Decadal trends in a coral community and evidence of changed disturbance regime

Abstract

A 23 year data set (1981-2003 inclusive) and the spatially explicit individual-based model ''Compete Ó '' were used to investigate the implications of changing disturbance frequency on cover and taxonomic composition of a shallow coral community at Lizard Island, Australia. Near-vertical in situ stereo-photography was used to estimate rates of coral growth, mortality, recruitment and outcomes of pair-wise competitive interactions for 17 physiognomic groups of hard and soft corals. These data were used to parameterise the model, and to quantify impacts of three acute disturbance events that caused significant coral mortality: 1982-a combination of coral bleaching and Crown-of-Thorns starfish; 1990-cyclone waves; and 1996-Crown-of-Thorns starfish. Predicted coral community trajectories were not sensitive to the outcomes of competitive interactions (probably because average coral cover was only 32% and there was strong vertical separation among established corals) or to major changes in recruitment rates. The model trajectory of coral cover matched the observed trajectory accurately until the 1996 disturbance, but only if all coral mortality was confined to the 3 years of acute disturbance. Beyond that date (1997)(1998)(1999)(2000)(2001)(2002)(2003), when the observed community failed to recover, it was necessary to introduce annual chronic background mortality to obtain a good match between modelled and observed coral cover. This qualitative switch in the model may reflect actual loss of resilience in the real community. Simulated over a century, an 8 year disturbance frequency most closely reproduced the mean community composition observed in the field prior to major disturbance events. Shorter intervals between disturbances led to reduced presence of the dominant hard coral groups, and a gradual increase in the slow growing, more resilient soft corals, while longer intervals (up to 16 years) resulted in monopolization by the fastest growing table coral, Acropora hyacinthus.

Recolonisation of Acropora hyacinthus following climate-induced coral bleaching on the Great Barrier Reef

“…However, relatively low growth rates have also been reported in the GBR (e.g. < 3 cm yr −1 by Wakeford et al 2008), emphasising the high variability in coral growth rates within and among reefs.…”

“…Nevertheless, a lower recovery capacity of Acropora has also been observed in some reefs, mainly attributable to environmental conditions or time between disturbances (van Woesik 2000, Berumen & Pratchett 2006). Direct observation as well as modelling have shown monopolization of A. hyacinthus over long intervals without disturbance and limited recovery with ongoing disturbance (Berumen & Pratchett 2006, Wakeford et al 2008). …”

“…A. hyacinthus also tends to dominate shallow, exposed reef habitats in the western Pacific, and exerts a major influence on community structure and dynamics there. High mortality and the rapid recovery of tabular corals due to high recruitment (Wallace 1985, Wakeford et al 2008 and fast growth create a complex mosaic of shaded and unshaded habitats (Sheppard 1981), making it possible for organisms with different life histories and physiological requirements to adapt to different stages of recovery (Baird & Hughes 2000).…”

Effectiveness of benthic foraminiferal and coral assemblages as water quality indicators on inshore reefs of the Great Barrier Reef, Australia

“…Although ecological studies on foraminiferal succession in the Pacific Region are lacking, ecological studies of symbiont-bearing foraminifera in the Florida reef tract have shown that populations recover from bleachingrelated mortality events or hurricane scouring in a matter of months to at most a year or two (Williams et al 1997;Hallock et al 2006, Baker et al 2009). This is in contrast with coral communities which may need decades to fully recover colony density and diversity (Done et al 2007;Wakeford et al 2008). Indeed, paleontological data suggest that foraminiferal assemblages are much more persistent over time than coral assemblages (Tager et al in press).…”

Genetic diversity and connectivity in a brooding reef coral at the limit of its distribution

“…Coral reefs have evolved under various intensities, modes and frequencies of disturbances, and reef coral populations have experienced severe reduction (Wakeford et al 2008) to complete eradication throughout their evolutionary history (e.g. Veron 1995).…”