Biological invasions in a changing world: responses of two invasive thistles to disturbance and climate change

Open Access
Zhang, Rui
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
July 22, 2011
Committee Members:
  • Katriona Shea, Dissertation Advisor
  • Katriona Shea, Committee Member
  • Eric S Post, Committee Chair
  • Ottar N Bjornstad, Committee Member
  • David A Mortensen, Committee Member
  • Carduus acanthoides
  • biological invasion
  • climate change
  • warming
  • disturbance
  • Carduus nutans
Biological invasion has been recognized as a significant component of global change due to its detrimental consequences on biodiversity, ecosystem functioning and human health. Invasion can also interact with other elements of global change, such as human-mediated disturbances and climate change. Therefore, understanding the responses of invasive species to disturbance and climate change is crucial both for predicting future invasion risks, and in order to develop appropriate management strategies for the future. Using both experimental and theoretical approaches, I examined invasions of two congeneric thistles, Carduus nutans and C. acanthoides in the context of disturbance and climate change. I aimed to assess the invasion potential of these two species under different disturbance regimes (specifically, different levels of intensity, frequency, and timing of mowing) and under projected climates for the future (specifically, increased temperature and increased precipitation). My disturbance results underline the importance of considering multiple aspects of disturbance in invasion studies, as interactions and interdependence of these aspects may lead to complex outcomes and potentially counter-intuitive conclusions (Chapter 2). Based on these empirical data, I demonstrate that the choice of management strategy may also depend on the time horizon of the management objective (Chapter 3). Strong tolerance of the two species to disturbance (i.e. regrowth ability after damage) is likely to contribute to their success in disturbed areas (Chapter 4). The two species also showed positive responses to climate change (Chapter 5). Besides enhanced demographic vital rates, dispersal of Carduus spp. was also enhanced under warming, mainly due to increased plant height. Consequently, population spread rates are expected to increase under future climatic scenarios. Phenology was affected by warming as well, altering both individual phenological dates, and durations of each developmental stage (Chapter 6). Uneven shifts in the phenological sequence of C. nutans led to extended growing duration, which contributed to increased plant height at the dispersing stage. However, increased height of C. acanthoides appeared to be due to faster growth rates under warming. Furthermore, I found that for C. nutans, plant structural defense (i.e. prickle density) was reduced under warming (Chapter 7) and maternal warming significantly affected germination and seedling emergence of the offspring (Chapter 8). Results from this thesis suggest that invasions of Carduus spp. are likely to increase both in local abundance and spatial spread to become more problematic with ongoing global change.