Context and challenge

Australia’s Great Barrier Reef (GBR) is the world’s largest coral reef ecosystem, and one of the country’s most important ecological and economic assets. Most of the GBR is enclosed within the Great Barrier Reef Marine Park (GBRMP), a multiple- use marine park comprising eight different usage zones (figure 1), with one-third zoned no-take. The Australian government, acting primarily through the GBRMP Authority, is responsible for management, undertaken in conjunction with other federal and Queensland agencies, Indigenous Traditional owners and various other stakeholders.

Figure 1. Great Barrier Reef Marine Park zoning (resulting from the 2003 Zoning Plan, in effect since 1 July 2004). Map supplied by Spatial Data Centre, Great Barrier Reef Marine Park Authority, © Commonwealth of Australia (GBRMPA).

Although the GBRMP was originally created to protect the reef from mining exploration, its coral reefs are now mainly threatened by recurrent bleaching, cyclones and outbreaks of crown-of-thorns starfish. Large areas, particularly the inshore and northern reefs, have lost large proportions of their live coral cover in recent years. Secondary threats include adverse water quality, unsustainable fishing, dredging and coastal development. Despite these pressures, the condition of the GBR is good compared with that of many other reef systems globally.

Approach

Conservation of the GBR’s coral habitat requires three types of connectivity to be protected. The first, and most important, is larval connectivity: most organisms on reefs have an obligate pelagic larval dispersive phase making connectivity a constant demographic necessity. Oceanic currents create spatiotemporally complex larval connectivity patterns that drive population dynamics on the GBR. These connectivity patterns are similar to terrestrial ecological corridors, but the dispersing organisms are not exposed to threats during dispersal, and so marine ecological corridors do not require protection. Instead, conservation outcomes are enhanced by networks of marine reserves that exchange large amounts of larvae, while fishery outcomes are improved when no-take zones are connected to fished areas. Understanding larval source populations is important for applying these design considerations into practice (figure 2). The second form of connectivity is ontogenetic migration, typically where species spend their early life-stages in estuarine/inshore habitats, before migrating offshore as adults; Figure 3 shows one example. The third is small-scale movement of adults for foraging or reproducing. Most coral reef species are benthic- associated, and so these movements occur at within-reef scales. However, pelagic species can undertake longer- distance adult movements between reefs.

The GBRMP was substantially rezoned and expanded in 2003, based on systematic planning principles. Eleven biophysical operating principles (BOPs) (GBRMPA, 2002) were devised to protect representative examples of each of the GBR’s 70 bioregions (30 reef habitat; 40 non-reef) (Fernandes et al., 2005). The maintenance of connectivity was also an explicit goal of the marine park – both the total size of the no- take marine reserves and their individual locations were taken into account. As an overarching goal, BOP 9 recommended that no-take zones be chosen to maintain connectivity across the GBR. However, minimal data about connectivity were available at the time of the rezoning, and so several of the BOPs were designed to prioritise potential proxies for each form of connectivity. BOPs 1 and 2 aimed to protect larval connectivity, particularly self-recruitment. For example, BOP 2 recommended that no-take zones be as large as possible, motivated by models indicating self-recruitment increased with reserve dimensions. BOP 4 recommended that no-take zones include whole reefs where possible, to protect connectivity for foraging and migrating adults.

Figure 2. Potentially robust source reefs identified by modeling coral larval dispersal from populations with known resilience to coral bleaching and Crown of Thorns Starfish (COTS) attacks (Hock et al., 2017).

Results

Little information on connectivity was available for the 2003 rezoning, so proxies were used to design networks of no-take zones that would ensure the exchange of larvae between them, as well as the export of larvae to fished areas. Subsequent empirical studies and biophysical modelling demonstrated that this approach was successful to some extent, with larval dispersal connecting no-take zones at a range of scales, from local self-recruitment (Harrison et al., 2012) to consistent bi- directional exchanges of over 250 km (Williamson et al., 2016; Bode et al., 2019). 

There are three possible reasons why a network of no-take zones that was not designed with explicit connectivity data was nevertheless able to achieve connectivity outcomes. First, the GBRMP contains a very large proportion of effective no-take zones (33% of the entire area). We would generally expect that higher levels of protection will achieve superior connectivity outcomes. Second, explicit connectivity proxies form the basis of several BOPs, and these likely improved connectivity outcomes beyond the simple null expectation.

The final reason is less obvious. The GBRMP is a global exemplar of a systematically planned network. Several BOPs (specifically 5 and 7) aimed to create a ‘representative’ network, with no-take zones distributed across bioregions, latitudes and cross-shelf position. While these goals do not mention connectivity, evidence suggests that representativeness allows no-take networks to effectively protect previously unknown biodiversity features (e.g. mesophotic reefs, as in Bridge et al., 2016). It is entirely possible that representativeness principles are also responsible for the protection of connectivity in the GBR.

Figure 3. ‘Crossing the Blue Highway’: The red emperor (Lutjanus sebae) spends different stages of its life cycle utilising different habitats across the GBR. © Russell Kelley/Australian Coral Reef Society, http://www.russellkelley.info/print/the-blue-highway/. See also: Cappo, M. & Kelley, R. (2001). Connectivity in the Great Barrier Reef World Heritage Area – An overview of pathways and processes. In Wolanski, E. Oceanographic processes of coral reefs: physical and biological links in the Great Barrier Reef: 161-187. CRC Press, New York.

References

Bode, M., Leis, J.M., Mason, L.B., Williamson, D.H., Harrison, H.B., Choukroun, S., and Jones, G.P. (2019). ‘Successful validation of a larval dispersal model using genetic parentage data’. PLoS Biology 17(7):e3000380. https://doi.org/10.1371/journal.pbio.3000380 .

Hock K, Wolff NH, Ortiz JC, Condie SA, Anthony KRN, Blackwell PG, et al. (2017) Connectivity and systemic resilience of the Great Barrier Reef. PLoS Biol 15(11): e2003355. https://doi.org/10.1371/journal.pbio.2003355. 

Fernandes, L., Day, J.O.N., Lewis, A., Slegers, S., Kerrigan, B., Breen, D.A.N., Cameron, D., Jago, B., Hall, J., Lowe, D. and Innes, J. (2005). ‘Establishing representative no-take areas in the Great Barrier Reef: Large-scale implementation of theory on marine protected areas’. Conservation Biology 19(6):1733–1744. https://doi.org/10.1111/j.1523-1739.2005.00302.x.

GBRMPA (Great Barrier Reef Marine Park Authority) (2002). GBRMPA Technical Information Sheet No.6: ‘Biophysical Operational Principles as recommended by the Scientific Steering Committee for the Representative Areas Program’. http://www.gbrmpa.gov.au/__data/assets/pdf_file/0011/6212/tech_ sheet_06.pdf

Harrison, H.B., Williamson, D.H., Evans, R.D., Almany, G.R., and Thorrold, S.R. (2012). ‘Larval export from marine reserves and the recruitment benefit for fish and fisheries’. Current Biology 22:1023– 1028. https://doi.org/10.1016/j.cub.2012.04.008.

Williamson, D.H., Harrison, H.B., Almany, G.R., Berumen, M.L., Bode, M., Bonin, M.C, Choukroun, S., Doherty, P.J., Frisch, A.J., Saenz-Agudelo, P., and Jones, G.P. (2016). ‘Large-scale, multidirectional larval connectivity among coral reef fish populations in the Great Barrier Reef Marine Park’. Molecular Ecology 25(24):6039– 6054. https://doi.org/10.1111/mec.13908.