Drivers of recovery and reassembly of coral reef communities

Abstract

Understanding processes that drive community recovery are needed to predict ecosystem trajectories and manage for impacts under increasing global threats. Yet, the quantification of community recovery in coral reefs has been challenging owing to a paucity of long-term ecological data and high frequency of disturbances. Here we investigate community re-assembly and the bio-physical drivers that determine the capacity of coral reefs to recover following the 1998 bleaching event, using long-term monitoring data across four habitats in Palau. Our study documents that the time needed for coral reefs to recover from bleaching disturbance to coral-dominated state in disturbance-free regimes is at least 9-12 years. Importantly, we show that reefs in two habitats achieve relative stability to a climax community state within that time frame. We then investigated the direct and indirect effects of drivers on the rate of recovery of four dominant coral groups using a structural equation modelling approach. While the rates of recovery differed among coral groups, we found that larval connectivity and juvenile coral density were prominent drivers of recovery for fast growing Acropora but not for the other three groups. Competitive algae and parrotfish had negative and positive effects on coral recovery in general, whereas wave exposure had variable effects related to coral morphology. Overall, the time needed for community re-assembly is habitat specific and drivers of recovery are taxa specific, considerations that require incorporation into planning for ecosystem management under climate change.

Keywords: bleaching; community; coral reef; disturbance; recovery; recruitment.

Figure 1.

Conceptual diagram illustrating in the lower half, a coral reef (bottom right) impacted by an acute disturbance, transitioning (grey arrows) to a non-coral dominated reef (bottom left) [3] and recovering to a high coral state going through the phases of coral populations growth. Boxes are ecological processes influencing recovery rate. The upper half of the diagram represents the coral recovery structural equation model (SEM). Each path (direct or connected) ending with an arrow head represents a model component. Black circles show interactions among variables. Coral legacies represent the coral percentage cover after the disturbance.

Figure 2.

Map of Palau with the location of long-term coral reef monitoring sites in each reef habitat.

Figure 3.

Temporal dynamics in benthic communities in the four habitats and two depths. n = 20 to 30 × 50 m transects for outer reefs, n = 10 to 30 for inner reefs, and n = 10 to 15 for patch reefs. The red star represents qualitative coral cover range estimation from 20 m depth to the shallows [31,39]. Black bars represent degree heating weeks (DHW) in 1 July–30 Sept 1998, 2010 and 2014 [40]. Black lines in the eastern outer reef habitat represent two typhoons occurrence (December 2012 and November 2013, [29]). Grey arrows show survey times (electronic supplementary material, table S1).

Figure 4.

Non-metric multidimensional scaling plots (nMDS) of coral community matrices within each habitat and depth. Overlaid vectors show coral taxa that correlate mostly with the change of communities. Photographs show coral reef communities in their most coral abundant state ((a) © Tane H. Sinclair-Taylor, (b–d) © PICRC).

Figure 5.

Coral recovery structural equation models (SEM) showing direct and indirect effects of coral demographics, spatial and environmental variables on the rate of recovery (IGR) of three coral groups. Continuous variables are averages during the recovery phase of each coral group. The thickness of paths is proportional to the given standardized path coefficients. Black and red arrows indicate positive and negative pathways, respectively. Non-significant pathways are not shown.