Connectivity networks reveal the risks of crown‐of‐thorns starfish outbreaks on the Great Barrier Reef

Abstract

Summary1. Many ecosystems suffer systemwide outbreaks of damaging species propagating from primary outbreak sites. Connectivity patterns can identify parts of the ecosystem that help turn local outbreaks into a systemwide contagion through a series of transmission events. Here, we show that patterns of larval connectivity among reefs can help explain periodic crown-ofthorns starfish (COTS) epidemics across the Great Barrier Reef (GBR). 2. We simulated potential dispersal of COTS larvae to obtain a connectivity network of coral reefs across the entire GBR. Network analysis revealed areas of high local connectivity where any outbreaks could be amplified locally, as well as those areas with potential to cause largescale epidemics with ecosystem-wide impacts. 3. We find that the regions where COTS epidemics are known to originate are predictable from their high local and systemwide connectivity. Extensive larval exchanges among reef clusters in these regions can start a chain reaction of COTS population build-up. The same regions also have high potential to reach and affect other parts of the GBR, thereby maximizing the likelihood that any outbreaks would eventually propagate throughout the ecosystem. 4. Hydrodynamic properties and geography of the GBR make it vulnerable to COTS epidemics. Using network analysis to identify regions with high-risk high-impact sources could help control these devastating events in future. 5. Synthesis and applications. The observed centre of origin for COTS epidemics (the Cooktown-Cairns region) can be predicted from its elevated short-and long-range levels of larval connectivity. Connectivity analysis of per-reef risks provides spatially explicit targets to guide surveillance and control measures that might help curtail COTS epidemics through prioritization of highly connected reefs. The analytical approach developed here for COTS connectivity can also be applied to identify well-connected patches and regions in other interconnected ecological systems.

30 Years of Research on Crown-of-thorns Starfish (1986-2016): Scientific Advances and Emerging Opportunities

“…However, the spread of population outbreaks, determined based on field surveys Miller 2000;Vanhatalo et al 2016) and modelling (Dight et al 1990a(Dight et al , 1990bReichelt et al 1990;Black and Moran 1991; Black 1993;Hock et al 2014;Vanhatalo et al 2016) shows that at least some larvae must be dispersed and settle on non-natal reefs, regardless of the presence of adult CoTS. Improvements in hydrodynamic models, combined with advances in computational power and new methods for analysing patterns of particle dispersal, are providing increasingly resolved and tractable models to inform patterns of initiation and spread for CoTS outbreaks (Dight et al 1990a(Dight et al , 1990bBlack et al 1995;Hock et al 2014;Hock and Mumby 2015). However, these models are potentially very sensitive to the precise timing of spawning and the relevant speed and direction of currents, and predictions arising from these models need explicit testing based on extensive spatial and temporal sampling to resolve the occurrence and timing of outbreaks.…”

“…Elevated nutrients may therefore, be a necessary precursor for outbreaks to become established, but there are other conditions that must also be met. Modelling studies by Wooldridge and Brodie (2015) suggested that it is inter-annual variation in levels of larval retention (see Hock et al 2014) that may explain when, and perhaps where, primary outbreaks become established. Importantly, hydrodynamic conditions would have promoted high levels of self-recruitment (rather than dispersal of larvae among widely separated reefs) in years prior to outbreaks arising in 1979(Wooldridge and Brodie 2015.…”

“…solaris on the GBR are initiated on mid-shelf reefs between Cooktown and Cairns, and mainly in the northern portion of this area (commonly referred to as the "initiation box"). This region is characterized by high densities of individual reefs and highly restricted water movement (Hock et al 2014), which will promote steady and sustained increases in local densities of A. cf. solaris during successive years of spawning.…”

“…This prohibits meaningful comparisons of putative predator densities or nutrient concentrations among reefs that do and do not sustain primary outbreaks. There is also very limited monitoring of biological communities and environmental conditions at spatial and temporal scales relevant to explain the initiation of CoTS outbreaks (sections 2.1-2.2).For secondary outbreaks, the predominant factor that will influence (or at least fundamentally constrain) when and where outbreaks arise is the extent of larval delivery via hydrodynamic connectivity (Black and Moran 1991;Hock et al 2014). That said, the delivery of high densities of CoTS larvae to individual reefs would not necessarily result in the establishment of population outbreaks if there were local constraints on larval survival and development or high rates postsettlement mortality (Fig.…”

“…Recent developments in the hydrodynamic modeling for the GBR (e.g., eReefs) have reinvigorated the attempts to understand CoTS population dynamics and provide management solutions. A connectivity network model for CoTS on the GBR based on hydrodynamic models generated from eReefs (Hock et al 2014) aimed to identify the most important areas for initiating and spreading CoTS outbreaks via their relative importance within the network. This approach has since been used as the basis of an adaptive management strategy, whereby reefs targeted for control are selected based upon their likelihood to spread future outbreaks (Hock et al 2016).…”