•Planar area and diameter of coral colonies are good proxies of shelter volume.•Shelter sizes can be inferred from the surface complexity of colonies.•Within reefscapes, abundance and size of ...colonies determine shelter distribution.•Quantifying shelter can enhance assessment of reefs’ capacity to support biodiversity.
Structural complexity plays a key role in the functioning of coral reef ecosystems. Reef-building corals are major contributors to this complexity, providing shelter and food for numerous invertebrates and fish species. Both structural complexity and shelter capacity of reefscapes are determined by several components such as spurs and grooves, slope, caves and holes, vegetation and coral colonies. Quantifying the shelter capacity from coral colonies to reefscapes is a fundamental step to estimating ecosystem potential to support biodiversity. Here, we applied underwater photogrammetry to quantify shelter volumes provided by individual coral colonies. Overall, 120 3D models of coral colonies from branching, massive, columnar and tabular growth forms were studied. Three reefscapes were also 3D modeled. The study encompasses three Indo-Pacific Islands: Reunion, Europa and New Caledonia. At the colony level, measurements of diameter, planar area, surface and shelter volume were computed. At the reefscape, the diameter and planar area of each colony were extracted from orthomosaics and then used to estimate shelter capacity. Linear models had high accuracy for predicting shelter volume (a 3D metric) from 2D metrics: the diameter= 83.1%, R2= 0.95; the planar area= 87.5%, R2= 0.95 and the colony surface= 87.3%, R2= 0.96. The surface complexity and the shelter volume of the colonies allowed inferring the size of shelters provided by coral growth forms. Quantitative descriptors (i.e. relative percentage of shelter by growth form, the abundance of coral colonies, “Shannon-Shelter Index”) revealed reefscape-scale shelter differences.
Our major finding is that planar area and diameter of coral colonies are satisfactory proxies for estimating shelter volume. These new proxies allow 2D metrics to quantify 3D shelter provision, which can support scientists and managers in conservation actions since such metrics are widely used in monitoring programs. Future investigations on the relationships between shelter provision and reef biodiversity will improve the understanding of these complex ecosystems.
The functioning of tropical coral reefs is imperilled by climate change, overfishing, and decreasing water quality. Maintaining their capacity to provide goods and services will critically depend on ...our ability to monitor their functioning at appropriate spatial and temporal scales. Given the constraints of traditional methods to respond to those needs, the potential of complementary tools such as Passive Acoustic Monitoring (PAM) is emerging. Coral reef soundscapes (i.e. ambient sound) encompass sounds produced by numerous organisms. Soundscape characteristics quantified by ecoacoustic indices have been found to reflect general ecosystem properties, such as diversity and abundance of fishes, and coral cover. The present study tested, on the virtually pristine coral reefs around Europa Island, South-West Indian Ocean, the capacity of acoustic indices to assess key ecosystem functions. Soundscapes were recorded during 2 h, and ecosystem functions were evaluated using video footage of the fish assemblages and underwater photogrammetry of the benthic landscapes. We found significant and strong correlations between six ecoacoustic indices and six key ecosystem functions, including habitat features and fish assemblage characteristics. The six ecoacoustic indices were representative of several combinations of frequency, amplitude and time analysis domains, highlighting the diversity of the functional information conveyed by soundscapes. Our findings reveal that a 2 h daytime recording on a coral reef could provide sufficient acoustic information to characterise major ecosystem functions of a site. This should facilitate the detection of functional disturbances at temporal and spatial scales adapted to the rapidity of upcoming climate changes. Our results also highlight the potential of ecoacoustics to bring novel and relevant insights in the functioning of ecosystems.
The choice of ecological monitoring methods and descriptors determines the effectiveness of a program designed to assess the state of coral reef ecosystems. Here, we comparer the relative performance ...of the traditional Line Intercept Transect (LIT) method with three methods derived from underwater photogrammetry: LIT on orthomosaics, photoquadrats from orthomosaics, and surface analyses on orthomosaics. The data were acquired at Reunion Island on five outer reef slopes and two coral communities on underwater lava-flows. Coral cover was estimated
in situ
using the LIT method and with LITs and photoquadrats digitized on orthomosaic. Surface analyses were done on the same orthomosaics. Structural complexity of the surveyed sites was calculated from digital elevation models using three physical descriptors (fractal dimension, slope, surface complexity), and used to explore their possible influence in coral cover estimates. We also compared the methods in terms of scientific outputs, the human expertise and time required. Coral cover estimates obtained with
in situ
LITs were higher than those obtained with digitized LITs and photoquadrats. Surfaces analyses on orthomosaics yielded the lowest but most the precise cover estimates (i.e., lowest sample dispersion). Sites with the highest coral cover also had the highest structural complexity. Finally, when we added scientific outputs, and requirements for human expertise and time to our comparisons between methods, we found that surface analysis on the orthomosaics was the most efficient method. Photoquadrats were more time-consuming than both
in situ
and digitized LITs, even though they provided coral cover estimates similar to those of digitized LITs and yielded more than one descriptor. The LIT
in situ
method remains the least time-consuming and most effective for species-level taxonomic identifications but is the most limited method in terms of data outputs and representativeness of the ecosystem.
Maintaining key functions of coral reefs is vital for the persistence of these ecosystems as well as for securing the goods and services that they provide in the Anthropocene. Underwater ...photogrammetry by Structure from Motion (SfM) allows the quantification of novel habitat descriptors that may be particularly relevant in assessing key reefscape traits, that is, physical and ecological characteristics of coral reef habitats. Here, we combined this new technology with fish surveys to explore how reefscape traits shape the functional structure of reef fish assemblages around three environmentally contrasted islands of the Indo‐Pacific (Europa Island, Reunion Island, and New Caledonia). At 24 sites, habitat descriptors were computed from digital elevation models (DEM) and orthomosaics, while reef fish assemblages were assessed by visual census and video footage. Four habitat descriptors were marginally correlated and presented low variance inflation factor (VIF) values, thus being the most complementary descriptors: surface complexity, total shelter capacity, Shannon Shelter Index, and total coral cover. Linear mixed models (LMM) were used to explore the relationships between these habitat descriptors and four key fish functional entities: prey, planktivores, grazers, and predators. For each model, the variance explained (i.e., marginal R2) was significantly higher when considering multiple predictors, including the novel three‐dimensional descriptors (i.e., total shelter capacity and Shannon Shelter Index). The habitat descriptors quantified from underwater photogrammetry outputs (i.e., DEM and orthomosaics) provide easily available data to assess key reefscape traits and predict fish assemblage structure in coral reef ecosystems. This trait‐based functional approach allows consistent assessment of the links between these descriptors from local to regional scales. Considering the global coral reef crisis and the increasing availability of world‐reef photogrammetric surveys, this new technology should be key to bringing solutions to 21st‐century conservation issues.
On the island of Réunion, a French overseas department located in the western Indian Ocean, a prefectorial decree taken by the authorities on July 26, 2013 has been reconducted for the past 10 years ...to forbid swimming and water-based activities, apart from the conditions set out in the injunction. This decision was the outcome of increased shark sightings and incidents causing, in particular, the deaths of 5 ocean users between 2011 and 2013. As a preliminary attempt to address the lack of empirical research on ocean users' social representations and their relationship to non-humans, this paper aims to primarily analyze thoroughly the issues of the acceptance of shark risk mitigation options. Data were gathered through a qualitative study that mobilized several ethnographic tools such as face-to-face semi-structured interviews, participant and non-participant observation as well as social media data survey. Results indicate that the many tensions generated by shark-human interactions (SHIs) on the island of Réunion are related to a lack of public consultation, thus calling the acceptance of the shark management strategy and the scientific recommendations into question. The findings may improve shark-human interactions management as we consider the management of risk as a process to develop a well-balanced way of living with the sharks where residual risk remains. This study contributes to the advancement of knowledge on the management of human-wildlife conflicts (HWCs) while the results tend to confirm the findings of many other studies showing that they are often rooted in human-human conflicts.
Sur l'île de La Réunion, département français d'outre-mer situé dans l'ouest de l'océan Indien, un arrêté préfectoral pris par les autorités le 26 juillet 2013 a été reconduit depuis 10 ans pour interdire la baignade et les activités nautiques, en dehors des conditions prévues par l'injonction. Cette décision fait suite à l'augmentation des observations de requins et des incidents, ayant notamment causés la mort de 5 usagers de l'océan entre 2011 et 2013. Afin de pallier le manque de recherches empiriques sur les représentations sociales des usagers de la mer et de leur rapport aux non-humains, cet article a pour objectif principal d'analyser en profondeur la question de l'acceptation sociale des mesures visant à réduire le risque requin. Les données ont été recueillies dans le cadre d'une étude qualitative qui a mobilisé plusieurs outils ethnographiques tels que des entretiens semi-directifs, des observations participantes et non participantes, ainsi qu'une enquête sur les données issues des réseaux sociaux. Les résultats indiquent que les nombreuses tensions générées par les interactions entre requins et humains sur l'île de La Réunion sont le fruit d'un manque de consultation du public, remettant ainsi en question l'acceptation de la stratégie de gestion du risque requin et les recommandations des scientifiques. Les résultats sont susceptibles d'améliorer la gestion des interactions entre les requins et les humains, dans la mesure où nous considérons la gestion du risque comme un processus permettant de développer un mode de vie équilibré avec les requins dans un contexte où un risque résiduel subsiste. Cette étude contribue à l'avancement des connaissances sur la gestion des conflits entre les sociétés humaines et la faune sauvage, puisque les résultats tendent à confirmer les découvertes de nombreuses autres études qui ont démontré qu'ils sont souvent enracinés dans des conflits entre humains.
On the island of Réunion, a French overseas department located in the western Indian Ocean, a prefectorial decree taken by the authorities on July 26, 2013 has been reconducted for the past 10 years ...to forbid swimming and water-based activities, apart from the conditions set out in the injunction. This decision was the outcome of increased shark sightings and incidents causing, in particular, the deaths of 5 ocean users between 2011 and 2013. As a preliminary attempt to address the lack of empirical research on ocean users' social representations and their relationship to non-humans, this paper aims to primarily analyze thoroughly the issues of the acceptance of shark risk mitigation options. Data has been gathered through a qualitative study that mobilized several ethnographic tools such as face-to-face semi-structured interviews, participant and non-participant observation as well as social media data survey. Results indicate that the many tensions generated by shark-human interactions (SHIs) on the island of Réunion are related to a lack of public consultation, thus calling the acceptance of the shark management strategy and the scientific recommendations into question. The findings may improve shark-human interactions management as we consider the management of risk as a process to develop a well-balanced way of living with the sharks where residual risk remains. This study contributes to the advancement of knowledge on the management of human-wildlife conflicts (HWCs) while the results tend to confirm the findings of many other studies showing that they are often rooted in human-human conflicts
On the island of Réunion, a French overseas department located in the western Indian Ocean, a prefectorial decree taken by the authorities on July 26, 2013 has been reconducted for the past 10 years ...to forbid swimming and water-based activities, apart from the conditions set out in the injunction. This decision was the outcome of increased shark sightings and incidents causing, in particular, the deaths of 5 ocean users between 2011 and 2013. As a preliminary attempt to address the lack of empirical research on ocean users' social representations and their relationship to non-humans, this paper aims to primarily analyze thoroughly the issues of the acceptance of shark risk mitigation options. Data were gathered through a qualitative study that mobilized several ethnographic tools such as face-to-face semi-structured interviews, participant and non-participant observation as well as social media data survey. Results indicate that the many tensions generated by shark-human interactions (SHIs) on the island of Réunion are related to a lack of public consultation, thus calling the acceptance of the shark management strategy and the scientific recommendations into question. The findings may improve shark-human interactions management as we consider the management of risk as a process to develop a well-balanced way of living with the sharks where residual risk remains. This study contributes to the advancement of knowledge on the management of human-wildlife conflicts (HWCs) while the results tend to confirm the findings of many other studies showing that they are often rooted in human-human conflicts.
Abstract
Species monitoring can be strongly limited by terrain accessibility, impeding our understanding of species ecology and thus challenging their conservation. This is particularly true for ...species living in the canopy, on cliffs or in dense vegetation. Remote sensing imagery may fill this gap by offering a cost‐effective monitoring approach allowing to improve species detection in inaccessible areas. We investigated the applicability of drone‐based monitoring for a Critically Endangered insular gecko (
Phelsuma inexpectata
) and two invasive alien gecko species representing a risk for the former (
P. grandis
and
P. laticauda
). We determined the approach distance before inducing behavioural response caused by the drone's presence. All three study species showed no escape behaviour to the drone's presence until very close distances (mean distance for
P. inexpectata
: 33.8 cm;
P. grandis
: 21.9 cm;
P. laticauda
: 26.4 cm). We then performed horizontal and vertical approaches with the drone, taking photos every meter starting at 10 m away from the canopy edge to determine an optimal distance for detection while ensuring species‐level identification. We examined a total of 328 photos. We found a bimodality in the number of detected geckos for two species, with different individuals recorded between short and intermediate distances. Therefore, we recommend taking photos at two distances of 2–2.5 and 5 m away from the canopy, ideally facing away from the sun and in low wind conditions. We encourage the application of our methodology for
Phelsuma
spp., but also for other species of similar size and ecology to improve detection in inaccessible areas.
Abstract in French is available with online material.
Résumé
Le suivi des espèces est fortement limité par l'accessibilité au terrain, ce qui freine notre compréhension de leur écologie et leur conservation. Ceci s'applique particulièrement aux espèces vivant dans la canopée, sur les falaises ou dans la végétation dense. La télédétection ouvre la possibilité de combler ces lacunes tout en améliorant la détectabilité de ces espèces dans les endroits inaccessibles. Nous avons exploré la faisabilité du suivi par drone d'un gecko insulaire en danger critique d'extinction (
Phelsuma inexpectata
) et deux autres invasifs qui représentent un risque pour ce dernier (
P. grandis
et
P. laticauda
). Nous avons déterminé la distance d'approche maximale avant d'observer une réaction à la présence du drone. Les trois espèces étudiées n'ont montré aucune réponse avant de très courtes distances (
P. inexpectata
: 33.8 cm;
P. grandis
: 21.9 cm;
P. laticauda
: 26.4 cm en moyenne). Nous avons ensuite effectué des approches horizontales et verticales avec le drone, prenant une photo chaque mètre entre 1 et 10 m du bords de la canopée afin de déterminer la distance optimale pour une identification d'espèce fiable. Nous avons examiné un total de 328 photos et trouvé une bimodalité dans le nombre de geckos détectés pour deux espèces, avec différents individus identifiés entre les plans rapprochés et les plans à distances intermédiaires. Nous recommandons la prise de deux photos, respectivement à des distances de 2–2.5 et 5 m de la canopée, idéalement dos au soleil et sous vent faible. Nous encourageons l'application de notre méthode pour toutes espèces de
Phelsuma
, mais aussi d'autres espèces de taille et d'écologie similaire afin d'améliorer la détection dans les endroits inaccessibles.
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