Adaptation and Ecological Significance of the Dioecious Plant Pimelea microcephala subsp. microcephala in the Arid Zone.

Abstract

Dioecy, the segregation of sexes to male and female individuals, is found in 6% of angiosperms, yet the percentage of dioecious species in arid zone flora are typically below this world average. Arid zones are theoretically inhospitable environments for dioecious species to survive and propagate. Despite this, a growing body of research suggests that the enforced outcrossing and resulting phenotypic variability granted by dioecy may enhance adaptation to these resource-poor environments. Such adaptable dioecious species might therefore make ideal candidates for long-term arid zone revegetation schemes in the face of future drier conditions. Chapter 1 highlights that arid zone dioecious plants are under-researched in the Australian literature. To understand the life history associations and distribution of dioecy in central southern Australia, Chapter 2 created an inventory of dioecious species and mapped the variation of proportions of dioecious species richness of 66 Interim Biogeographical Regionalisation of Australia subregions. Dioecy was associated with woodiness, abiotic pollination and abiotic dispersal, as seen in other floras. Proportions of dioecy were lowest in arid zones (average ~3.8%), highest in coastal zones (average ~6.5%) and were explained best by regression models of woodiness and annual temperature range. The apparent coastal association may be due to range overlap among endemic coastal species and wide-ranging species that occupy arid and coastal subregions. The wide-ranging dioecious species Pimelea microcephala subsp. microcephala was selected to determine the species tolerance and adaptation to aridification, thus informing its potential use to support native fauna in revegetation programs. Chapters 3 and 4, utilised four populations across a rainfall gradient (250–390 mm/year) to investigate variation in population and sex-level parameters respectively. Size classes, spatial structures and proportions of juveniles were approximately equal between sites and did not vary according to rainfall, indicating that populations should be able to maintain succession under further aridification. The only trait to vary with annual rainfall was male, but not female reproductive effort, which was negatively associated with rainfall. Investigation of the nutritional quality of pollen and nectar and diversity of nutrient-consuming insects (Chapter 5) indicates that insect visitation is equal between sexes regardless of rainfall. Floral visitors receive nectar from both sexes, where males provided sugar-dominant nectar (alongside low-protein pollen, 2.4% DW), while females provided protein-rich nectar, making both sexes attractive. Fruits also contain beneficial nutrients, including water (60% FW), sugar (2.5% FW) and antioxidants (0.3% FW) and are consumed and dispersed by at least four native bird species. Low levels of the mammalian toxin simplexin (5% FW) were also detected, which may benefit the use of this species for conservation if its presence helps to deter excessive browsing of this shrub in revegetated areas. This study revealed the potential importance of dioecious plants for conservation, and through the largest investigation of the continuous proportions of dioecy in Australia indicates that further research is needed. Future research may be assisted by the inventory and map created in Chapter 2 to expand work into climate correlates, phylogenies and endemism, to enable better comparisons between Australia and other countries.