Connecting the dots helps manage COTS

Connectivity networks and the control of Crown-Of-Thorns starfish on the GBR


KEY MESSAGES:
  • Crown of thorns starfish are a major threat to the GBR
  • Outbreaks spread like an epidemic as starfish larvae drift from reef to reef
  • Studying how reefs are connected by larval dispersal is generating valuable insights on how COTS outbreaks can be anticipated and managed

Crown of Thorns starfish (Image: Mark Priest)

Crown of Thorns starfish (Image: Mark Priest)

Outbreaks of coral-eating Crown-of-Thorns Starfish (COTS) can seriously affect the health of the Great Barrier Reef (GBR). Periodically there are local explosions in the numbers of these large, thorny starfish. Aggregations of adult starfish can literally strip reefs of their coral cover. It’s been estimated that as much as 40% of coral cover loss can be attributed to COTS-related causes.

Outbreaks spread among the reefs in an epidemic-like fashion as COTS larvae are transported by ocean currents. Studying how reefs are connected by larval dispersal help us to represent the GBR as an ecosystem-sized connectivity network. Such network models can then be used to determine which reefs will be more likely to spread COTS larvae, as well as which reefs will be at a greater risk of outbreaks due to their exposure to COTS larvae.

Analysis of COTS connectivity networks reveal high concentrations of well-connected reefs in regions of the GBR where major COTS outbreak events have historically originated, most notably around Cooktown and Cairns in the north and the Swains in the south (Hock et al, 2014).

A powerful combination of high connectivity among the nearby reefs and a potential to link up a wider region could turn a local build-up of COTS populations into a large-scale cascade of outbreaks. This correspondence between the connectivity metrics and field observations suggests that the GBR is exceptionally sensitive to COTS dynamics on reefs in these regions. Monitoring COTS populations at specific reefs in these areas could therefore provide early warning signs that would alert the managers to the initiation of future outbreak cascades.

Figure 1: Connectivity patterns indicate the potential of individual reefs to spread COTS larvae towards the major tourism sites in the northern GBR. Geographical proximity of the reefs to tourism sites is not always the best predictor of this potential because of a strongly directional transport of COTS larvae by the ocean currents. Reefs with tourism sites represented as hexagons; other reefs represented as coloured circles according to their predicted potential to spread COTS larvae towards tourism sites; thickness of links indicates the predicted levels of larval transport between reefs. (Reprinted with permission from Hock et al, 2016).

Figure 1: Connectivity patterns indicate the potential of individual reefs to spread COTS larvae towards the major tourism sites in the northern GBR. Geographical
proximity of the reefs to tourism sites is not always the best predictor of this potential because of a strongly directional transport of COTS larvae by the ocean
currents. Reefs with tourism sites represented as hexagons; other reefs represented as coloured circles according to their predicted potential to spread COTS larvae towards tourism sites; thickness of links indicates the predicted levels of larval transport between reefs. (Reprinted with permission from Hock et al, 2016).

While connectivity could help us predict the onset of future outbreak cascades, the pressing issue right now is that the GBR is currently in the middle of a major COTS outbreak cascade. Field control methods to remove adult COTS during outbreaks are resource- and time-intensive, and the logistics of intervening on remote reefs as well as the sheer size of the affected area emphasise the need to deploy the control efforts in a targeted manner in order to maximise their impact.

COTS connectivity patterns reveal the potential pathways and reefs through which the outbreaks could spread, and possibly endanger major tourism sites in the northern GBR (Figure 1; Hock et al, 2016). Management resources can then be deployed to these reefs in an attempt to interfere with the range expansion and limit future distribution of COTS.

Decisions on which populations to target can be also be adaptively adjusted as new information on outbreak distribution becomes available, further improving the allocation of COTS control efforts.

While it might not be possible to completely stop the COTS outbreaks from spreading with the current field control methods, connectivity patterns can nevertheless help the managers decide where to deploy the best available practices while considering system-wide consequences of local management actions.

This research was performed in the Marine Spatial Ecology Lab at the University of Queensland and CEED, and realised through collaborations with CSIRO Oceans & Atmosphere, Australian Institute of Marine Science (AIMS), and the Great Barrier Reef Marine Park Authority (GBRMPA).


More info: Karlo Hock k.hock1@uq.edu.au

References

Hock K, NH Wolff, SA Condie, KRN Anthony & PJ Mumby (2014). Connectivity networks reveal the risks of crown-of-thorns starfish outbreaks on the Great Barrier Reef. Journal of Applied Ecology 51:1188-1196. http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12320/abstract

Hock K, NH Wolff, R Beeden, J Hoey, SA Condie, KRN Anthony, HP Possingham & PJ Mumby (2016). Controlling range expansion in habitat networks by adaptively targeting source populations. Conservation Biology http://onlinelibrary.wiley.com/doi/10.1111/cobi.12665/abstract

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