Australia’s marine ecosystems are experiencing demonstrable declines in biodiversity and ecosystem condition driven by multiple and interacting pressures. Recent assessments have documented continent-wide declines in shallow reef life alongside widespread declines across Australian fish stocks (Edgar et al. 2023; Edgar et al. 2018). Climate change, including marine heatwaves and associated ecological change (Oliver et al. 2021; Wernberg et al. 2016), habitat degradation resulting from land-based impacts (Brown et al. 2019), bottom trawling and other industrial fishing impacts (Clark et al. 2016; Lack 2010), marine debris and plastic pollution (Hardesty et al. 2017), and expanding industrial use of marine environments continue to place additional pressure on Australian marine ecosystems. In this context, ensuring that highly protected marine parks are sufficiently large, representative and effectively managed is an important component of conserving biodiversity, rebuilding depleted populations and maintaining ecosystem resilience.

Scientific assessments consistently conclude that protecting at least 30% of marine ecosystems within highly protected no-take areas is necessary to maintain biodiversity, ecological processes, fisheries productivity and climate resilience (O'Leary et al. 2016; Jones et al. 2020; Sala et al. 2021). Importantly, these recommendations do not refer simply to protecting 30% of the ocean overall. Conservation outcomes depend on ensuring protection is distributed across the full diversity of marine ecosystems. Consequently, contemporary conservation science emphasises protecting at least 30% of each marine bioregion, habitat type and ecosystem within highly protected no-take areas, ensuring protection is representative across the full diversity of marine ecosystems (ANZECC Task Force on Marine Protected Areas 1999; Margules & Pressey 2000; Pressey et al. 2007; Sala et al. 2021).

Highly protected no-take marine parks are among the most extensively studied conservation interventions in the marine environment. Across hundreds of marine protected areas worldwide, highly protected no-take reserves consistently support greater biodiversity, biomass, abundance, body size and trophic complexity than adjacent fished areas (Lester et al. 2009; Edgar et al. 2014; Sala & Giakoumi 2018). In one of the largest global syntheses, Lester et al. (2009) reported that no-take reserves supported 446% greater biomass, 166% higher abundance, 28% larger body sizes and 21% higher species richness than adjacent fished areas. Similar patterns have subsequently been documented across diverse ecosystems and geographic regions, with effective no-take reserves supporting greater abundance of top predators, more natural food-web structure and enhanced ecosystem functioning than less protected or fished locations (Edgar et al. 2014; MacNeil et al. 2015; Sala & Giakoumi 2018). These ecological benefits can also extend beyond reserve boundaries through spillover of adults and export of larvae, contributing to fisheries productivity while protecting breeding populations within reserves (Roberts et al. 2001; Harrison et al. 2012; Lynham et al. 2022).

Protection level is a strong predictor of ecological performance, with outcomes generally improving as extractive activities are reduced and reaching their greatest magnitude in highly protected no-take areas. Comparative assessments show that fully protected reserves consistently outperform partially protected areas across a range of ecological indicators, including biomass, abundance, species richness and trophic structure (Sciberras et al. 2015; Sala & Giakoumi 2018; Zupan et al. 2018). In contrast, partially protected areas frequently deliver weaker and more variable outcomes and, in some cases, ecological conditions are difficult to distinguish from those observed in fished locations (Sciberras et al. 2015). While partially protected areas can contribute to broader marine planning objectives, the weight of evidence indicates that the strongest, most reliable and most enduring conservation outcomes are achieved where extractive activities are excluded entirely (Sala & Giakoumi 2018).

Highly protected marine parks are increasingly recognised as important tools for enhancing climate resilience and supporting ecosystem recovery in a rapidly changing ocean. By maintaining biodiversity, ecological redundancy, trophic complexity and ecosystem function, no-take marine parks can improve the capacity of marine ecosystems to withstand and recover from disturbance (Roberts et al. 2017; Sala et al. 2021). These benefits are particularly important as marine heatwaves, species redistributions and other climate-driven impacts become more frequent and severe. While marine parks cannot prevent the direct effects of ocean warming, they can reduce local pressures and help maintain the ecological attributes that underpin resilience and recovery. Recent syntheses similarly conclude that long-term, highly protected marine parks can contribute to resilience in multi-stressor environments by reducing local stressors and supporting ecosystem recovery, even where they cannot eliminate climate-driven threats (Filbee-Dexter et al. 2024).

Collectively, the evidence demonstrates that achieving biodiversity conservation, fisheries benefits and climate resilience requires not only sufficient area under protection, but sufficient levels of protection distributed representatively across Australia's marine bioregions and ecosystems.

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