The geographical pollinator mosaic

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Pollinators are distributed unevenly in time and space. Indeed, like all animals, pollinators have restricted ranges, which are often determined by both physical factors, such as altitude, temperature, and rainfall, and by biotic factors, such as vegetation structure and availability of flowering plants (Chapters 6 and 15). The landscape in which plants evolve presents a geographical mosaic of pollinators that is hard to visualize, but is no less important for plant evolution than the geographical mosaics of soils, climate, and herbivores. Grant and Grant (1965) referred to this mosaic as the ''pollinator climate.'' The extent to which the geographical pollinator mosaic influences plant evolution depends on its stability in time and space, the sharpness of its boundaries, and the extent to which plants are generalized or specialized in their pollination systems. Like the Grants, Waser (2001, p. 327) emphasized that gradients in a pollinator mosaic will often be characterized by ''quantitative differences among populations in the relative abundance of different pollinators, rather than from qualitative turnover in pollinators'' (see also Aigner 2005).

The geographical pollinator mosaic is the basis for all allopatric and parapatric divergence in pollination systems. Yet, spatial and temporal variation in pollinator faunas remains poorly documented (see Chapter 15). Grant was keenly aware of its importance, and emphasized the significance of patterns in the distributions of hummingbirds, hawk moths, bumble bees, and flies within North America for plant evolution (Grant and Grant 1965; Grant 1983, 1994a). Unfortunately, much of this perspective seems to have been lost with the more recent emphasis on studies at single localities and reductionist approaches to studying pollination systems in general. Darwin himself was guilty on this score, as his famous orchid studies were conducted mostly at a single locality near his house. Despite being invaluable in showing the functional significance of floral traits, these studies did not provide the same insights into diversification that were afforded by Darwin's comparative studies of animals in their geographical context.

The existence of a pollinator mosaic can be illustrated graphically using data on the distribution of long-proboscid flies in South Africa. Figure 16.1 shows the main spatial and temporal components of this mosaic. This particular mosaic is significant for floral evolution, because plants tend to become involved in highly specialized relations with long-proboscid flies (Goldblatt and Manning 2000; Johnson and Steiner 2000). As individual fly species vary extensively in proboscis length, behaviour,

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Figure 16.1 Distribution of long-proboscid fly species in southern Africa, illustrating a geographical pollinator mosaic. Note that the mosaic also involves a strong temporal dimension. The approximate number of plant species in each pollination guild associated with a fly species is given in the legend. Philoliche spp. belong to the Tabanidae, whereas the remaining species are nemestrinids. Based on information in Goldblatt and Manning (2000), Potgieter and Edwards (2005), and unpublished data of the author.

Figure 16.1 Distribution of long-proboscid fly species in southern Africa, illustrating a geographical pollinator mosaic. Note that the mosaic also involves a strong temporal dimension. The approximate number of plant species in each pollination guild associated with a fly species is given in the legend. Philoliche spp. belong to the Tabanidae, whereas the remaining species are nemestrinids. Based on information in Goldblatt and Manning (2000), Potgieter and Edwards (2005), and unpublished data of the author.

colour preferences, and flight period, local adaptation of a plant species for pollination by a particular fly species causes distinct changes in floral morphology and phenology (Johnson and Steiner 1997; Goldblatt and Manning 2000). Examples of specia-tion events linked to shifts across boundaries in this mosaic are discussed in Section 16.6.

Elevational differences in pollinator faunas have been documented in several studies (Cruden 1972; Arroyo et al. 1982; Warren et al. 1988). In the most widely cited example, Cruden (1972) showed that bees became less important pollinators and birds increased in importance with increasing elevation in Mexico. Similarly, studies in alpine regions of several continents have shown that flies become relatively common, but bees diminish in importance, as altitude increases (Muller 1880; Arroyo et al. 1982; Primack 1983; Warren et al. 1988), although Galen (1989) found the opposite trend with visitors to Polemonium viscosum in a North American alpine zone.

At an even finer scale, pollinator abundance can vary spatially between sites separated by only hundreds or tens of metres (see also Chapter 8). The South African butterfly Aeropetes tulbaghia seldom ventures far from steep rocky habitats, so that pollination success of one of its host plants, the orchid Disa uniflora, can vary three- or fourfold between sites a few hundred metres apart, a pattern which is consistent from year to year (Johnson and Bond 1992). Abundance of insect pollinators also varies according to the spatial distribution of primary nectar sources, which can have major implications for spatial variation in pollination success of less abundant or non-rewarding plants (Laverty 1992; Johnson et al. 2003).

Are biotic selection environments sufficiently stable in time and space to be important for floral character evolution and, ultimately, speciation? Mosaic stability probably varies in direct proportion to scale. Subcontinental distribution patterns of pollinators are probably stable for long enough to influence speciation. On the other hand, current mosaics in parts of the world that have been subjected to frequent climatic perturbation, such as Europe and northern North America, may not reflect those present during speciation. Johnson and Steiner (2000) suggested that the instability of pollination mosaics in postglacial environments may account for the relative lack of floral specialization evident in pollination systems in the northern temperate zone. This link between mosaic stability and specialization was supported by Valiente-Banuet's (2004) demonstration that populations of the cactus Pachycereus pecten-abor-iginum are specialized for bat pollination in southern Mexico, where bats are resident, and more generalist in northern Mexico, where bats are migrant.

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