9.3.7 Cumulative impacts

The threats assessed individually in Figure 9.1 do not affect the Reef in isolation. Cumulative impacts result from the interaction of effects from one or more threats, as well as from past, present and reasonably foreseeable future influences.2149 Responses to a single threat can be complex, such as indirect or not linear, variable in space and time, and compounded by other stressors or ecological processes.2154 In isolation, a single threat can affect specific life stages or ecological processes that may also interact with other environmental factors, which are often difficult to detect. Some threats build up slowly over decades and have a chronic effect in the ecosystem, while exposure to others can occur as acute events over short periods of time.1404 

The Reef and its Catchment are exposed, directly or indirectly, to diverse and extensive human activities, including ports, shipping, fishing, tourism, land-based runoff and coastal development. The impacts of these activities do not occur in isolation, and often interact with threats, such as ongoing sea-temperature increase, altered weather patterns, and outbreaks of coral predators and diseases.2155 These cumulative impacts can interact with each other in different ways. For example, they can be additive (the combined effect is the sum of each independently), synergistic (the combined effect is greater than the sum of each independently) or antagonistic (the combined effect is less than the sum of each independently).1572 These multiple impacts can also occur at the same time or can arise if the pressures occur at different times when they occur in the same space (due to the long-term effects of past pressures).

A dead boulder coral covered in turf algae, with fleshy algae in the background.
Multiple stressors favour algae in competition with coral. © Johnny Gaskell 2020

A wide variety of threats, either in isolation or in conjunction with others, are affecting the Region’s natural, Indigenous, social, historic, and other heritage values (Chapters 2, 3, 4, 5 and 6). For example, chronic impacts from land-based runoff are often associated with declining trends in reef-building coral cover, but cyclones and thermal stress have also emerged as a major driver of changes in inshore coral reef ecosystems dynamics.190 Similarly, impacts from heavy rainfall and floods have resulted in the decline of seagrass abundance. The extent of decline in some inshore regions and the recent impact of marine heatwaves are likely impairing recovery, even when meadows exhibit a substantial level of resilient traits.145

The complexity and uncertainty created by multiple threats can hinder the capacity of managers to identify and address the stressors that are contributing to changes in the Region’s values and affecting the Reef’s resilience. These stressors are particularly difficult to identify in habitats such as coral reefs and seagrass meadows, where acute and chronic pressures simultaneously affect the reproduction, growth and mortality of habitat-forming species.2154,2156 Given the spatial scale and environmental diversity of the Reef, the resilience of communities at any point in space or time will almost certainly vary based on their exposure to pressures, the sensitivity of species to those pressures and the scale of disturbance that may affect population-level feedback processes.1404 Sophisticated modelling approaches, calibrated and validated with empirical data, are required to capture the  ecosystem-level effects of multiple stressors and to provide enhanced guidance for the strategic planning and spatial prioritisation of management actions and interventions.2154

A health coral reef with a big scar through the middle due to a vessel grounding.
Vessel grounding scar on a reef. © Johnny Gaskell 2023
  • 145. McKenzie, L.J., Collier, C.J., Langlois, L.A. and Yoshida, R.L. 2023, Marine Monitoring Program: annual report for inshore seagrass monitoring 2021–22. Report for the Great Barrier Reef Marine Park Authority, Great Barrier Reef Marine Park Authority, Townsville.
  • 190. Thompson, A., Davidson, J., Logan, M. and Thompson, C. 2024, Marine Monitoring Program: Annual report for inshore coral reef monitoring 2022–23. Report for the Great Barrier Reef Marine Park Authority, Townsville.
  • 1404. Thompson, A., Martin, K. and Logan, M. 2020, Development of the coral index, a summary of coral reef resilience as a guide for management, Journal of Environmental Management 271: 111038.
  • 1572. Trebilco, R., Fischer, M., Hunter, C., Hobday, A., Thomas, L., et al. 2021, Australia State of the Environment 2021: Marine. Independent report to the Australian Government Minister for the Environment, DCCEEW, Canberra.
  • 2149. Great Barrier Reef Marine Park Authority 2018, Cumulative impact management policy, Great Barrier Reef Marine Park Authority, Townsville.
  • 2154. Bozec, Y.M., Hock, K., Mason, R.A., Baird, M.E., Castro‐Sanguino, C., Condie, S.A., Puotinen, M., Thompson, A., Mumby, P.J 2022, Cumulative impacts across Australia’s Great Barrier Reef: a mechanistic evaluation, Ecological Monographs 92(1): e0149.
  • 2155. Grech, A., Pressey, R.L., Day, J.C. 2016, Coal, cumulative impacts, and the Great Barrier Reef, Conservation Letters 9(3): 200-207.
  • 2156. Griffiths, L.L., Connolly, R.M., Brown, C.J. 2020, Critical gaps in seagrass protection reveal the need to address multiple pressures and cumulative impacts, Ocean & Coastal Management 183: 104946.