Reproductive biology and mating system of Banksia tricuspis (Proteaceae)

Abstract

An understanding of the role of genetic and ecological factors that influence demographic change is paramount for the conservation of plant populations. These genetic and ecological factors often act in concert to influence reproductive success and thus the ability of a species to maintain its presence in the community. Paramount among the array of factors influencing a species' fitness are mating system considerations and their interaction with ecological processes. Both mating systems and ecological processes are influenced by intrinsic and extrinsic forces that may operate synergistically to constrain pollen transfer, gametic union and the provisioning of progeny. Plants respond to these constraining forces, which typically vacillate, through mechanisms that minimise random effects while maximising the quality and quantity of progeny.Mating system values indicated that B. tricuspis is an obligate outbreeder. The extent to which populations of this Banksia indulged in mixed mating was associated with habitat disturbance and its impact on pollinators. A reduction in outcrossing in one disturbed population was ascribed to an increase in inbreeding promoted through pollinator/pollen limitation. Conversely, a reduction in the diversity of pollinator types which enhanced the opportunity for outbreeding was the favoured interpretation of elevated outcrossing in another disturbed population.Disparity in outcrossing among inflorescences was driven by pollinator/pollen limitation considerations. Inconsistency among outcrossing values between inflorescences was ascribed to incongruity in floral attractiveness and sympatric assortative pollinator/pollen limitations. Augmentation of the floral display, as inferred through greater inflorescence size and presentation height, was positively associated with maternal fecundity and progeny fitness. This advance in quality and quantity was attained despite greater attractive displays increasing the probability of geitonogamous and consanguineous matings. This accomplishment indicated that attractiveness of the floral display in B. tricuspis enhanced the opportunity for sorting among possible mates, a proposition supported by the increased abandonment of zygotes with increasing display effort. The increases in maternal fecundity and progeny provisioning with increasing inflorescence size and height indicated that maternal resource considerations were not limiting reproductive success.Attractiveness of the floral display was also credited with the moderation of pollinator/pollen limitations within an inflorescence. The apparent increase in floral display with progress of the 'advancing front' towards the peduncle significantly altered pollinator foraging behaviour and differentially enhanced the opportunity for pollen receipt and export, especially of pollen types which promoted outbreeding. Commensurate with this moderation in pollinator/pollen limitation was an increase in maternal fecundity and progeny provisioning, although selection among zygotes was tempered and outbreeding declined slightly. The availability of nutritional resources within an inflorescence and sink strength relationships are submitted as proximate causes for this moderation, which conforms with the notion of mate choice and the proposition that the genetic threshold determining selection alternates with resource provisions.Scenarios propelled by selection among compatible mates which are commensurate with maternal resource considerations imply a high level of intrinsic control over reproductive success in B. tricuspis. Reproductive synchrony, continuity and predicability in fecundity and the compensatory repartitioning of resources following floral damage also signify a high level of intrinsic control. This control is required to mitigate extrinsic proximate constraints which impinge on reproductive success thereby synchronising maternal investment with resources and maximising fitness. The randomness of pollination was advanced as the proximate constraint affecting reproductive success in this species. This constraint was mitigated firstly by a mating system which sorted among compatible mates and tolerated deleterious matings, and secondly through an decrease in the opportunity for deleterious matings afforded by increases in the attractiveness of the floral display.The production of 'surplus' flowers by B. tricuspis was proposed as the principal adaptive response to the forces constraining reproductive success. 'Surplus' flowers primarily served, at various levels of organisation with the species, to enhance attractiveness, thereby moderating the influence of pollinator/pollen limitations on plant fitness. Maternal fecundity and progeny fitness within and between inflorescences was augmented by increases in attractiveness while conversely, at the population level, decreases in attractiveness deleteriously impinged on fitness. 'Surplus' flowers also conferred other functional advantages in this species which operated synergistically with the enhancement to floral attractiveness. These advantages were primarily related to the opportunities that 'surplus' flowers conferred for selective abortion and bet hedging.Caution is required in attributing 'surplus' flowers primarily to the mitigation of pollinator/pollen limitation considerations as other forces may represent stronger selective process. While the attractiveness of the floral display undoubtedly increases fitness, it also inherently increases the opportunity for floral damage by birds and insects as the discrimination among displays by damaging agents was driven by attractiveness cues. The impact of floral herbivores ranged from minimal for parrots and cockatoos to severe for insects, with the pattern of floral damage by both birds being determined by the pattern of insect floral herbivory. The predicability in floral damage between seasons and serial adjustment between successive reproductive development stages in B. tricuspis, as evident by compensatory responses in fecundity, moderates the influence of floral damage as a selective force influencing reproductive success.The production of 'surplus' flowers in B. tricuspis has facilitated the development of intrinsic maternal processes which, in reply to genetic and ecological stimuli, operate to maximise fitness through mitigating the deleterious impacts on reproductive success of floral damage and pollinator/pollen limitations. These maternal processes strategically maximise the utilisation of maternal resources and allow progeny with low fitness expectations to be tolerated. Ultimately, these maternal processes and deterministic genetic and ecological stimuli promote reproductive assurance which contributes to population and species persistence through advances in fitness.