2. Biodiversity

Biodiversity is a critical component of the Reef’s outstanding universal and natural heritage value. The Region is host to one of the world’s most diverse and remarkable ecosystems and contains the largest coral reef system in the world. Biodiversity was assessed in terms of habitats and populations of species, and while there are a range of positive or encouraging results, these are balanced by some serious ongoing issues. Turning declines around is often neither easy nor fast.

Across the entire Region, the condition of habitats (as a group) was rated poor, the same overall rating as 2019. This assessment reflects that habitat loss and degradation or alteration in a number of areas continue to have persistent and substantial effects on populations of some dependent species. Overall, populations of species within the Region also remain in poor condition, and over half of the assessed component species and species groups are graded as being in poor or very poor condition. Species assessments reflect both the long-lasting effects of significant past population declines (for example, from historical commercial harvesting) and the impact of deterioration of the habitats species depend on.

Coral reef habitats within the Region are assessed to be in poor (borderline good) condition, reflecting an improvement since 2019. Significant and large-scale impacts on coral reef habitats continue to be observed, primarily as a result of climate-related ocean warming. Coral reefs remain highly vulnerable to climate change, crown-of-thorns outbreaks and poor water quality in inshore areas. Mass bleaching of corals occurred in both 2020 and 2022 in response to extreme sea temperatures, however, these events did not result in the extensive mortalities seen during 2016 and 2017. From a populations of species perspective, corals improved overall, but remain in a poor condition. The 2019 to 2023 window of reduced acute disturbances has enabled recovery in cover of fast‑growing coral species in some areas. However, it is likely that alterations to habitat and species diversity remain in a number of areas. This is the context in which the additional effects of the summer 2023–24 mass coral bleaching event will unfold.

Grades provided are for the entire Region for all habitats and species, yet the size of the Region is extensive and there is variability in condition. So for the first time in Outlook reporting, the condition of coral reef habitats is also assessed at a regional scale to provide some insight on variation between the northern, central and southern regions. Even at a regional level, considerable variability exists between inshore and offshore areas and within and between individual reefs. Reefs that have escaped impacts from bleaching, crown-of-thorns outbreaks and reduced water quality remain in good condition. 

The overall condition of seagrass meadows has improved to good (borderline poor), reflecting recovery in northern regions. However, in contrast to the overall trend, seagrass meadows in the south have continued to decline as a result of flooding impacts in recent years, while little is known about the condition of deep-water seagrass meadows. Similarly, the condition of seagrasses as a group has improved from poor to good since 2019, which again reflects the recovery of seagrasses in northern regions, while seagrasses in the south remain in poor condition.

Coastal habitats, including mangroves, islands and mainland beaches and coastlines, remain in good condition across the Region. Observed changes in coastlines and coastal habitats in recent years may be the result of natural fluctuations in climatic and oceanic processes, although these habitats will be particularly vulnerable to projected future sea level rise. Other habitats remain in good or very good condition.

Marine turtles are in poor condition, and population declines are reported in most species other than southern green turtles and eastern Queensland flatbacks. Dugongs also remain in poor condition, showing no consistent trend at a Reef-wide scale. Northern dugong populations are considered stable, but there is a high probability that southern populations have deteriorated. Overall, sharks and rays, sea snakes, seabirds, shorebirds and other (non-coral) invertebrates are also all assessed or inferred to remain in poor condition, based on variable, and often limited, available data.

Other species components are assessed as being in good or very good condition. Benthic algae, bony fishes and whales are all graded as good condition and have a stable trend. The overall condition of whales partially reflects the strong recovery of the east coast humpback whale population; the conditions of other whale species are largely inferred. Notably, the condition of populations of estuarine crocodiles is considered to have improved and retains a good condition grade.

Key knowledge gaps remain for many species and habitats that are not comprehensively monitored. The lack of information on many groups of species means that declines may not be evident until critical thresholds are exceeded. For some of the deeper and less accessible habitats, such as Halimeda bioherms and meadows, the continental slope and lagoon floor, the condition is inferred. Although knowledge of these habitats has improved since 2019, they are not frequently or comprehensively monitored. Information is needed to inform assessment of condition and trends in these habitats. 

The Region’s biodiversity has undergone relatively few intense disturbances, such as temperature extremes or cyclones since 2019, although impacts from high temperatures and successive cyclones in late 2023 and early 2024 are not yet fully understood. However, the threats posed by climate change have not abated, and most habitats and a range of species remain extremely vulnerable. Future deterioration in their condition is likely if threats persist and windows for recovery are not adequate.