Stony coral tissue loss disease indirectly alters reef communities

Abstract

Many Caribbean coral reefs are near collapse due to various threats. An emerging threat, stony coral tissue loss disease (SCTLD), is spreading across the Western Atlantic and Caribbean. Data from the U.S. Virgin Islands reveal how SCTLD spread has reduced the abundance of susceptible coral and crustose coralline algae and increased cyanobacteria, fire coral, and macroalgae. A Caribbean-wide structural equation model demonstrates versatility in reef fish and associations with rugosity independent of live coral. Model projections suggest that some reef fishes will decline due to SCTLD, with the largest changes on reefs that lose the most susceptible corals and rugosity. Mapping these projected declines in space indicates how the indirect effects of SCTLD range from undetectable to devastating.

Fig. 1.. Stony coral tissue loss disease progressed throughout the U.S. Virgin Islands over a period of 27 months.

Temporal progression of SCTLD through the 34 TCRMP sites represented by an interpolated heatmap (A) and inset showing study site locations within the broader Caribbean region (B). SCTLD was first documented in St. Thomas, and then spread through St. John, followed by St. Croix. In little more than 2 years, SCTLD had infected all 34 TCRMP sites in the USVI. This figure uses an ArcGIS base map with the following attribution: “Esri, TomTom, Garmin, FAO, NOAA, USGS, and EPA.”

Fig. 2.. Stony coral tissue loss disease altered benthic communities in the U.S. Virgin Islands.

Results from multilevel Bayesian regression models of benthic organism percent cover over time since initial SCTLD infection, based on TCRMP data. (A) Standardized effect size estimates with 95% credible intervals (CIs) are shown in thick black lines, and 65% CIs are shown in thin black lines of each stat-eye plot. Plots where CIs cross zero, shown with a vertical dashed black line, did not respond to SCTLD, whereas the further the CI is from zero, the stronger the response to SCTLD. (B) Ten-year predictions of absolute cover of SCTLD-susceptible and SCTLD-resistant species. Points represent real observations in the TCRMP data, whereas lines represent mean predicted values for each island in the TCRMP dataset, and shaded regions show 95% confidence intervals of these predictions.

Fig. 3.. Existing networks between benthic organisms, rugosity, and fish are expected to influence SCTLD’s impact on Caribbean coral reefs.

Path diagram showing all statistically significant relationships identified with the SEM of NCRMP data. Region, year, and depth are not shown but were included as random variables. Path weights are proportional to the mean effect sizes of the group-centered predictor variables. All benthic space holders (SCTLD-resistant coral, SCTLD-susceptible coral, cyanobacteria, fire coral, macroalgae, and crustose coralline algae) were included as predictors for all other variables. Rugosity was included as a predictor of the sessile benthic organisms and all three fish variables. Significant positive relationships are represented by blue arrows, insignificant relationships are not shown, and negative relationships are shown in red.

Fig. 4.. Following the introduction of SCTLD, reef fish responses are predicted to vary across functional groups.

Predictions of absolute and relative changes after the introduction of SCTLD. Each thin line represents predictions at a site from the NCRMP dataset, and thick lines show regional mean values. Predictions were generated by applying rates of change in benthic cover associated with SCTLD observed in the TCRMP dataset to initial conditions in the NCRMP dataset, given relationships between variables that were quantified by the SEM. Predictions were generated out to 4 years after the initial SCTLD infection.

Fig. 5.. Community impacts of SCTLD are predicted to be highly geographically variable.

Map of predicted changes to SCTLD-susceptible coral, rugosity, coral-associated fishes, herbivorous fishes, and all other fishes after 4 years of SCTLD presence. Point size represents the magnitude of predicted change after 4 years, expressed as the number of SDs from the initial values. Larger points represent the greatest predicted changes, red points represent sites where decreases are predicted, and black points represent sites with predicted increases. Latitude and longitude lines for each region (Dry Tortugas, Puerto Rico, and USVI) are shown in the leftmost map of each region. This figure uses an ArcGIS base map with the following attribution: “Esri, HERE, Garmin, GEBCO, EPA, FAO, FDEP, Foursquare, Kadaster Netherlands, METI/NASA, NaturalVue, NPS, SafeGraph, USVI GIS Division, USDA, and USGS.”