Relative density and dispersion pattern of two southern African Asteraceae affect fecundity through heterospecific interference and mate availability, not pollinator visitation rate

Since co-flowering plants often share pollinators, their fecundity is likely affected by pollen transfer within and among plant species. Changes in pollinator visitation rates (e.g. through facilitation or competition for pollinators) are usually thought to exert the predominant influence on plant fecundity. However, the transfer of conspecific relative to heterospecific pollen between individuals may also play an important role. Indeed, the relative importance of these determinants of fecundity is expected to depend on the density, relative abundance and spatial arrangement of con- and heterospecifics. We investigated the effects of con- and heterospecific density and spatial distribution pattern on pollinator visitation and plant fecundity within and between two annual, self-incompatible co-flowering species [Dimorphotheca pinnata and Ursinia cakilefolia (Asteraceae)] by manipulating their relative abundance, overall patch density and dispersion patterns in experimental arrays in Namaqualand, South Africa. We quantified pollinator visitation rates and fruit set in arrays of varying density and aggregation. This enabled us to determine which mechanism(s) were driving variation of fecundity, particularly through their influence on visitation rates, mate availability and heterospecific pollen interference. To test whether autogamy offers reproductive assurance when individuals are scattered among a dense population of heterospecifics, we included an autogamous species (U.anthemoides) in a separate experiment. We found that increased fecundity with increasing conspecific density was not the result of higher visitation rates, but rather increased mate availability. Furthermore, increased spatial aggregation of conspecifics at low density significantly increased fecundity through reduced heterospecific interference. In contrast to results for self-incompatible species, fruit set in U.anthemoides was consistently high and unaffected by scattered distribution patterns. This suggests that autogamy may offer reproductive assurance when mates are limited and the potential for interspecific pollen transfer (IPT) is high.Synthesis. In this study of annual daisies, variation in fruit set is primarily driven by factors that affect the transfer of conspecific relative to heterospecific pollen, independent of pollinator visitation rate. Our findings demonstrate that mate limitation and IPT negatively affect fruit set and that these effects can be mitigated by intraspecific aggregation and the ability to autonomously self-pollinate.