Future trajectories for warming temperate reefs

Anthropogenic climate change is impacting marine systems globally, from species-level changes in distribution and abundance to ecosystem-wide transformations. Temperate marine systems are warming at accelerating rates, facilitating the poleward movement of warm-water species which are tropicalising higher-latitude reefs. This is resulting in profound shifts in dominant foundation species and the reorganising of associated assemblages with cascading implications for ecosystem structure and functioning. Although evidence of tropicalisation is increasingly documented, what is needed is a better understanding of the processes underpinning tropicalisation, as well as the potential outcomes for the future functioning of tropicalised temperate reefs. In this thesis, I sought to provide insight into these aspects of tropicalisation. Firstly, analysis of a decade of underwater visual census data encompassing subtropical to cool-temperate shallow reef ecosystems in southeastern Australia revealed that sea temperature was a strong determinant of abundances of juveniles of range-expanding tropical and subtropical reef fish species occurring outside of their normal range limits (Chapter 2). I also showed that juveniles of some temperate reef fish species had strong links to temperate habitat-forming species that may not persist in the future under climate warming. These relationships were then explored further by examining the role of species habitat use traits in mediating range expansions by tropical and subtropical reef fishes in this same region (Chapter 3). Here I found a positive relationship between habitat niche breadth and abundances for both range-expanding and resident temperate species at warm-latitude reefs. In Chapter 4 I used a combination of compound-specific and bulk tissue stable isotope analyses and a space-for-time substitution approach set in a tropical-temperate transition zone in eastern Australia to investigate the potential impacts of tropicalisation for species trophic interactions, trophic structure and energy flow through food webs. Coexistence of tropical, subtropical and temperate herbivorous fish species in warming temperate reefs can be facilitated by trophic niche segregation and complementary resource use. Here I also showed that ecosystem state shifts away from kelp-dominated to tropicalised reefs results in a shift in the ultimate source of primary productivity supporting food webs, from reliance on plankton as the dominant energy pathway in temperate reefs to mixed reliance on both plankton and benthic energy pathways in tropicalised reefs. In Chapter 5, experimental herbivory assays revealed a previously underappreciated role of resident temperate herbivores in contributing to tropicalisation by facilitating expansion of a range-expanding subtropical coral in temperate reefs. The findings presented in this thesis help to improve predictions about where, how and when tropical and subtropical species will move into temperate regions as these warm and the impacts on the structure and trophodynamics in these ecosystems.