Spatial connectivity of coral and Symbiodinium populations on the Great Barrier Reef
Project Leader: Prof Terry Hughes, JCU & Dr Julian Caley, AIMS
Increasing understanding of patterns of spatial connectivity within and between reefs of the Great Barrier Reef - effectively, the capacity for replenishment of reefs following patchy disturbance - is important if the Reef is to be managed for resilience. This MTSRF-funded project has provided insights into how populations of the ubiquitous brooding coral Seriatopora hystrix are related between exposed and sheltered habitats at small spatial scales within the Palm and Lizard Island groups. Exposed-habitat populations were found to be more closely related between the two Island groups (which are ~500 km apart), than were corals from sheltered and exposed habitats within each Island group (1-10 km apart). These preliminary results suggest that there may be high rates of self-recruitment and population inbreeding in sheltered habitats within protected bays, indicating that genotypes in these habitats may be particularly vulnerable to disturbance and extinction. In other surveys, the Symbiodinium strain hosted by the coral Acropora millepora was found to be constant through space and time within the Palm Island group. More than 99% of coral colonies sampled (n = >400) hosted exclusively Symbiodinium strain C2 (sensu van Oppen) regardless of whether they were healthy or bleached. However, the population-level genetic composition of Symbiodinium C2 at seven reef sites differed significantly between sampling years and between healthy and bleached sub-populations. These population genetic shifts could potentially be attributed to genetic drift in asexually maintained populations, sexual recombination, acquisition of new Symbiodinium genotypes from the external environment, and/or natural selection. Elucidating the causal mechanism(s) requires further research into the basic life history characteristics of Symbiodinium, including the timing and location of sexual reproduction.
Sea water heating rate and host density are critical precursors for coral disease outbreaks
Project Leader: Dr Julian Caley, AIMS & Prof Bette Willis, JCU
Links have been made between anomalously high summer temperatures and outbreaks of the group of coral diseases known as White Syndrome on Indo-Pacific reefs. However, further advances in understanding their aetiologies and in developing management actions to mitigate their impacts are hampered by not knowing where or when outbreaks will occur. Before 2009, the only known outbreaks of White Syndrome on the Great Barrier Reef were documented in 2002. Outbreak sites tended to experience high values of heating rate (a metric developed as a measure of thermal stress) and also had high cover of corals within the genus Acropora, the primary hosts. MTSRF funding enabled the development of an empirical regression model that explains 93% of the variation in the abundance of White Syndrome observed during 2002, and suggests that abundance of White Syndrome increases exponentially as both heating rate and host cover increase. We used the model to hindcast the likelihood of outbreaks on Australian reefs for 2002 and each following summer (combining data from 45 sites surveyed from 2003 to 2008), producing no false negative or false positive predictions during this period. The model identified reefs with high heating rates in 2009 and forecast high outbreak likelihood in both the north-central and southern Great Barrier Reef. Targeted surveys to evaluate the efficacy of the model detected greatest abundance of White Syndrome at a site with high heating rate but medium host cover. The imperfect fit of the forecast with survey data highlights the need to reconsider host density threshold requirements for outbreaks. This work has shown that forecasting coral disease outbreaks in an era of changing climate is feasible (at least for selected disease syndromes) and requires integration of biological and physical data. The predictive tools being developed through this project are already helping managers of Australian reefs to target research and monitoring to enable informed responses to future outbreaks of coral disease.
Quantifying levels of herbivory in macro-algal phase shifts
Project Leader: Prof Terry Hughes, JCU
Inshore reefs are increasingly showing their susceptibility to human-induced changes. The vulnerability of marginal systems has been highlighted by one algal species, Lobophora, a brown leathery alga which appears to be highly resistant to grazing. As a result, this species may have the capacity to dominate inshore reefs regardless of herbivore presence, and surveys have shown that it was this species that drove recent algal outbreaks in the Keppel Islands and other inshore areas of the Great Barrier Reef. Further analyses identified extensive spatial variation in the capacity for algal removal. In the north (Cairns - Low Isles region), algal removal by herbivores from inshore reefs was strong, with good prospects for continued coral development. By contrast, just 5% of that effective herbivore activity was observed at the Keppel Islands, due to low herbivore numbers and low feeding rates. Our observations suggest that southern reefs, and the Keppels in particular, are more likely to become dominated by macro-algae, and once in place the algae are likely to remain. While protection of herbivorous fishes is a clear and logical management priority, this may not
be sufficient, as herbivore effectiveness depends on both fish abundance and fish behaviour. The ongoing challenge is to understand the environmental factors that moderate herbivore activity on inshore reefs.
Evaluating long term recovery and resilience of reef fish communities to climate change
Project Leader: Prof Terry Hughes, JCU
Climate change poses a major threat to the Great Barrier Reef, and the most immediate threat comes from sustained and ongoing increases in sea surface temperatures. The future of coral reef ecosystems is heavily dependent upon the rate at which populations and communities can recover from successive disturbances associated with periodic temperature extremes. Ongoing monitoring of fishes and corals has been conducted in the central Great Barrier Reef following significant coral bleaching and associated habitat degradation in 2001-02. Significant bleaching and subsequent coral mortality reduced coral cover to <5% across several reefs in this region, and cover
has generally remained low for the subsequent eight years. Even at locations where there has been spectacular recovery of coral cover, the structure of coral assemblages is markedly different in the aftermath of the 2001-02 bleaching event. These changes in cover and composition of reef-building corals have had significant effects on fi sh communities. Most notably, coral-feeding butterflyfishes all but disappeared following extensive coral loss, and recovery has been very slow and highly variable among species. Directional shifts in the structure of butterflyfish assemblages have been strongly influenced by changes in the structure of coral assemblages. These results suggest that there is limited resilience among fish and coral assemblages in the central Great Barrier Reef, and that further increases in the frequency and/or severity of warm water bleaching will cause major (potentially catastrophic) changes in the structure of local reef assemblages. Even if reef assemblages are resilient, and simply require more than eight years to become re-established, it seems unlikely that contemporary reef assemblages could be sustained in the face of future global warming. Further research is urgently required to assess what factors are limiting the resilience of reef assemblages in the central Great Barrier Reef.
Preliminary status and trends report completed in June 2007 by C. Cvitanovic, R. J. Fox and D. R. Bellwood, School of Marine and Tropical Biology, James Cook University.