René Galzin

Rare species support vulnerable functions in high-diversity ecosystems

The most unusual, and thus irreplaceable, functions performed by species in three different species-rich ecosystems are fulfilled by only the rare species in these ecosystems.

RELATING BEHAVIOR TO HABITAT: SOLUTIONS TO THEFOURTH‐CORNER PROBLEM

This paper addresses the following question: how does one relate the biological and behavioral characteristics of animals to habitat characteristics of the locations at which they are found? Ecologists often assemble data on species composition at different localities, habitat descriptions of these localities, and biological or behavioral traits of the species. These data tables are usually analyzed two by two: species composition against habitat characteristics, or against behavioral data, using such methods as canonical analysis. We propose a solution to the problem of estimating the parameters describing the relationship between habitat characteristics and biology or behavior, and of testing the statistical significance of these parameters; this problem is referred to as the fourth-corner problem, from its matrix formulation. In other words, the fourth-corner method offers a way of analyzing the relationships between the supplementary variables associated with the rows and columns of a binary (presence or absence) data table. The test case that motivated this study concerns a coral reef fish assemblage (280 species). Biological and behavioral characteristics of the species were used as supplementary variables for the rows, and characteristics of the environment for the columns. Parameters of the association between habitat characteristics (distance from beach, water depth, and substrate variables) and biological and behavioral traits of the species (feeding habits, ecological niche categories, size classes, egg types, activity rhythms) were estimated and tested for significance using permutations. Permutations can be performed in different ways, corresponding to different ecological hypotheses. Results were compared to predictions made independently by reef fish ecologists, in order to assess the method as well as the pertinence of the variables subjected to the analysis. The new method is shown to be applicable to a wide class of ecological problems.

Relationships between coral reef substrata and fish

Abstract. The objective of this work is to identify which substrata characteristics (such as coral morphology, coral diversity, coral species richness, percentage coverage by live coral or by algae) influence the structure and abundance of fish communities. The study was carried out at Reunion Island, Indian Ocean, where six sites were sampled in two zones (reef flat and outer reef slope). Quantitative data were collected by visual census techniques, along a linear transect of 50 m for the substratum, and a belt of 50×2 m for the fish communities. Correspondence analysis (CA) and an optimising cluster analysis, called dynamic clustering method (DCM) were used to describe and compare fish assemblages with the benthic composition. The relationships between benthic and fish communities were examined using the classes revealed by the partitioning of the substratum with DCM. This partitioning allowed us to derive four classes of substratum: the non-disturbed reef flat, the non-disturbed outer reef slope, the perturbed reef habitat and the reef pass. The analysis of the partitioning based on the coral variables suggests that there are significant relationships between benthic and fish assemblages.

Climate Warming, Marine Protected Areas and the Ocean-Scale Integrity of Coral Reef Ecosystems

Coral reefs have emerged as one of the ecosystems most vulnerable to climate variation and change. While the contribution of a warming climate to the loss of live coral cover has been well documented across large spatial and temporal scales, the associated effects on fish have not. Here, we respond to recent and repeated calls to assess the importance of local management in conserving coral reefs in the context of global climate change. Such information is important, as coral reef fish assemblages are the most species dense vertebrate communities on earth, contributing critical ecosystem functions and providing crucial ecosystem services to human societies in tropical countries. Our assessment of the impacts of the 1998 mass bleaching event on coral cover, reef structural complexity, and reef associated fishes spans 7 countries, 66 sites and 26 degrees of latitude in the Indian Ocean. Using Bayesian meta-analysis we show that changes in the size structure, diversity and trophic composition of the reef fish community have followed coral declines. Although the ocean scale integrity of these coral reef ecosystems has been lost, it is positive to see the effects are spatially variable at multiple scales, with impacts and vulnerability affected by geography but not management regime. Existing no-take marine protected areas still support high biomass of fish, however they had no positive affect on the ecosystem response to large-scale disturbance. This suggests a need for future conservation and management efforts to identify and protect regional refugia, which should be integrated into existing management frameworks and combined with policies to improve system-wide resilience to climate variation and change.

High mortality during settlement is a population bottleneck for a tropical surgeonfish

Replenishment of benthic marine populations typically involves “settlement” from pelagic larval to benthic juvenile habitats. Mortality during this transition has been unknown because of the difficulty of measuring propagule supply in open water. For three weeks, we compared the nocturnal passage of presettlement fishes across the barrier reef encircling Moorea Island (French Polynesia) with the abundance of benthic recruits in the back-reef lagoon on the following morning. During this time, >40,000 presettlement unicornfish, Naso unicornis entered our study area of ∼1 km2 with half arriving on just two nights. Using coupled Beverton-Holt functions to describe the decay of each cohort, we were able to predict the daily abundance of recruits and their final age structure from the presettlement inputs. The best model estimated that ∼61% of the potential settlers were lost between their nocturnal arrival and the following morning, independent of cohort size. Postsettlement mortality was density dependent, varying between 9% and 20% per day. We attribute all mortality to predation and suggest that high risk associated with settlement has shaped colonization strategies. Because fishing targets the survivors of this population bottleneck, aquarium fisheries may be more sustainable when sourced from pelagic juveniles.

Recurrent disturbances, recovery trajectories, and resilience of coral assemblages on a South Central Pacific reef

Coral reefs are increasingly threatened by various disturbances, and a critical challenge is to determine their ability for resistance and resilience. Coral assemblages in Moorea, French Polynesia, have been impacted by multiple disturbances (one cyclone and four bleaching events between 1991 and 2006). The 1991 disturbances caused large declines in coral cover (~51% to ~22%), and subsequent colonization by turf algae (~16% to ~49%), but this phase-shift from coral to algal dominance has not persisted. Instead, the composition of the coral community changed following the disturbances, notably favoring an increased cover of Porites, reduced cover of Montipora and Pocillopora, and a full return of Acropora; in this form, the reef returned to pre-disturbance coral cover within a decade. Thus, this coral assemblage is characterized by resilience in terms of coral cover, but plasticity in terms of community composition.

Density dependence in marine protected populations: a review

Summary The cessation or reduction of fishing in marine protected areas (MPAs) should promote an increase in abundance and mean size and age of previously exploited populations. Thus density-dependent changes in life-history characteristics should occur when populations are allowed to recover in MPAs. In this review, we synthesize the existing information on resource limitation in marine ecosystems, densitydependent changes in life-history traits of exploited populations and evidence for biomass export from MPAs. Most evidence for compensatory changes in biological variables has been derived from observations on populations depleted by high fishing mortality or on strong year classes, but these changes are more evident in juveniles than in adults and in freshwater rather than in marine systems. It is unclear if adults of exploited marine populations are resource limited. This may suggest that exploited populations are controlled mainly by density-independent processes, which could be a consequence of the depleted state of most exploited populations. MPAs could be a useful tool for testing these hypotheses. If we assume that resources become limiting inside MPAs, it is plausible that, if suitable habitats exist, mobile species will search for resources outside of the MPAs, leading to export of biomass to areas which are fished. However, it is not possible to establish from the available data whether this export will be a response to resource limitation inside the MPAs, the result of random movements across MPA boundaries or both. We discuss the implications of this process for the use of MPAs as fisheries management tools.

Parasite diversity\host age and size relationship in three coral-reef fishes from French Polynesia

The parasite communities of three coral-reef fish species (Stegastes nigricans, Dascyllus aruanus and Cephalopholis argus) were on Tiahura reef, French Polynesia. The age and growth of each fish was analysed by otolith increment counts and a significant correlation between these variables was found. Stegastes nigricans was parasitised by six adult parasite species, D. aruanus by two adult parasite species and C. argus by five adult parasite species. The most common parasite species were found in all fish size classes. Ectoparasites showed a positive relationship between their abundance and host body length for all three reef fish species. A positive relationship was found only between host size and parasite abundance for common endoparasite species. Parasite species richness, Brillouins diversity index, and host size and age were positively related. Finally, we discuss the influence of different biological (host diet, host immune response, parasite life-cycle) and ecological factors on parasite community structure in these three reef fishes. Host diet quality seems to be one of the major factors affecting the endoparasite community structure in these reef fishes. Ectoparasite communities seem to be influenced more by biological factors such as, for example, host immunity for the caligid larvae or parasite life-cycle for the gnathiid praniza larvae. In addition, the effect of ecological factors such as cleaning symbiosis on these ectoparasites cannot be dismissed.

Trophic functioning of the Tiahura reef sector, Moorea Island, French Polynesia

Abstract. Energy-balanced steady-state models of the fringing and barrier reefs of Tiahura, Moorea Island, French Polynesia, are presented. A total of 43 and 46 trophic groups were identified on the two reef habitats respectively. The models’ outputs indicate that most of the substantial primary productivity is processed and recycled (59–69% of NPP) in the web through detritus based, microbially mediated food webs, with a substantial but secondary flux through grazer-based webs. This mechanism produces long pathways with low trophic efficiencies at the higher trophic levels. The trophic structure of both reef habitats efficiently conserves energy and materials within the reef ecosystem through two forms of internal recycling: a relatively large cycle produced through detritus and a microbial food web, and a relatively short one directly produced through predation. The models outputs suggest that bottom-up and top-down control are each ecologically important in both reef habitats.

Experimental assessment of sensory modalities of coral-reef fish larvae in the recognition of their settlement habitat

One of the great mysteries of coral-reef fish ecology is how larvae locate the relatively rare patches of coral-reef habitat on which they settle. The present study aimed to estimate, by experiments in aquaria, the sensory modalities of coral-reef fish larvae for senses used in searching for their species’ settlement habitat. Larval recognition of settlement habitat can be based on the detection of conspecifics and/or of characteristics of coral habitat using visual, chemical and mechanical cues. For this study, larvae were captured with crest nets and were then introduced into experimental tanks that allowed testing of each type of cue separately (visual, chemical or mechanical cues). Among the 18 species studied, 13 chose their settlement habitat due to the presence of conspecifics and not based on the characteristics of coral habitat, and 5 species did not move toward their settlement habitat (e.g. Scorpaenodes parvipinnis, Apogon novemfasciatus). Among the different sensory cues tested, two species used the three types of cues (Parupeneus barberinus and Ctenochaetus striatus: visual, chemical and mechanical cues), six used two types (e.g. Myripristis pralinia: visual and chemical cues; Naso unicornis: visual and mechanical cues), and five used one type (e.g. Chrysiptera leucopoma: visual cues; Pomacentrus pavo: chemical cues). These results demonstrate that many coral-reef fish larvae could in practice use sensory cues for effective habitat selection at settlement, and have the ability to discriminate species-specific sensory cues.