The pollination niche and its role in the diversification and maintenance of the southern African flora

Abstract

The flora of southern Africa has exceptional species richness and endemism, making it an ideal system for studying the patterns and processes of evolutionary diversification. Using a wealth of recent case studies, I examine the evidence for pollinator-driven diversification in this flora. Pollination systems, which represent available niches for ecological diversification, are characterized in southern Africa by a high level of ecological and evolutionary specialization on the part of plants, and, in some cases, by pollinators as well. These systems are asymmetric, with entire plant guilds commonly specialized for a particular pollinator species or functional type, resulting in obvious convergent floral evolution among guild members. Identified modes of plant lineage diversification involving adaptation to pollinators in these guilds include (i) shifts between pollination systems, (ii) divergent use of the same pollinator, (iii) coevolution, (iv) trait tracking, and (v) floral mimicry of different model species. Microevolutionary studies confirm that pollinator shifts can be precipitated when a plant species encounters a novel pollinator fauna on its range margin, and macroevolutionary studies confirm frequent pollinator shifts associated with lineage diversification. As Darwin first noted, evolutionary specialization for particular pollinators, when resulting in ecological dependency, may increase the risk of plant extinction. I thus also consider the evidence that disturbance provokes pollination failure in some southern African plants with specialized pollination systems.Keywords: adaptive radiation; coevolution; mutualism; pollination ecotypes; pollination syndrome; speciationResearch in these fields [pollination and dispersal biology] is going to be the most fruitful approach to understanding evolutionary trends in higher plants. (Stebbins 1970)

Intraspecific divergence and convergence of floral tube length in specialized pollination interactions

“…The primary conceptual model of pollinator-driven floral divergence has its roots in the writings of Grant [3] and Stebbins [7] and has been referred to as the Grant-Stebbins model of floral divergence [4] or the pollinator-shift model of floral divergence [8,9]. Here, geographically separated plant populations of the same species may experience different pollinator communities, and specialization on those different communities is thought to drive allopatric divergence [10][11][12].…”

“…Extensive variation in pollinator morphology and behaviour is thought to account for much of the richness in floral form. Pollinator species or functional groups of pollinators can be viewed as ecological niches, the exploitation of which often requires adaptive evolutionary shifts in floral design, resulting in floral diversification across heterogeneous pollinator landscapes [3,4]. This association between plants and their pollinators is thought to have been a key contributor towards the impressive radiation of angiosperms [5,6].…”

Pollination: Impact, role-players, interactions and study – A South African perspective

“…13 It has been argued that the formation of specific floral structures in plants is largely driven by means of natural selection from their respective pollinators. 9 Better pollinator-flower compatibility would, therefore, result in higher selection, through increased fertilisation events of these individuals. The pollinator-plant interaction is important as pollinator or floral adaptations can drive speciation as suggested by the diverse floras of the Cape region, South Africa.…”

“…There is an estimated 25 000 bee species in the world of which approximately two-thirds are taxonomically described. 9 Bees are important pollinators of tropical forest trees 17 and play an essential role in the pollination of smaller trees, shrubs and herbaceous plants 9 . Many crop plants are bee-pollinated, such as sunflower, tomato, canola, cowpea and coffee.…”

“…8 Mutualism between the plant and the pollinator is based on rewards (pollen, nectar and oil) that the pollinator receives from the plant, and the plant gains the service of successful pollination, thereby securing its reproductive success. [7][8][9] This mutualism can be facultative or obligate, depending on whether the plant is self-compatible or whether it is monoecious or dioecious. 7 Parasitic interactions are also possible when a potential pollinator takes pollen and nectar from the plant without playing any role in its pollination.…”