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Climate Change

Can a complex model hold the fate of the crown-of-thorns starfish?

Article: Morello EB, Plaganyi EE, Babcock RC, Sweatman H, Hillary R, Punt A (2014) Model to manage and reduce crown-of-thorns starfish outbreaks. Mar Ecol Prog Ser 512: 167-183. Doi: 10.3354/meps10858

Cover Image: Amsler Kurt. “Crown of thorns starfish showing purple colouration”. Photograph. Ardea. <www.ardea.com>.

 

Introduction

Starfish may seem like cute, docile creatures, but this is not the case with the crown-of-thorns starfish (COTS). This venomous, spined starfish devours any coral in its path and, as of now, they are the leading cause of coral decline on the Great Barrier Reef. The frequency of COTS outbreaks have increased in the last fifty years, indicating reefs are unstable. While the initial cause of the outbreaks is still uncertain, we do know that an outbreak occurs when the COTS population consumes coral at a greater rate than the corals can grow in a year (or season). Three types of outbreaks can happen: a primary outbreak, where a small, localized population of COTS doubles in density, a secondary outbreak, where the COTS larvae from the primary outbreak spawn in a separate area (currents and wave action can carry larvae long distances), and finally a chronic outbreak occurs when the COTS simply gets out of control, growing substantially faster than the coral with no hope of the coral recovering.

Chronic outbreaks can obviously have serious consequences, therefore, researchers from Australia set out to develop a model to manage and reduce the COTS outbreaks.

 

Methods

While the cause of COTS outbreaks is still uncertain, three main hypotheses have been presented by other researchers:

  1. The rapid growth rate and high fecundity causes the COTS to have wide fluctuations in population numbers without any external factors being involved.
  2. The COTS larval survival is dependent on several factors, such as salinity and water temperature, phytoplankton biomass, and nutrient levels, which when all working together can cause a proliferation of larval success.
  3. COTS outbreaks seem to occur when predatation or predator numbers (includes triggerfish, pufferfish, molluscs, and other scavenger species) are low. This leads to the belief that popular fishing grounds enhance the survival of COTS by removing their predators.

These three hypotheses were put together in the conceptual framework of this project in order to build a model that represents the COTS population, its external factors, and its predator-prey relationships.

The first step in building this model was to create a diagram that focuses on the trophic relationship of the COTS (Fig. 1). The diagram will use the outcome of each step in the trophic cascade, rather than specifically seeing how the COTS population is affected by their consumption of coral. Data over the past 40 years was used to understand the population numbers of COTS and both fast and slow growing corals at Lizard Island, Australia. By fitting this data with the trophic relationships, the effects of the COTS can be quantified into three separate scenarios; (1) the effect of predation of adult COTS by large fishes, (2) the effect of predation on juvenile COTS by smaller invertebrate species, and (3) the effect of manual removal/poison injections of COTS in the absence of all predators.

Fig 1

 

 

 

 

 

 

 

 

 

 

 

Results

Fig 5

Fig 4. Projections (2011-2031) for Scenario 3, the effect of manual removal of COTS.

The built model not only adequately represented the data collected on Lizard Island, but also showed the projections for the three different scenarios that backed the proposed hypothesis of why COTS outbreaks occur. The first scenario suggested that consumption of COTS by large predatory fishes would reduce the COTS outbreak likelihood. The model was unable to distinguish a major difference between this scenario and the other two proposed (Fig 2, 3, and 4). Furthermore, it was shown that once an outbreak was already in the midst of occurring, injecting or manually removing COTS had minimal impact on the overall population (even when over 50% of the COTS population had been removed), leaving the corals still in danger. The COTS are just too hungry!

 

Fig 3

Fig 2. Results for the historical period (1970-2010) and of the projections (2011-2031) carried out to evaluate Scenario 1, showing interaction parameters of predation by large fish on COTS.

Fig 4

Fig 3. Projections (2011-2031) for Scenario 2, the effect of predation of different proportions on benthic invertebrates on COTS.

 

Significance

The COTS outbreaks have become a huge management problem across the world, especially on the Great Barrier Reef. Countless studies have been done and despite not knowing what causes the outbreaks, an effective management strategy still needs to be put in place. The aim of this study was to create a model, fitted to actual data, to describe the predator and prey relationship of the COTS. Assessing the impacts of removal strategy (both of predators and of the COTS itself) will allow us to further our understanding and hopefully manage the COTS population. While this study only used three scenarios, the built model has the ability to adapt and fit several different data representations. The inclusion of other predators (small fish that will feed on both juvenile and adult COTS), the role of no-take zones, nutrient loading and unloading, plus management tools such as disease and virus introductions into this model will help predict what strategies will work and what will not. With some configuring and adjusting, this model holds a bright future for the protection of coral reefs.

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