An Investigation of Environmental Drivers of Crown of Thorns Starfish Outbreaks on Guam

Abstract

One of the least understood and prevalent threats to Guam’s reefs is Acanthaster planci, more commonly known as Crown-of-Thorns Starfish (COTS). COTS are prolific corallivorous asteroid echinoderms. At high population densities, COTS can rapidly decimate coral reef ecosystems in a matter of months. Several devastating COTS outbreaks in Guam dating back to 1967 brought the understanding that COTS outbreaks are a real threat to Guam’s coral reefs. Considerable research has been done on COTS biology and several hypotheses have been put forward to explain the occurrence of outbreaks but the cause(s) still remain contentious. This study aims to relate the physical environment to COTS outbreaks in order to find supporting evidence that COTS outbreaks are linked to environmental conditions. We hypothesized that precipitation and low wave events were the two factors that set the stage for the occurrence and proliferation of COTS outbreaks in Guam. To investigate this, we calculated and mapped COTS populations to assess spatio-temporal variability. A seasonal climatology was constructed and surface mapped from regional model data to determine the mean seasonal environmental conditions. An event-based analysis was then performed on two sites, Gun Beach and Uruno Point, to discover any consistencies between the presence of COTS and environmental parameters at the sites. Overall, we were unable to find any consistent or significant spatio-temporal relationships that support our hypothesis. This study highlights the data sparseness of Guam and sets the stage for future research into COTS outbreak ecology in Guam. Further research and long-term fine-scale data will be necessary to accurately pinpoint the relationship between the physical environment and COTS outbreaks on Guam.