Climate-driven coral reorganisation influences aggressive behaviour in juvenile coral-reef fishes

Globally, habitat degradation is altering the abundance and diversity of species in a variety of ecosystems. This study aimed to determine how habitat degradation, in terms of changing coral composition under climate change, affected abundance, species richness and aggressive behaviour of juveniles of three damselfishes (Pomacentrus moluccensis, P. amboinensis and Dischistodus perspicillatus, in order of decreasing reliance on coral). Patch reefs were constructed to simulate two types of reefs: present-day reefs that are vulnerable to climate-induced coral bleaching, and reefs with more bleaching-robust coral taxa, thereby simulating the likely future of coral reefs under a warming climate. Fish communities were allowed to establish naturally on the reefs during the summer recruitment period. Climate-robust reefs had lower total species richness of coral-reef fishes than climate-vulnerable reefs, but total fish abundance was not significantly different between reef types (pooled across all species and life-history stages). The nature of aggressive interactions, measured as the number of aggressive chases, varied according to coral composition; on climate-robust reefs, juveniles used the substratum less often to avoid aggression from competitors, and interspecific aggression became relatively more frequent than intraspecific aggression for juveniles of the coral-obligate P. moluccensis. This study highlights the importance of coral composition as a determinant of behaviour and diversity of coral-reef fishes

Ecological limitations to the resilience of coral reefs

The decline of coral reefs has been broadly attributed to human stressors being too strong and pervasive, whereas biological processes that may render coral reefs fragile have been sparsely considered. Here we review several ecological factors that can limit the ability of coral reefs to withstand disturbance. These include: (1) Many species lack the adaptive capacity to cope with the unprecedented disturbances they currently face; (2) human disturbances impact vulnerable life history stages, reducing reproductive output and the supply of recruits essential for recovery; (3) reefs can be vulnerable to the loss of few species, as niche specialization or temporal and spatial segregation makes each species unique (i.e., narrow ecological redundancy); in addition, many foundation species have similar sensitivity to disturbances, suggesting that entire functions can be lost to single disturbances; and (4) feedback loops and extinction vortices may stabilize degraded states or accelerate collapses even if stressors are removed. This review suggests that the degradation of coral reefs is due to not only the severity of human stressors but also the “fragility” of coral reefs. As such, appropriate governance is essential to manage stressors while being inclusive of ecological process and human uses across transnational scales. This is a considerable but necessary upgrade in current management if the integrity, and delivery of goods and services, of coral reefs is to be preserved

Coral reef degradation alters the isotopic niche of reef fishes

The degradation of coral reefs is widely reported, yet there is a poor understanding of the adaptability of reef fishes to cope with benthic change. We tested the effects of coral reef degradation on the feeding plasticity of four reef fish species. We used isotopic niche sizes and mean δ15N and δ13C values of each species in two coral reefs that differed in benthic condition. The species chosen have contrasting feeding strategies; Chaetodon lunulatus (corallivore), Chrysiptera rollandi (zooplanktivore), Halichoeres melanurus (invertivore) and Zebrasoma velifer (herbivore). We predicted that the corallivore would have a lower mean δ15N value and a smaller isotopic niche size in the degraded reef, that the herbivore and the invertivore might have a larger isotopic niche size and/or a different mean δ13C value, whereas the zooplanktivore might be indifferent since the species is not linked to coral degradation. Some results matched our predictions; C. lunulatus had a smaller niche size on the degraded reef, but no difference in mean δ15N and δ13C values, and H. melanurus displayed an increase in niche size and a lower mean δ15N value on the degraded reef. Some other results were contrary to our predictions; whereas Z. velifer and C. rollandi had smaller mean δ13C values but no difference in niche size. Our findings suggest there may be feeding plasticity to maintain a similar diet despite contrasting habitat characteristics, with different amplitude depending on species. Such findings suggest that certain species guilds would probably adapt to changes linked to habitat degradation

Social drivers forewarn of marine regime shifts

Some ecosystems can undergo regime shifts to alternative compositions of species. Although ecological indicators can identify approaching regime shifts, we propose that rapid changes in the social drivers underlying ecosystem change may provide additional and potentially earlier indicators of impending shifts. We demonstrate this by reconstructing the underlying social drivers of four iconic marine regime shifts: Pacific kelp forests, Northwest Atlantic continental shelf, Jamaican coral reefs, and the Chesapeake Bay estuary. In all cases, a range of social drivers – including opening of lucrative markets, technological innovations, and policies that enhanced the driver – ultimately prompted these ecosystem shifts. Drawing on examples emerging from environmental management practice, we present three practical recommendations for using social drivers as early indicators: monitor social change, determine social trigger points, and identify policy responses. We argue that accounting for the underlying social drivers of ecosystem change could improve decision making

Fishery benefits from exploiting spawning aggregations not solely dependent on enhanced fish density

The vulnerability of spawning aggregations to exploitation varies among fisheries as a result of differences in the population-density changes associated with this behaviour. However, vulnerability to fishing is also influenced by technology, environmental factors, and fish and fisher behaviours. Focusing on a fishery for the rabbitfish Siganus sutor at Praslin Island, Seychelles, we examined how catch rate varied across spawning and non-spawning habitats in relation to in situ population-density changes and other factors known to influence catchability. Catch rates in spawning habitat were disproportionate to density changes, being only fourfold greater than catch rates in non-spawning habitat, despite the fact that spawning-aggregation formation involved nine- to thirteen-fold increases in population density. Catch rates in spawning habitat were also highly variable across the spawning season (0–23.4 fish trap-hour−1). Current strength was of similar importance to density as a catch-rate predictor, with the highest catch rates in spawning habitats confined to months with the strongest currents. Therefore, in addition to density-dependent catchability, other factors that influence catch rates must be examined to avoid overestimation of the vulnerability of populations to aggregation fishing. The dynamics of catchability at spawning sites can limit the ability of fishers to predict and maximise returns based on increases in fish density

Structural complexity mediates functional structure of reef fish assemblages among coral habitats

Coral community composition varies considerably due to both environmental conditions and disturbance histories. However, the extent to which coral composition influences associated fish assemblages remains largely unknown. Here an ecological trait-based ordination analysis was used to compare functional richness (range of unique trait combinations), functional evenness (weighted distribution of fishes with shared traits), and functional divergence (proportion of total abundance supported by species with traits on the periphery of functional space) of fish assemblages among six distinct coral habitats. Despite no significant variation in species richness among habitats, there were differences in the functional richness and functional divergence, but not functional evenness, of fish assemblages among habitats. Structural complexity of coral assemblages was the best predictor of the differences in functional richness and divergence among habitats. Functional richness of fish assemblages was highest in branching Porites habitats, lowest in Pocillopora and soft coral habitats, and intermediate in massive Porites, staghorn Acropora, and mixed coral habitats. Massive and branching Porites habitats displayed greater functional divergence in fish assemblages than the Pocillopora habitat, whilst the remaining habitats were intermediate. Differences in functional richness and divergence were largely driven by the presence of small schooling planktivores in the massive and branching Porites habitats. These results indicate that differential structural complexity among coral communities may act as an environmental filter, affecting the distribution and abundance of associated species traits, particularly those of small-bodied schooling fishes

Visual versus video methods for estimating reef fish biomass

Estimates of fish biomass collated at the community level are reliable indicators of fish and ecosystem health. Data to calculate fish biomass is routinely collected using either underwater visual census (UVC) or stereo diver operated video (DOV), although the compatibility of UVC and DOV based estimates are yet to be assessed. Accordingly, we calculated and compared community level measures of coral reef fish biomass at Ningaloo reef (Western Australia) using both UVC and DOV. The UVC based biomass estimates were 788 kg/Ha, which was ∼50% greater than those from DOV (500 kg/Ha). Differences between the methods were primarily due to DOV measuring the length of only ∼40% of fish detected by video, preventing fish specific weight calculations for all fish encountered. When the size of unmeasured fish was assumed to be the median value of fish measured by DOV, revised DOV+ estimates of community biomass (778 kg/Ha) were similar to those from UVC. However, even when unmeasured fish were included in DOV calculations, biomass of some families (serranids) were still higher when using UVC. Conversely, DOV adjusted estimates of pomacentrid biomass were higher than those from UVC, due to DOV measuring fewer small bodied fish (<3 cm), thus having a larger median size for the high number of unmeasured pomacentrids compared to UVC. Our results suggest that community measures of fish biomass from DOV and UVC are broadly comparable once weights of unmeasured fish are incorporated into DOV estimates. This may increase the spatial and temporal scales at which fish biomass can be monitored, although compatibility of data will depend on the composition and size distribution of the fish assemblages

Ecosystem regime shifts disrupt trophic structure

Regime shifts between alternative stable ecosystem states are becoming commonplace due to the combined effects of local stressors and global climate change. Alternative states are characterised as substantially different in form and function to pre-disturbance states, disrupting the delivery of ecosystem services and functions. On coral reefs, regime shifts are typically characterised by a change in the benthic composition from coral- to macroalgal-dominance. Such fundamental shifts in the benthos are anticipated to impact associated fish communities that are reliant on the reef for food and shelter, yet there is limited understanding of how regime shifts propagate through the fish community over time, relative to initial or recovery conditions. This study addresses this knowledge gap using long-term data of coral reef regime shifts and recovery on Seychelles reefs following the 1998 mass bleaching event. It shows how trophic structure of the reef fish community becomes increasingly dissimilar between alternative reef ecosystem states (regime-shifted vs recovering) with time since disturbance. Regime-shifted reefs developed a concave trophic structure, with increased biomass in base trophic levels as herbivorous species benefitted from increased algal resources. Mid trophic level species, including specialists such as corallivores, declined with loss of coral habitat, while biomass was retained in upper trophic levels by large-bodied, generalist invertivores. Recovering reefs also experienced an initial decline in mid trophic level biomass, but moved towards a bottom-heavy pyramid shape, with a wide range of feeding groups (e.g. planktivores, corallivores, omnivores) represented at mid trophic levels. Given the importance of coral reef fishes in maintaining the ecological function of coral reef ecosystems and their associated fisheries, understanding the effects of regime shifts on these communities is essential to inform decisions that enhance ecological resilience and economic sustainability

Mass coral bleaching causes biotic homogenization of reef fish assemblages

Global climate change is altering community composition across many ecosystems due to nonrandom species turnover, typically characterized by the loss of specialist species and increasing similarity of biological communities across spatial scales. As anthropogenic disturbances continue to alter species composition globally, there is a growing need to identify how species responses influence the establishment of distinct assemblages, such that management actions may be appropriately assigned. Here, we use trait‐based analyses to compare temporal changes in five complementary indices of reef fish assemblage structure among six taxonomically distinct coral reef habitats exposed to a system‐wide thermal stress event. Our results revealed increased taxonomic and functional similarity of previously distinct reef fish assemblages following mass coral bleaching, with changes characterized by subtle, but significant, shifts toward predominance of small‐bodied, algal‐farming habitat generalists. Furthermore, while the taxonomic or functional richness of fish assemblages did not change across all habitats, an increase in functional originality indicated an overall loss of functional redundancy. We also found that prebleaching coral composition better predicted changes in fish assemblage structure than the magnitude of coral loss. These results emphasize how measures of alpha diversity can mask important changes in the structure and functioning of ecosystems as assemblages reorganize. Our findings also highlight the role of coral species composition in structuring communities and influencing the diversity of responses of reef fishes to disturbance. As new coral species configurations emerge, their desirability will hinge upon the composition of associated species and their capacity to maintain key ecological processes in spite of ongoing disturbances

Mesopredator trophodynamics on thermally stressed coral reefs

Ecosystems are becoming vastly modified through disturbance. In coral reef ecosystems, the differential susceptibility of coral taxa to climate-driven bleaching is predicted to shift coral assemblages towards reefs with an increased relative abundance of taxa with high thermal tolerance. Many thermally tolerant coral species are characterised by low structural complexity, with reduced habitat niche space for the small-bodied coral reef fishes on which piscivorous mesopredators feed. This study used a patch reef array to investigate the potential impacts of climate-driven shifts in coral assemblages on the trophodynamics of reef mesopredators and their prey communities. The ‘tolerant’ reef treatment consisted only of coral taxa of low susceptibility to bleaching, while ‘vulnerable’ reefs included species of moderate to high thermal vulnerability. ‘Vulnerable’ reefs had higher structural complexity, and the fish assemblages that established on these reefs over 18 months had higher species diversity, abundance and biomass than those on ‘tolerant’ reefs. Fish assemblages on ‘tolerant’ reefs were also more strongly influenced by the introduction of a mesopredator (Cephalopholis boenak). Mesopredators on ‘tolerant’ reefs had lower lipid content in their muscle tissue by the end of the 6-week experiment. Such sublethal energetic costs can compromise growth, fecundity, and survivorship, resulting in unexpected population declines in long-lived mesopredators. This study provides valuable insight into the altered trophodynamics of future coral reef ecosystems, highlighting the potentially increased vulnerability of reef fish assemblages to predation as reef structure declines, and the cost of changing prey availability on mesopredator condition