Clinal variation in life-history traits of the invasive plant species Echium plantagineum L.
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Range expansion during the invasion of a novel environment requires that invading species adapt to geographical variation in climate and maintain positive population growth in the face of environmental heterogeneity. Thus, invasive species are expected to undergo adaptive evolutionary changes as they encounter novel selection pressures. The aim of this thesis was to identify adaptive changes in plant growth and reproductive traits throughout the lifecycle of a model invasive species to determine which traits are vital to the success of invasive species encountering novel environments. The model species used in this study was the widespread European invader, Echium plantagineum, which has invaded over 33 million ha across Australia, causing ~$30 million (AUD) damage per annum. I investigated geographic variation in life-history traits of 34 populations of E. plantagineum across a 1,000 km arid-mesic gradient throughout south-eastern NSW, Australia. Seeds were collected for each population along the arid-mesic gradient, germinated in the laboratory and grown in the glasshouse in a common environment. I found that E. plantagineum has rapidly adapted to environmental selection pressures throughout its range, resulting in two major clines linked to plant flowering time and seed size of progeny. Compared with populations from mesic habitats along the arid-mesic gradient, plant populations from arid environments had significantly higher relative growth rate and leaf production which was associated with much earlier flowering time and reduced time between stem production and flower production. Plants from arid regions also produced significantly larger seeds compared with plants from mesic habitats. Interestingly, seeds from all E. plantagineum populations along the arid-mesic gradient germinated rapidly (within 48 hours of water exposure) allowing them to quickly and opportunistically take advantage of available resources. Considered together, these adaptations allow E. plantagineum to grow rapidly, reproduce and produce progeny before conditions become unsuitable. The findings in this thesis provide compelling evidence for the rapid development, within 150 years, of clines in reproductive strategies linked to flowering and seed size evolution. My results support the notion that the successful invasive spread of species can be increased through genetic divergence of populations along arid-mesic climatic gradients. The climate of south-eastern Australia is predicted to change to become hotter and drier inducing many species to adapt or perish. The range and distribution of E. plantagineum is unlikely to be altered by these climatic changes as pre-adapted genotypes currently exist in the range margins and have persisted in arid regions for over 100 years. Consequently, further work is required to investigate the evolutionary capacity of other native and invasive species to determine how ecosystem dynamics and composition may change in the future.
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