Change in resilience of two coral reef communities over twenty five years : influence of disturbance and other mechanisms
Creators: name: Wakeford, M
Publisher: name: University Of Tasmania
Year: 2023

description: Mechanisms underlying the dynamics of two shallow coral reef communities at Lizard Island, Great Barrier Reef, Australia, over a twenty five year period (1981-2005 inclusive) were investigated using time series of photographs and a spatial model. At the first study site (sub-tidal, ~2 m depth) methods were developed to generate a spatial model based on local processes and disturbances. Rates for coral growth, mortality, recruitment and outcomes of pair-wise competitive interactions determined from the photographs were used to parameterise the model. Three acute disturbance events occurred at the site and determine the disturbance schedule of the model: 1982- a combination of coral bleaching and crown-of-thorns starfish; 1990 - cyclone waves; and 1996 - crown-of-thorns starfish. Predicted coral community trajectories at the first site were not sensitive to the outcomes of competitive interactions, probably due to the high proportion of substratum not covered by coral (average 68%) and strong vertical separation among established corals), or to major changes in recruitment rates. The model trajectory accurately matched the observed trajectory until the 1996 disturbance, but only if coral mortality was confined to the three years of acute disturbance. Beyond that date (1997-2003), when the observed community failed to recover, it was necessary to introduce annual background mortality to obtain a good match between modelled and observed coral cover. This qualitative switch in the model may reflect a 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. At the second site (inter-tidal) a second spatial model was generated based on rates of coral growth, mortality, recruitment and outcomes of pair-wise competitive interactions measured at that site. The disturbance schedule (duration, timing and impact of disturbance events) varied between sites and illustrates how the manifestation of reef-wide disturbances (namely bleaching, crown-of-thorns starfish and cyclones) was highly variable at small spatial scales. Three putative causes of disturbance were as follow at the site: 1982 to 1989 - a combination of coral bleaching and crown-of-thorns starfish; 1990 to 1992 cyclone damage / structural weakening; and 1996 to 1999 - crown-of-thorns starfish. While coral species represented at the two sites (separated by several kilometres) were similar, their starting states and 25 year trajectories describing temporal change in community structure were very different. Obtaining a reasonable fit of the model to observed trajectories, required inclusion of site-specific disturbance schedules that reflected local site-specific variations in shelter and depth. Manipulation of the model revealed that differences in disturbance schedule between sites were more important than differences in growth, recruitment or interaction outcomes in driving community dynamics. Importantly, despite differences in disturbance schedules and community dynamics, the model indicates a qualitative switch from acute to chronic disturbance around the turn of the century, which is independent of local community structure and site-specific disturbance schedules.