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Nagelkerken, Ivan Booth, David (University of Technology Sydney) Mitchell, Angus School of Biological Sciences 2023 https://hdl.handle.net/2440/139879 Anthropogenic warming has facilitated the redistribution of species globally. Many tropical species have shifted their ranges poleward either to escape thermally unsuitable conditions or to invade previously inaccessible temperate environments. Climate-driven species redistributions have facilitated range-shifting and local species to interact for available resources. These novel species interactions can modify the pace of species range extensions. In Australia, over 150 tropical fish species have been observed moving poleward into temperate marine ecosystems but often fail to establish due to hostile conditions (winter temperatures and environmental novelty). However, ocean warming could minimise lethal winter effects on range-extending tropical fishes, allowing them to establish at temperate latitudes in the near future. This thesis provides empirical and experimental evidence that current and future climatic conditions (ocean acidification and warming) and novel species interactions can modify the pace of tropicalisation in temperate marine ecosystems and highlights the capacity of local temperate fish communities to respond to tropical fish range extensions. By using a climate-manipulated laboratory experiment, I reveal that ocean acidification might slow tropicalisation through degradation of the shoaling performance of novel tropical-temperate fish shoals (Chapter 2). In addition, I show that future winter conditions negatively affect tropical fish behavioural repertoire and physiological function in temperate ecosystems (Chapters 3 and 4). In contrast, I demonstrate that the temperate fish behaviour remained unchanged but experienced increased growth and physiological performance under future winter conditions compared to current and future summer conditions (Chapters 3 and 4), which could seasonally modify tropical-temperate interactions. I further reveal that tropical fish entering novel temperate environments trade off foraging efficiency for predator vigilance, independent of novel shoaling interactions (mixed-species shoaling). However, novel species interactions can enhance tropical fish foraging, which may allow them to perform better at their cold-temperate range edges (Chapter 5). While present-day lethal winter temperatures and novel temperate environmental conditions slow tropicalisation, the overall effect of ocean warming and positive species interactions will facilitate the tropicalisation of temperate waters in a future ocean. However, future winter temperatures and ocean acidification could limit tropical fish performance at temperate latitudes. Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 2023 en Ocean acidification ocean warming fish behaviour novel species interactions fish ecophysiology tropicalisation range-extensions Ocean Acidification and Seasonal Temperatures Counter Positive Novel Species Interaction and Warming Effects on Tropicalising Temperate Fish Communities Thesis This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legalsopen.access