2. Biodiversity

Mangrove forests are specialised plant communities with canopies of trees or shrubs, adapted to cope with the salty, oxygen-limited soils of the upper intertidal zone. These forests cover an area of approximately 2387 square kilometres in the Region.¹¹⁰ The Region’s mangrove forests are diverse on a world scale and support a high biodiversity of associated species, dominated by invertebrates (crustaceans, worms, molluscs, and other benthic animals) and fishes.¹¹¹ They are important to the cultural heritage of First Nations peoples, who have made use of these abundant natural resources for thousands of years.¹¹² Mangrove forests are an integral component of a mosaic of coastal ecosystems, including estuaries, seagrass beds, and saltmarshes (Section 3.5.1), that provide ecologically and economically important linkages between terrestrial and marine environments.¹¹³ While the connections between mangrove forests and fisheries are complex, these habitats are known to play a crucial role in providing food, shelter, and breeding grounds for species of commercial and recreational fishing value, such as bream, mullet, mangrove jack, barramundi, and prawns.¹¹⁴ 

Mangrove forests also provide a range of ecosystem services, including coastal stabilisation storm buffering, filtration of land-based runoff, nutrient cycling and export to marine ecosystems, primary production, and carbon sequestration.^36,115,116 As ‘blue carbon’ ecosystems, their capacity to accumulate and store organic sediments makes them important carbon sinks, contributing to the net removal of carbon dioxide from the atmosphere.^117,118 It has been estimated that every hectare of mangrove wetland stores about 550 tonnes of carbon.³⁶

Mangrove forests continue to occupy most of their historical extent,¹¹⁰ although coastal development and natural processes have led to both losses and gains through time.^119,120 Coastal developments including berms, ponded pasture, dams, seawalls, and roads on coastal plains, can lead to loss or degradation of mangroves and associated wetlands ¹²¹ (Section 6.4). Historical removal is a common legacy issue on degraded agricultural land, presenting opportunities for carbon capture through restoration in these areas.¹²² Although overall extent has changed little since 2015,¹¹⁰ expansion of mangroves has been observed in low wooded islands of the Howick Group in the northern Great Barrier Reef. This expansion equates to 10,233 tonnes of new mangrove biomass since the 1970s and is thought to be a consequence of changing environmental controls including relative sea-level rise over the past century.¹²³ The ability of mangrove forests to keep pace with rising sea levels is limited.¹²⁴ 

Climate change is an increasing threat for mangrove forests, with sea level rise and coastal squeeze expected to impact mangrove forests in future decades (Section 6.3.2). Cyclone impacts generally remained low since 2019, though the impacts of cyclones in late 2023 and early 2024 have not yet been quantified. Evidence that greater damage after severe cyclones proportionately impedes recovery processes ¹²⁵ suggests that future return intervals for severe cyclones will be a key determinant of future trends. Since 2019, important advancements in reporting on condition of mangrove forests include incorporation of a satellite-derived canopy-cover index ¹²⁶ into the National Ocean Ecosystem Account.¹²⁷ Mangrove condition indexes have also been incorporated into the Reef 2050 Regional Report Cards.^128,129 The condition of mangrove forests across the Region is considered good, with an overall stable trend between 2005 and 2021.¹³⁰ 

Little change has occurred to the overall extent of mangrove forests. Gains and losses are occurring due to natural and anthropogenic processes. Evidence regarding the Reef-wide distribution and individual causes of these changes is currently limited. Acute cyclone impacts were relatively low from 2019 to 2023, but the impacts of successive cyclones during the 2023–24 wet season have not yet been assessed. Mangrove forests have expanded on some islands in the northern Great Barrier Reef.