Nicolas Guillemot

Functional Redundancy Patterns Reveal Non-Random Assembly Rules in a Species-Rich Marine Assemblage

The relationship between species and the functional diversity of assemblages is fundamental in ecology because it contains key information on functional redundancy, and functionally redundant ecosystems are thought to be more resilient, resistant and stable. However, this relationship is poorly understood and undocumented for species-rich coastal marine ecosystems. Here, we used underwater visual censuses to examine the patterns of functional redundancy for one of the most diverse vertebrate assemblages, the coral reef fishes of New Caledonia, South Pacific. First, we found that the relationship between functional and species diversity displayed a non-asymptotic power-shaped curve, implying that rare functions and species mainly occur in highly diverse assemblages. Second, we showed that the distribution of species amongst possible functions was significantly different from a random distribution up to a threshold of ∼90 species/transect. Redundancy patterns for each function further revealed that some functions displayed fast rates of increase in redundancy at low species diversity, whereas others were only becoming redundant past a certain threshold. This suggested non-random assembly rules and the existence of some primordial functions that would need to be fulfilled in priority so that coral reef fish assemblages can gain a basic ecological structure. Last, we found little effect of habitat on the shape of the functional-species diversity relationship and on the redundancy of functions, although habitat is known to largely determine assemblage characteristics such as species composition, biomass, and abundance. Our study shows that low functional redundancy is characteristic of this highly diverse fish assemblage, and, therefore, that even species-rich ecosystems such as coral reefs may be vulnerable to the removal of a few keystone species.

Does herbivorous fish protection really improve coral reef resilience? A case study from new caledonia (South Pacific).

Parts of coral reefs from New Caledonia (South Pacific) were registered at the UNESCO World Heritage list in 2008. Management strategies aiming at preserving the exceptional ecological value of these reefs in the context of climate change are currently being considered. This study evaluates the appropriateness of an exclusive fishing ban of herbivorous fish as a strategy to enhance coral reef resilience to hurricanes and bleaching in the UNESCO-registered areas of New Caledonia. A two-phase approach was developed: 1) coral, macroalgal, and herbivorous fish communities were examined in four biotopes from 14 reefs submitted to different fishing pressures in New Caledonia, and 2) results from these analyses were challenged in the context of a global synthesis of the relationship between herbivorous fish protection, coral recovery and relative macroalgal development after hurricanes and bleaching. Analyses of New Caledonia data indicated that 1) current fishing pressure only slightly affected herbivorous fish communities in the country, and 2) coral and macroalgal covers remained unrelated, and macroalgal cover was not related to the biomass, density or diversity of macroalgae feeders, whatever the biotope or level of fishing pressure considered. At a global scale, we found no relationship between reef protection status, coral recovery and relative macroalgal development after major climatic events. These results suggest that an exclusive protection of herbivorous fish in New Caledonia is unlikely to improve coral reef resilience to large-scale climatic disturbances, especially in the lightly fished UNESCO-registered areas. More efforts towards the survey and regulation of major chronic stress factors such as mining are rather recommended. In the most heavily fished areas of the country, carnivorous fish and large targeted herbivores may however be monitored as part of a precautionary approach.

Baseline study of the spatio-temporal patterns of reef fish assemblages prior to a major mining project in New Caledonia (South Pacific)

From 2008 onwards, the coral reefs of Koné (New Caledonia) will be subjected to a major anthropogenic perturbation linked to development of a nickel mine. Dredging and sediment runoff may directly damage the reef environment whereas job creation should generate a large demographic increase and thus a rise in fishing activities. This study analyzed reef fish assemblages between 2002 and 2007 with a focus on spatio-temporal variability. Our results indicate strong spatial structure of fish assemblages through time. Total species richness, density and biomass were highly variable between years but temporal variations were consistent among biotopes. A remarkable spatio-temporal stability was observed for trophic (mean 4.6% piscivores, 53.1% carnivores, 30.8% herbivores and 11.4% planktivores) and home range structures of species abundance contributions. These results are discussed and compared with others sites of the South Pacific. For monitoring perspectives, some indicators related to expected disturbances are proposed.

Modulation of Habitat-Based Conservation Plans by Fishery Opportunity Costs: A New Caledonia Case Study Using Fine-Scale Catch Data

Numerous threats impact coral reefs and conservation actions are urgently needed. Fast production of marine habitat maps promotes the use of habitat-only conservation plans, where a given percentage of the area of each habitat is set as conservation objectives. However, marine reserves can impact access to fishing grounds and generate opportunity costs for fishers that need to be minimized. In New Caledonia (Southwest Pacific), we used fine-scale fishery catch maps to define nineteen opportunity costs layers (expressed as biomass catch loss) considering i) total catches, ii) target fish families, iii) local marine tenure, and iv) gear type. The expected lower impacts on fishery catch when using the different cost constraints were ranked according to effectiveness in decreasing the costs generated by the habitat-only scenarios. The exercise was done for two habitat maps with different thematic richness. In most cases, habitat conservation objectives remained achievable, but effectiveness varied widely between scenarios and between habitat maps. The results provide practical guidelines for coral reef conservation and management. Habitat-only scenarios can be used to initiate conservation projects with stakeholders but the costs induced by such scenarios can be lowered by up to 50–60% when detailed exhaustive fishery data are used. When using partial data, the gain would be only in the 15–25% range. The best compromises are achieved when using local data.

Alert thresholds for monitoring environmental variables: a new approach applied to seagrass beds diversity in New Caledonia.

Monitoring ecological variables is mandatory to detect abnormal changes in ecosystems. When the studied variables exceed predefined alert thresholds, management actions may be required. In the past, alert thresholds have been typically defined by expert judgments and descriptive statistics. Recently, approaches based on statistical power were also used. In New Caledonia, seagrass monitoring is a priority given their vulnerability to natural and anthropic disturbances. To define a suitable monitoring strategy and alert thresholds, we compared a Percentile Based Approach (PBA) and a sensitivity analysis of power (SAP). Both methods defined statistically relevant alert thresholds, but the SAP approach was more robust to spatial and temporal variability of seagrass cover. Moreover, this method characterized the sensitivity of threshold values to sampling efforts, a useful knowledge for managers.

Power analysis as a tool to identify statistically informative indicators for monitoring coral reef disturbances

Extensive biological field surveys are costly and time consuming. To optimize sampling and ensure regular monitoring on the long term, identifying informative indicators of anthropogenic disturbances is a priority. In this study, we used 1800 candidate indicators by combining metrics measured from coral, fish, and macro-invertebrate assemblages surveyed from 2006 to 2012 in the vicinity of an ongoing mining project in the Voh-Koné-Pouembout lagoon, New Caledonia. We performed a power analysis to identify a subset of indicators which would best discriminate temporal changes due to a simulated chronic anthropogenic impact. Only 4% of tested indicators were likely to detect a 10% annual decrease of values with sufficient power (>0.80). Corals generally exerted higher statistical power than macro-invertebrates and fishes because of lower natural variability and higher occurrence. For the same reasons, higher taxonomic ranks provided higher power than lower taxonomic ranks. Nevertheless, a number of families of common sedentary or sessile macro-invertebrates and fishes also performed well in detecting changes: Echinometridae, Isognomidae, Muricidae, Tridacninae, Arcidae, and Turbinidae for macro-invertebrates and Pomacentridae, Labridae, and Chaetodontidae for fishes. Interestingly, these families did not provide high power in all geomorphological strata, suggesting that the ability of indicators in detecting anthropogenic impacts was closely linked to reef geomorphology. This study provides a first operational step toward identifying statistically relevant indicators of anthropogenic disturbances in New Caledonia’s coral reefs, which can be useful in similar tropical reef ecosystems where little information is available regarding the responses of ecological indicators to anthropogenic disturbances.