I reconstructed the historical pollination landscape by detecting the genetic echo of past pollination, and by determining historical pollination rates from rehydrated herbarium specimens. Thus, I demonstrated that plant-pOllinator mutualisms have recently collapsed in small, urban conservation areas. Does a decline in pollination really matter ? Patterns of variation in the composition of the community of oil-secreting plants suggest that the anthropogenic extirpation of oil collecting bees from small conservation areas has been followed by the extirpation of dependent plant species. Patterns of infraspecific variation in the floral traits in sunbird and oil-bee pollinated plants suggest an alternative to extinction. Pollinator loss may ultimately result in the evolution floral traits that decrease dependence on the absent pollinator.


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