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Rising anthropogenic CO₂ emissions are anticipated to drive change to ocean ecosystems, but a conceptualization of biological change derived from quantitative analyses is lacking. Derived from multiple ecosystems and latitudes, our metaanalysis of 632 published experiments quantified the direction and magnitude of ecological change resulting from ocean acidification and warming to conceptualize broadly based change. Primary production by temperate noncalcifying plankton increases with elevated temperature and CO₂, whereas tropical plankton decreases productivity because of acidification. Temperature increases consumption by and metabolic rates of herbivores, but this response does not translate into greater secondary production, which instead decreases with acidification in calcifying and noncalcifying species. This effect creates a mismatch with carnivores whose metabolic and foraging costs increase with temperature. Species diversity and abundances of tropical as well as temperate species decline with acidification, with shifts favoring novel community compositions dominated by noncalcifiers and microorganisms. Both warming and acidification instigate reduced calcification in tropical and temperate reef-building species. Acidification leads to a decline in dimethylsulfide production by ocean plankton, which as a climate gas, contributes to cloud formation and maintenance of the Earth’s heat budget. Analysis of responses in short- and long-term experiments and of studies at natural CO₂ vents reveals little evidence of acclimation to acidification or temperature changes, except for microbes. This conceptualization of change across whole communities and their trophic linkages forecast a reduction in diversity and abundances of various key species that underpin current functioning of marine ecosystems.

Aim: Understanding the relative importance of climatic and non-climatic distribution drivers for co-occurring, functionally similar species is required to assess potential consequences of climate change. This understanding is, however, lacking for most ecosystems. We address this knowledge gap and forecast changes in distribution for habitat-forming seaweeds in one of the world's most species-rich temperate reef ecosystems. Location: The Great Southern Reef. The full extent of Australia's temperate coastline. Methods: We assessed relationships between climatic and non-climatic environmental data known to influence seaweed, and the presence of 15 habitat-forming seaweeds. Distributional data (herbarium records) were analysed with MAXENT and generalized linear and additive models, to construct species distribution models at 0.2° spatial resolution, and project possible distribution shifts under the RCP 6.0 (medium) and 2.6 (conservative) emissions scenarios of ocean warming for 2100. Results: Summer temperatures, and to a lesser extent winter temperatures, were the strongest distribution predictors for temperate habitat-forming seaweeds in Australia. Projections for 2100 predicted major poleward shifts for 13 of the 15 species, on average losing 78% (range: 36%-100%) of their current distributions under RCP 6.0 and 62% (range: 27%-100%) under RCP 2.6. The giant kelp (Macrocystis pyrifera) and three prominent fucoids (Durvillaea potatorum, Xiphophora chondrophylla and Phyllospora comosa) were predicted to become extinct from Australia under RCP 6.0. Many species currently distributed up the west and east coasts, including the dominant kelp Ecklonia radiata (71% and 49% estimated loss for RPC 6.0 and 2.6, respectively), were predicted to become restricted to the south coast. Main conclusions: In close accordance with emerging observations in Australia and globally, our study predicted major range contractions of temperate seaweeds in coming decades. These changes will likely have significant impacts on marine biodiversity and ecosystem functioning because large seaweeds are foundation species for 100s of habitat-associated plants and animals, many of which are socio-economically important and endemic to southern Australia.

Ecological complexity represents a network of interacting components that either propagate or counter the effects of environmental change on individuals and communities1–3. Yet, our understanding of the ecological imprint of ocean acidification (elevated CO2) and climate change (elevated temperature) is largely based on reports of negative effects on single species in simplified laboratory systems4,5. By combining a large mesocosm experiment with a global meta-analysis, we reveal the capacity of consumers (fish and crustaceans) to resist the impacts of elevated CO2. While individual behaviours were impaired by elevated CO2, consumers could restore their performances in more complex environments that allowed for compensatory processes. Consequently, consumers maintained key traits such as foraging, habitat selection and predator avoidance despite elevated CO2 and sustained their populations. Our observed increase in risk-taking under elevated temperature, however, predicts greater vulnerability of consumers to predation. Yet, CO2 as a resource boosted the biomass of consumers through species interactions and may stabilize communities by countering the negative effects of elevated temperature. We conclude that compensatory dynamics inherent in the complexity of nature can buffer the impacts of future climate on species and their communities.

Contrary to expectation, some fish species living around CO 2 vents—natural ‘laboratories’ for studying the effects of ocean acidification—show increased abundance due to indirect positive effects of acidification on habitat and food resources. Ocean ecosystems are predicted to lose biodiversity and productivity from increasing ocean acidification 1 . Although laboratory experiments reveal negative effects of acidification on the behaviour and performance of species 2 , 3 , more comprehensive predictions have been hampered by a lack of in situ studies that incorporate the complexity of interactions between species and their environment. We studied CO 2 vents from both Northern and Southern hemispheres, using such natural laboratories 4 to investigate the effect of ocean acidification on plant–animal associations embedded within all their natural complexity. Although we substantiate simple direct effects of reduced predator-avoidance behaviour by fishes, as observed in laboratory experiments, we here show that this negative effect is naturally dampened when fish reside in shelter-rich habitats. Importantly, elevated CO 2 drove strong increases in the abundance of some fish species through major habitat shifts, associated increases in resources such as habitat and prey availability, and reduced predator abundances. The indirect effects of acidification via resource and predator alterations may have far-reaching consequences for population abundances, and its study provides a framework for a more comprehensive understanding of increasing CO 2 emissions as a driver of ecological change.

1. Policy initiatives that seek to recover lost habitats require the capacity to anticipate and suppress the mechanisms that drive loss. The replacement of forested landscapes by simple landscapes comprising of opportunistic or 'weedy' species represents an increasingly common phenomenon across human-dominated systems. The failure of subtidal forests to recover from natural and human disturbance and their ultimate replacement by degraded habitats is recognized globally. The current lack of knowledge on whether such shifts can be reversed jeopardizes considerations of restoration policy within increasingly human-dominated landscapes. 2. We critically assessed the model that recovery of canopies within remnant kelp forests in degraded landscapes (i.e. turf-forming algae that carpet space) is slower than in adjacent forested landscapes, but may be increased by removing turfs. 3. After generating experimental disturbance, canopies recovered to their former state within forested landscapes, but not in remnant forests in degraded landscapes. Removal of turfs from spaces between remnant forests, however, enabled canopies to recruit and subsequently develop covers that matched those in remnant forests. 4. Whilst the supply of canopy-forming propagules to degraded landscapes is likely to decline with gap expansion, we show that improvements to forest resilience and restoration are possible via policies that result in a reduction of turf covers. These results also support the model that regime-shifts need not be a product of synchronized loss, but can occur as a result of reduced rates of canopy-recruitment over broad areas and many years. Indeed, patterns of canopy-loss over several decades redouble attention to the human-mediated conditions that enable turfs to retain space (i.e. elevated nutrient and sediment loads via coastal runoff). 5. Synthesis and applications. We demonstrate that future restoration is a possible outcome of polices that promote ecosystem recovery. In doing so, we reduce uncertainty about policy initiatives that aim to upgrade the recycling potential of wastewater treatment plants (e.g. nearly 45% of South Australia's metropolitan wastewater) to improve the quality of water needed to restore subtidal forests. Uncertainty about resilience-building and restoration management are redressed by demonstrating that the feedbacks maintaining regime-shifted landscapes are not necessarily permanent.

The problem of linking fine-scale processes to broad-scale patterns remains a central challenge of ecology. As rates of abiotic change intensify, there is a critical need to understand how individual responses aggregate to generate compensatory dynamics that stabilize community processes. Notably, while local and global resource enhancement (e.g., nutrient and CO₂ release) can reverse dominance relationship between key species (e.g., shifts from naturally kelp-dominated to turf-dominated systems), herbivores can counter these shifts by consuming the additional productivity of competing species (e.g., turfs). Here, we test whether consumer plasticity in energy intake to maintain growth in varying environments can underpin changes in consumption that buffer varying levels of productivity. In response to carbon and nutrient enrichment, herbivores increased consumption of higher-quality food, which acted as a buffer against enhanced production, while maintaining organismal processes across varying abiotic conditions (i.e., growth). These results not only suggest plasticity in feeding behavior, but also in energy acquisition and utilization to maintain organismal processes. Such plasticity may not only underpin organismal homeostasis, but also compensatory dynamics that emerge from the aggregate of these responses to buffer change in community processes.

Primary producers rarely exist under their ideal conditions, with key processes often limited by resource availability. As human activities modify environmental conditions, and therefore resource availability, some species may be released from these limitations while others are not, potentially disrupting community structure. In order to examine the limitations experienced by algal functional groups that characterise alternate community structures (i.e. turf-forming algae and canopy-forming kelp), we exposed these groups to contemporary and enriched levels of carbon dioxide (CO 2 ) and nutrients. Turfs responded to the individual enrichment of both CO 2 and nutrients, with the greatest shift in the biomass and carbon:nitrogen (C:N) ratios observed under their combined enrichment. In contrast, kelp responded to enriched nutrients, but not enriched CO 2 . We hypothesise that the differing limitations reflect the contrasting physiologies of these functional groups, specifically their methods of C acquisition, such as the possession and/or efficiency of a carbon concentrating mechanism (CCM). Importantly, our results reveal that these functional groups, whose interactions structure entire communities, experience distinct resource limitations, with some potentially limited by a single type of resource (i.e. kelp by nutrients), while others may be co-limited (i.e. turf by CO 2 and nutrients). Consequently, the identification of how alternate conditions modify resource availability and limitations may facilitate anticipation of the future sustainability of major ecosystem components and the communities they support.

Ocean acidification affects species populations and biodiversity through direct negative effects on physiology and behaviour. The indirect effects of elevated CO 2 are less well known and can sometimes be counterintuitive. Reproduction lies at the crux of species population replenishment, but we do not know how ocean acidification affects reproduction in the wild. Here, we use natural CO 2 vents at a temperate rocky reef and show that even though ocean acidification acts as a direct stressor, it can indirectly increase energy budgets of fish to stimulate reproduction at no cost to physiological homeostasis. Female fish maintained energy levels by compensation: They reduced activity (foraging and aggression) to increase reproduction. In male fish, increased reproductive investment was linked to increased energy intake as mediated by intensified foraging on more abundant prey. Greater biomass of prey at the vents was linked to greater biomass of algae, as mediated by a fertilisation effect of elevated CO 2 on primary production. Additionally, the abundance and aggression of paternal carers were elevated at the CO 2 vents, which may further boost reproductive success. These positive indirect effects of elevated CO 2 were only observed for the species of fish that was generalistic and competitively dominant, but not for 3 species of subordinate and more specialised fishes. Hence, species that capitalise on future resource enrichment can accelerate their reproduction and increase their populations, thereby altering species communities in a future ocean.

Ecologically dominant species often define ecosystem states, but as human disturbances intensify, their subordinate counterparts increasingly displace them. We consider the duality of disturbance by examining how environmental drivers can simultaneously act as a stressor to dominant species and as a resource to subordinates. Using a model ecosystem, we demonstrate that CO2-driven interactions between species can account for such reversals in dominance; i.e., the displacement of dominants (kelp forests) by subordinates (turf algae). We established that CO2 enrichment had a direct positive effect on productivity of turfs, but a negligible effect on kelp. CO2 enrichment further suppressed the abundance and feeding rate of the primary grazer of turfs (sea urchins), but had an opposite effect on the minor grazer (gastropods). Thus, boosted production of subordinate producers, exacerbated by a net reduction in its consumption by primary grazers, accounts for community change (i.e., turf displacing kelp). Ecosystem collapse, therefore, is more likely when resource enrichment alters competitive dominance of producers, and consumers fail to compensate. By recognizing such duality in the responses of interacting species to disturbance, which may stabilize or exacerbate change, we can begin to understand how intensifying human disturbances determine whether or not ecosystems undergo phase shifts.

Increasing carbon emissions not only enrich oceans with CO₂ but also make them more acidic. This acidifying process has caused considerable concern because laboratory studies show that ocean acidification impairs calcification (or shell building) and survival of calcifiers by the end of this century. Whether this impairment in shell building also occurs in natural communities remains largely unexplored, but requires re-examination because of the recent counterintuitive finding that populations of calcifiers can be boosted by CO₂ enrichment. Using natural CO₂ vents, we found that ocean acidification resulted in the production of thicker, more crystalline and more mechanically resilient shells of a herbivorous gastropod, which was associated with the consumption of energy-enriched food (i.e. algae). This discovery suggests that boosted energy transfer may not only compensate for the energetic burden of ocean acidification but also enable calcifiers to build energetically costly shells that are robust to acidified conditions. We unlock a possible mechanism underlying the persistence of calcifiers in acidifying oceans.