3.4.10 Connectivity

The Reef and adjacent Catchment are a linked system with high internal and external connectivity.36,838,919 This biophysical connectivity within and between land, freshwater and marine ecosystems is pivotal to the health and functioning of the Region’s ecosystems. Connectivity is provided in various ways, for example, through water flows from the catchment to sea, flooded landscapes,364 genetics,920 species migration,919 and downstream transport of sediments that nourish beaches and mangrove forests.36,123,838

Larger animals, such as fish and sharks, actively move between connected habitats for reproduction and foraging,364,921 while prevailing water currents are important for connectivity in planktonic organisms 838 and between islands. Marine herbivores can help to connect seagrass habitats, while seabirds play an important role in dispersal of island vegetation. For species with significant connections outside the Region, such as shorebirds (Section 2.4.13) and humpback whales (Section 2.4.14), the condition of distant habitats can directly affect populations in the Region. Connectivity is also a vital consideration in the movement of diseases 922 and parasites 923 into and across the Region.

Connectivity is influenced by many spatial and temporal events and processes and may be interrupted by natural and anthropogenic drivers.838 For example, most Catchment development (Section 6.4) has occurred in low-lying coastal floodplain areas, leading to losses in connectivity through hydraulic barriers, excessive aquatic plant growth or establishment of invasive species.36,924 Shifts in the types of sediment delivered from the Catchment can have radical downstream impacts (Section 6.5.2).

Connectivity between coral reefs and other habitats across the Region supports the maintenance of genetic diversity within and between populations, a critical factor in the ability of species’ to adapt to environmental change.29 The influence of connectivity on the resilience and adaptation potential of coral populations is an important area of current research interest.925 New techniques, particularly genetic analysis (Box 2.1), are providing evidence of population connectivity and breaks in connectivity that indicate genetic separation and could have evolutionary consequences.838,926 For example, genetic studies on corals have confirmed expectations from connectivity models, showing predominant north-to-south gene flow over the recent evolutionary past.926

The Reef continues to be a system with high internal and external connectivity

The central role of ocean currents in structuring patterns of connectivity is illustrated by the identification of 16 spatially separated fish communities along the Reef: 7 occurring in shallow waters (less than 15 metres depth) and 9 in deep water.252 During the fish spawning season, the East Australian Current (Section 3.2.1) is at its strongest, which may enhance or restrict larval dispersal.252 Patterns in larval connectivity are highly correlated with the El Niño–Southern Oscillation: larval dispersal is predominantly poleward in central and southern regions during El Niño conditions and predominantly equatorward under extreme La Niña conditions.927 Potential changes to the El Niño–Southern Oscillation (Box 6.1) may therefore affect population dynamics and patterns of biodiversity in the Region.927 Reef fish community composition is influenced by connectivity both between reefs and among reefs within a region.928

Despite the widespread distributions of many tropical reef species across the Indo-Pacific, their dispersal capability is relatively restricted.928 Strong cross-shelf trends in dispersal of coral larvae mean that inshore and mid-shelf reefs typically receive more larvae than outer-shelf reefs.929 Dispersal of coral larvae operates within distinct communities: northern, central and southern.930 Models show that inshore reefs have more self-recruitment and lower levels of larval export than deeper reefs.931 Despite disruptions to larval supply caused by widespread coral bleaching, a study found that thermal refugia have the potential to deliver larvae to 58 per cent of the Reef.158 However, the persistence of these refugia is uncertain 667 (Section 6.3.2). A recent modelling study found that 2 degrees Celsius of ocean warming reduced inter-reef connectivity due to reduced dispersal of coral larvae, resulting in higher local retention.932 

The Reef continues to be a system with high internal and external connectivity across its diverse ecosystems. Legacy impacts (such as hydrological barriers in the Catchment) have reduced this connectivity, and the emerging impacts of climate change are leading to further declines in some areas and for some key species groups such as corals.

  • 29. Pinsky, M.L., Clark, R.D. and Bos, J.T. 2023, Coral reef population genomics in an age of global change, Annual Review of Genetics 57: 87-115.
  • 36. Queensland Museum 2022, Wetlands of Queensland: Queensland Museum discovery guide/published in partnership with the Department of Environment and Science, South Brisbane.
  • 123. Hamylton, S., Kelleway, J., Rogers, K., McLean, R., Tynan, Z.N., et al. 2023, Mangrove expansion on the low wooded islands of the Great Barrier Reef, Proceedings Royal Society B 290(2010): 20231183.
  • 158. Cheung, M.W.M., Hock, K., Skirving, W. and Mumby, P.J. 2018, Cumulative bleaching undermines systemic resilience of the Great Barrier Reef, Marine Ecology Progress Series 604(23): 263-268.
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  • 838. Davis, A. and Pearson, R. 2024, 2022 Scientific Consensus Statement: Summary | Evidence Statement for Question 1.4: How are the GBR’s key ecosystem processes connected from the catchment to the reef and what are the primary factors that influence these connections? in 2022 Scientific Consensus Statement on land-based impacts on Great Barrier Reef water quality and ecosystem condition, eds J. Waterhouse, M. Pineda and K. Sambrook, Commonwealth of Australia and Queensland Government.
  • 919. Waterhouse, J., Pearson, R., Lewis, S., Davis, A. and Waltham, N. 2024, 7. Great Barrier Reef ecohydrology, in Oceanographic Processes of Coral Reefs. Physical and Biological Links in The Great Barrier Reef, eds E. Wolanski and M.J. Kingsford, CRC Press, Boca Raton, pp. 105-125.
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  • 922. Dobbelaere, T., Holstein, D.M., Muller, E.M., Gramer, L.J., McEachron, L., et al. 2022, Connecting the dots: Transmission of stony coral tissue loss disease from the Marquesas to the Dry Tortugas, Frontiers in Marine Science 9: 778938.
  • 923. Hendrick, G.C., Nicholson, M.D., Narvaez, P., Sun, D., Packard, A., et al. 2023, Diel fish migration facilitates functional connectivity of coral reef and seagrass habitats via transport of ectoparasites, Marine Ecology Progress Series 731: 249-265.
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