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  1. Abstract Aim

    Habitat complexity plays an important role in the structure and function of ecosystems worldwide. On coral reefs, habitat complexity influences ecosystem services such as harvestable fish biomass and attenuation of wave energy. Here, we test how three descriptors of surface complexity—rugosity, fractal dimension, and height range—trend with the geological age of reefs (0.2–5.1 million years old), depth (1–25 m), wave exposure (1–306 kW/m), coral cover (0–80%), and three habitat types (aggregated reef, rock and boulder, and pavement).

    Location

    We surveyed across 234 sites and 4 degrees of latitude in the eight main Hawaiian Islands.

    Time Period

    April 2019 – July 2019.

    Major Taxa Studied

    Reef building corals.

    Methods

    We estimate three surface descriptors (rugosity, fractal dimension and height range) using structure‐from‐motion photogrammetry. We evaluate hypothesized relationships between these descriptors and geological reef age, depth, wave exposure, coral cover and reef habitat type using generalized linear models that account for survey design.

    Results

    The rugosity of reef habitats decreased with geological reef age; fractal dimension (and coral cover) decreased with wave exposure; and height range decreased with depth. Variations in these patterns were explained by the different habitat types and the way they are formed over time. Nonetheless, the three surface descriptors were geometrically constrained across all habitat types, and so habitats occupied distinctly different regions of habitat complexity space.

    Main Conclusions

    This study showed how broad environmental characteristics influence the structural complexity of habitats, and therefore geodiversity, which is an important first step toward understanding the communities supported by these habitats and their ecosystem services.

     
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  2. Abstract

    The health of coral reef benthic and fish communities is implicitly connected, yet typically studied and managed separately. By developing a coupled reef population model that connects coral populations and reef fish biomass through the habitat complexity that corals build and fish live among, we aim to address this gap by holistically quantifying ecological feedbacks and responses to ecological stressors. We explored the impacts of fishing effort in conjunction with three types of ecological disturbances as they propagated through a coral reef ecosystem: (1) a disturbance that disproportionately affected small, bio‐energetically vulnerable colonies, (2) a disturbance that predominantly affected large, mechanically vulnerable colonies, and (3) a disturbance that affected colonies of all sizes randomly. We found that joint coral and fish population recovery was fastest and most complete under events affecting small colonies, followed by recovery from disturbances affecting random sizes, and lastly large‐colony disturbances. These results suggest that the retention versus loss of large coral colonies with high reproductive potential critically influenced population recovery. Low fishing levels maintained fish and coral populations and allowed for recovery after disturbances, whereas high fishing levels prevented recovery due to greater fish‐dependent coral mortality. Finally, we tested various formulations of the relationship between coral size and habitat complexity (i.e., exponential, linear, logarithmic) that constrain fish carrying capacity. All formulations led to similar population projections in most disturbance scenarios, but there were exceptions where the timing and trajectory of recovery differed, such as faster and greater recovery potential when complexity is logarithmic with respect to coral size. These findings suggest that fishing and habitat complexity mediate the recovery of coral reef populations, emphasizing the importance of describing linkages between coral size distribution and reef habitat structure. Furthermore, our results highlight the utility of the coupled‐model framework for understanding and managing the impact of disturbances at ecosystem scales.

     
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  3. Abstract

    In their recent synopsis, Loke and Chisholm (Ecology Letters, 25, 2269–2288, 2022) present an overview of habitat complexity metrics for ecologists. They provide a review and some sound advice. However, we found several of their analyses and opinions misleading. This technical note provides a different perspective on the complexity metrics assessed.

     
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  4. Abstract

    Insights into assemblages that can persist in extreme environments are still emerging. Ocean warming and acidification select against species with low physiological tolerance (trait‐based ‘filtering’). However, intraspecific trait variation can promote species adaptation and persistence, with potentially large effects on assemblage structure. By sampling nine coral traits (four morphological, four tissue and one skeletal) along an offshore–inshore gradient in temperature and pH, we show that distantly related coral species undergo consistent intraspecific changes as they cross into warm, acidic environments. Intraspecific variation and species turnover each favoured colonies with greater tissue biomass, higher symbiont densities and reduced skeletal investments, indicating strong filtering on colony physiology within and across species. Physiological tissue traits were highly variable within species and were independent of morphology, enabling morphologically diverse species to cross into sites of elevated temperature and acidity. Widespread intraspecific change can therefore counter the loss of biodiversity and morphological structure across a steep environmental gradient.

     
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  5. Abstract

    Life‐history traits are promising tools to predict species commonness and rarity because they influence a population's fitness in a given environment. Yet, species with similar traits can have vastly different abundances, challenging the prospect of robust trait‐based predictions. Using long‐term demographic monitoring, we show that coral populations with similar morphological and life‐history traits show persistent (decade‐long) differences in abundance. Morphological groups predicted species positions along two, well known life‐history axes (the fast‐slow continuum and size‐specific fecundity). However, integral projection models revealed that density‐independent population growth (λ) was more variable within morphological groups, and was consistently higher in dominant species relative to rare species. Within‐group λ differences projected large abundance differences among similar species in short timeframes, and were generated by small but compounding variation in growth, survival, and reproduction. Our study shows that easily measured morphological traits predict demographic strategies, yet small life‐history differences can accumulate into large differences in λ and abundance among similar species. Quantifying the net effects of multiple traits on population dynamics is therefore essential to anticipate species commonness and rarity.

     
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  6. Abstract

    Humans have long sought to restore species but little attention has been directed at how to best select a subset of foundation species for maintaining rich assemblages that support ecosystems, like coral reefs and rainforests, which are increasingly threatened by environmental change.

    We propose a two‐part hedging approach that selects optimized sets of species for restoration. The first part acknowledges that biodiversity supports ecosystem functions and services, and so it ensures precaution against loss by allocating an even spread of phenotypic traits. The second part maximizes species and ecosystem persistence by weighting species based on characteristics that are known to improve ecological persistence—for example abundance, species range and tolerance to environmental change.

    Using existing phenotypic‐trait and ecological data for reef building corals, we identified sets of ecologically persistent species by examining marginal returns in occupancy of phenotypic trait space. We compared optimal sets of species with those from the world's southern‐most coral reef, which naturally harbours low coral diversity, to show these occupy much of the trait space. Comparison with an existing coral restoration program indicated that current corals used for restoration only cover part of the desired trait space and programs may be improved by including species with different traits.

    Synthesis and applications. While there are many possible criteria for selecting species for restoration, the approach proposed here addresses the need to insure against unpredictable losses of ecosystem services by focusing on a wide range of phenotypic traits and ecological characteristics. Furthermore, the flexibility of the approach enables the functional goals of restoration to vary depending on environmental context, stakeholder values, and the spatial and temporal scales at which meaningful impacts can be achieved.

     
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  7. Coral bleaching and mortality can show significant spatial and taxonomic heterogeneity at local scales, highlighting the need to understand the fine-scale drivers and impacts of thermal stress. In this study, we used structure-from-motion photogrammetry to track coral bleaching, mortality, and changes in community composition during the 2019 marine heatwave in Kāneʻohe Bay, Hawaiʻi. We surveyed 30 shallow reef patches every 3 weeks for the duration of the bleaching event (August-December) and one year after, resulting in a total of 210 large-area, high-resolution photomosaics that enabled us to follow the fate of thousands of coral colonies through time. We also measured environmental variables such as temperature, sedimentation, depth, and wave velocity at each of these sites, and extracted estimates of habitat complexity (rugosity R and fractal dimension D) from digital elevation models to better understand their effects on patterns of bleaching and mortality. We found that up to 80% of corals experienced moderate to severe bleaching in this period, with peak bleaching occurring in October when heat stress (Degree Heating Weeks) reached its maximum. Mortality continued to accumulate as bleaching levels dropped, driving large declines in more heat-susceptible species (77% loss of Pocillopora cover) and moderate declines in heat-tolerant species (19% and 23% for Porites compressa and Montipora capitata , respectively). Declines in live coral were accompanied by a rapid increase in algal cover across the survey sites. Spatial differences in bleaching were significantly linked to habitat complexity and coral species composition, with reefs that were dominated by Pocillopora experiencing the most severe bleaching. Mortality was also influenced by species composition, fractal dimension, and site-level differences in thermal stress. Our results show that spatial heterogeneity in the impacts of bleaching are driven by a mix of environmental variation, habitat complexity, and differences in assemblage composition. 
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  8. null (Ed.)