Partitioning of pollinators during flowering in an African Acacia community

Ecology, Dec, 1998 by Graham N. Stone, Pat Willmer, J. Alexandra Rowe

Tanzanian Acacia trees (Leguminosae; Mimosoideae; genus Acacia) provide a clear example of daily patterning, both of pollen release and pollinator activity (Stone et al. 1996). While members of each of four Acacia species showed highly synchronized anther dehiscence, daily peaks of pollen release were regularly spaced across species, as predicted for generation of the observed patterning by competitive character displacement (Pleasants 1980, Waser 1983, Williams 1995). Such patterning could also result from ecological sorting (through differential colonization and/or extinction rates) of species that differ in their timing of pollen release for other reasons (Armbruster 1985, 1986). The long evolutionary history of East African savannahs and the geographically stable structure of Acacia communities (Ross 1981, Kingdon 1990) both suggest that long-term evolutionary responses, rather than ecological sorting, are responsible for the patterns seen in Tanzania. Three factors may predispose Acacia species to the exploitation of the daily time resource axis. First, they commonly grow in mixed-species assemblages and several species flower together in space and time after seasonal rainfall (Ross 1981). Second, the scented and showy inflorescences of Acacia species are visited by a wide diversity of flower visitors, at least some of which are shared by more than one Acacia species (Bernhardt 1987, Tybirk 1989, 1993, Stone et al. 1996). Third, the structure of Acacia flowers is very similar across sympatric species, and there is no a priori reason why pollen collected from one species cannot be deposited on flowers of another. Segregated pollen flow among species through deposition of pollen on different regions of shared pollinators, known for a few other plant pollination guilds (Dressier 1968, Armbruster et al. 1994), is thus unavailable to Acacia.

In this study, we present a more detailed analysis of flowering in the same Tanzanian Acacia community discussed in Stone et al. (1996), and ask the following questions:

1) What are the intra- and interspecific patterns of pollen release in seasonal and daily time, and are these patterns consistent among sites and among years?

2) Do correlations between microclimatic variables and the timing of pollen release provide any indication of which cues could structure differences among species, if present?

3) Do interspecific differences in the timing of daily pollen release provide evidence of character displacement among co-flowering Acacia species?

4) Does the provision of different floral rewards (pollen vs. nectar) by Acacia species generate differences in the assemblages of flower visitors they attract? Do co-flowering Acacia species share flower visitors likely to be significant pollen vectors, and hence possible agents of selection for divergence in the timing of daily pollen release?

5) Do differences in timing of pollen release among co-flowering Acacia species generate corresponding structure in the activity of shared pollinators?