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Human activities threaten reef ecosystems globally, forcing ecological change at rates and scales regarded as unprecedented in the Holocene. These changes are so profound that a cessation of reef accretion (reef 'turn-off') and net erosion of reef structures is argued by many as the ultimate and imminent trajectory. Here, we use a regional scale reef growth dataset, based on 76 core records (constrained by 211 radiometric dates) from 22 reefs along and across the inner-shelf of the Great Barrier Reef, Australia, to examine the timing of different phases of reef initiation ('turn-on'), growth and 'turn-off' during the Holocene. This dataset delineates two temporally discrete episodes of reef-building over the last 8500 years: the first associated with the Holocene transgression-early highstand period [∼8.5-5.5 k calibrated years (cal y)]; the second since ∼2.3 k cal y. During both periods, reefs accreted rapidly to sea level before entering late evolutionary states - states naturally characterized by reduced coral cover and low accretion potential - and a clear hiatus occurs between these reef-building episodes for which no records of reef initiation exist. These transitions mimic those projected under current environmental disturbance regimes, but have been driven entirely by natural forcing factors. Our results demonstrate that, even through the late Holocene, reef health and growth has fluctuated through cycles independent of anthropogenic forcing. Consequently, degraded reef states cannot de facto be considered to automatically reflect increased anthropogenic stress. Indeed, in many cases degraded or nonaccreting reef communities may reflect past reef growth histories (as dictated by reef growth-sea level interactions) as much as contemporary environmental change. Recognizing when changes in reef condition reflect these natural 'turn-on'- growth -'turn-off' cycles and how they interact with on-going human disturbance is critical for effective coral reef management and for understanding future reef ecological trajectories. [ABSTRACT FROM AUTHOR]

It is thought that recovery of marine habitats from uncontrollable disturbance may be faster in marine reserves than in unprotected habitats. But which marine habitats should be protected, those areas at greatest risk or those at least risk? We first defined this problem mathematically for 2 alternate conservation objectives. We then analytically solved this problem for both objectives and determined under which conditions each of the different protection strategies was optimal. If the conservation objective was to maximize the chance of having at least 1 healthy site, then the best strategy was protection of the site at lowest risk. On the other hand, if the goal was to maximize the expected number of healthy sites, the optimal strategy was more complex. If protected sites were likely to spend a significant amount of time in a degraded state, then it was best to protect low-risk sites. Alternatively, if most areas were generally healthy then, counterintuitively, it was best to protect sites at higher risk. We applied these strategies to a situation of cyclone disturbance of coral reefs on Australia's Great Barrier Reef. With regard to the risk of cyclone disturbance, the optimal reef to protect differed dramatically, depending on the expected speed of reef recovery of both protected and unprotected reefs. An adequate consideration of risk is fundamental to all conservation actions and can indicate surprising routes to conservation success. [ABSTRACT FROM AUTHOR]

High-latitude coral reefs (HLRs) are potentially vulnerable marine ecosystems facing well-documented threats to tropical reefs and exposure to suboptimal temperatures and insolation. In addition, because of their geographic isolation, HLRs may have poor or erratic larval connections to tropical reefs and a reduced genetic diversity and capacity to respond to environmental change. On Australia's east coast, a system of marine protected areas (MPAs) has been established with the aim of conserving HLRs in part by providing sources of colonizing larvae. To examine the effectiveness of existing MPAs as networks for dispersal, we compared genetic diversity within and among the HLRs in MPAs and between these HLRs and tropical reefs on the southern Great Barrier Reef (GBR). The 2 coral species best represented on Australian HLRs (the brooding Pocillopora damicornis and the broadcast-spawning Goniastrea australensis ) exhibited sharply contrasting patterns of diversity and connectedness. For P. damicornis, the 8-locus genetic and genotypic diversity declined dramatically with increasing latitude (Na = 3.6–1.2, He = 0.3–0.03, Ng:N = 0.87–0.06), although population structure was consistent with recruitment derived largely from sexual reproduction (Go:Ge = 1.28–0.55). Genetic differentiation was high among the HLRs (FST[SD]= 0.32 [0.08], p < 0.05) and between the GBR and the HLRs (FST= 0.24 [0.06], p < 0.05), which indicates these temperate populations are effectively closed. In contrast for G. australensis , 9-locus genetic diversity was more consistent across reefs (Na = 4.2–3.9, He = 0.3–0.26, Ng:N = 1–0.61), and there was no differentiation among regions (FST= 0.00 [0.004], p > 0.05), which implies the HLRs and the southern GBR are strongly interconnected. Our results demonstrate that although the current MPAs appear to capture most of the genetic diversity present within the HLR systems for these 2 species, their sharply contrasting patterns of connectivity indicate some taxa, such as P. damicornis , will be more vulnerable than others, and this disparity will provide challenges for future management. [ABSTRACT FROM AUTHOR]