Ecological structure and processes on disturbed coral reefs

Disturbances affecting ecosystems are becoming more diverse, frequent and intense, resulting in many examples of degraded ecosystems worldwide. Degradation involves fundamental changes to the system's community structure and function, and can have negative implications environmentally, socially and economically. Conditions, processes and management strategies that prompt recovery are therefore desirable, but details remain equivocal. Coral reefs are one such complex, diverse and threatened ecosystem. A primary consequence of coral reef degradation has been the development of alternative benthic conditions, however our understanding of their implications for other aspects of reef assemblages is limited. This thesis aims to understand the role of reef condition in shaping ecological communities and influencing key ecological processes on disturbed coral reefs.\ud \ud To investigate processes operating in a particular system, it is first important to characterise the extent of its spatial variability. Reefs of the inner Seychelles islands were chosen because they were severely disturbed by the 1998 bleaching event, and now exist in a range of benthic conditions that might be representative of reef conditions elsewhere in the future. In chapter 2, I quantitatively characterised 21 reefs within a 3600 km² area of the inner Seychelles by considering their underlying substrata, benthos, and associated fish assemblages. High variability in benthic composition was found among the reefs, with a gradient from high coral cover (up to 58 %) and high structural complexity to high macroalgae cover (up to 95 %) and low structural complexity at the extremes. This gradient was associated with declining species richness of fishes, reduced diversity of fish functional groups, and lower abundance of corallivorous fishes. Reefs grouping at the extremes of complex coral habitats or low-complexity macroalgal habitats displayed markedly different fish communities, with only two species of benthic invertebrate feeding fishes in greater abundance in the macroalgal habitat. Moreover, much of the variability seems to be independent of fishing pressure, indicating that there are deeper processes involved. These results have negative implications for the continuation of many coral reef ecosystem processes and services if more reefs shift to extremely degraded conditions dominated by macroalgae.\ud \ud In systems that have suffered local population-extinctions of key organisms (e.g. branching Acropora corals), recruitment to replenish depauperate populations is essential. However, isolated reefs such as the Seychelles may only receive a limited supply. Other potential bottlenecks to re-establishing a thriving coral population include low settlement rates, and high mortality of new recruits or juvenile corals. In chapter 3 I investigated spatial variation in local abundance of scleractinian corals at three distinct life history stages (recruits, juveniles, and adults) on reefs with differing benthic conditions. On rubble-dominated reefs, which were characterised by low cover of macrobenthic organisms and unconsolidated substrata, cover of adult corals was very low compared to coral-dominated reefs despite similar densities of juvenile corals and incoming coral recruits. A bottleneck caused by low juvenile colony survivorship on unconsolidated rubble-dominated reefs is possible. Macroalgal-dominated reefs received the same number of recruits as coral-dominated reefs (i.e. larval supply was not compromised), but these recruits were either unable to settle successfully on the natural substrate, or survive through to their juvenile life stage. The presence of abundant macroalgae on reefs appears to be having detrimental effects on coral replenishment following disturbances. Bottlenecks to recovery of coral assemblages were present on reefs that had shifted to more degraded conditions.\ud \ud Shifts between alternative ecosystem states often involve alterations to vegetation type and quantity. Herbivory intensity, and the identity of herbivores, is therefore expected to influence the likelihood of such shifts. On coral reefs, assessments of macroalgal herbivory using bioassay experiments are primarily from systems with relatively high coral cover. In chapter 4, I investigated whether the species that remove macroalgae on coral-dominated reefs will still be present and performing significant algal removal on macroalgal-dominated reefs. That is, whether continued functionality can be ensured in degraded systems. Using filmed Sargassum bioassays, I found significantly higher Sargassum biomass loss on coral-dominated reefs. However, once standardised for the availability of macroalgae, the rates of removal were similar between the two reef conditions. Sargassum-assay consumer assemblages differed between reef conditions; nominally grazing herbivores, Siganus puelloides and Chlorurus sordidus, and the browser, Siganus sutor, dominated feeding on coraldominated reefs, whereas browsing herbivores, Naso elegans, N. unicornis, and Leptoscarus vaigiensis prevailed on macroalgal-dominated reefs. It appeared that macroalgal density in the surrounding habitat had a strong influence on the species driving the process of macroalgal removal. This suggests that although the function of macroalgal removal may continue, the species responsible may change with context, differing between systems that are regenerating versus degrading.\ud \ud Predictions of future ecosystem communities aid management and conservation initiatives yet are currently difficult to provide, especially at local scales. Using patterns in coral reef fish assemblages that are associated with reefs along a continuum of habitat quality, chapter 5 investigated 1) whether observed relationships hold over time, and 2) whether there are particular aspects of benthic change that the fish respond to. Significant and relatively consistent spatial relationships between the two years were identified for three aspects of the fish assemblage – species richness and the abundances of both corallivores and browsing herbivores. However, there was no consistent relationship for the remaining 10 fish assemblage variables. Nevertheless, a reef's trajectory along three different axes of benthic change (coral:macroalgae, cca:sand, other macroalgae:turf) was associated with reciprocal changes in nine (of 13) aspects of the reef fish assemblage. This suggests that while there are spatial patterns in some groups that may be used for predictions, caution should be exercised with many other ecosystem components, as unpredictable behaviour is likely.\ud \ud My research emphasises the long-term detrimental effects of reef degradation, especially phase shifts to macroalgal-dominance, on important ecological processes. Moreover, the more degraded examples of Seychelles coral reefs (rubble- and macroalgal-dominated) appear to be reinforcing their current condition, developing feedbacks that may be hard to break. The work also highlights areas or time periods where management strategies may be most effective