Jesús Ernesto Arias-González

Functional over-redundancy and high functional vulnerability in global fish faunas on tropical reefs

Significance Our results indicate that, even in highly diverse systems like coral reefs, we can no longer assume that the erosion of species diversity can be discounted by the high probability of functional redundancy: i.e., that several species can support the same function. Indeed, we show that fish species tend to disproportionately pack into a few particular functions while leaving many functions highly vulnerable, i.e., they are supported by just one species. Even the Coral Triangle, which has a high concentration of tropical-reef fishes, may experience a loss of functional diversity following fisheries pressure and local species extirpation. Our results suggest that the promised benefits of functional insurance from high species diversity may not be as strong as we once hoped. When tropical systems lose species, they are often assumed to be buffered against declines in functional diversity by the ability of the species-rich biota to display high functional redundancy: i.e., a high number of species performing similar functions. We tested this hypothesis using a ninefold richness gradient in global fish faunas on tropical reefs encompassing 6,316 species distributed among 646 functional entities (FEs): i.e., unique combinations of functional traits. We found that the highest functional redundancy is located in the Central Indo-Pacific with a mean of 7.9 species per FE. However, this overall level of redundancy is disproportionately packed into few FEs, a pattern termed functional over-redundancy (FOR). For instance, the most speciose FE in the Central Indo-Pacific contains 222 species (out of 3,689) whereas 38% of FEs (180 out of 468) have no functional insurance with only one species. Surprisingly, the level of FOR is consistent across the six fish faunas, meaning that, whatever the richness, over a third of the species may still be in overrepresented FEs whereas more than one third of the FEs are left without insurance, these levels all being significantly higher than expected by chance. Thus, our study shows that, even in high-diversity systems, such as tropical reefs, functional diversity remains highly vulnerable to species loss. Although further investigations are needed to specifically address the influence of redundant vs. vulnerable FEs on ecosystem functioning, our results suggest that the promised benefits from tropical biodiversity may not be as strong as previously thought.

Predicted impact of the invasive lionfish Pterois volitans on the food web of a Caribbean coral reef

The invasion of lionfish in the Caribbean is causing grave concern because of its deleterious impacts on coral reef food-webs. We have used an Ecopath-with-Ecosim model to predict the impacts of lionfish invasion on a coral reef community based on pre-invasion fish community data. Forty-six groups were defined, and an initial Ecopath model was balanced with a near-zero biomass of lionfish. In Ecosim, the near-zero biomass was eradicated by applying a very high fishing pressure in the first year of simulation. We subsequently (re-)introduced lionfish with a very low biomass, and allowed them to increase to very high abundance. With a near-zero lionfish biomass, the great majority of mesocarnivorous/omnivorous coral reef fish were predicted to be dominant while sharks were predicted to be the apex predators. Different management scenarios were established in the ecosystem to explore the eradication and resilience of lionfish. The management scenarios showed that if all adult lionfish were exploitable it will in theory be possible to fish the lionfish to a very low level, but the fishing pressure will have to be maintained, or the lionfish will recover. If the largest individuals are unexploitable it will be much more difficult to control the lionfish population.

Global mismatch between species richness and vulnerability of reef fish assemblages

The impact of anthropogenic activity on ecosystems has highlighted the need to move beyond the biogeographical delineation of species richness patterns to understanding the vulnerability of species assemblages, including the functional components that are linked to the processes they support. We developed a decision theory framework to quantitatively assess the global taxonomic and functional vulnerability of fish assemblages on tropical reefs using a combination of sensitivity to species loss, exposure to threats and extent of protection. Fish assemblages with high taxonomic and functional sensitivity are often exposed to threats but are largely missed by the global network of marine protected areas. We found that areas of high species richness spatially mismatch areas of high taxonomic and functional vulnerability. Nevertheless, there is strong spatial match between taxonomic and functional vulnerabilities suggesting a potential win-win conservation-ecosystem service strategy if more protection is set in these locations.

Additive diversity partitioning of fish in a Caribbean coral reef undergoing shift transition.

Shift transitions in dominance on coral reefs from hard coral cover to fleshy macroalgae are having negative effects on Caribbean coral reef communities. Data on spatiotemporal changes in biodiversity during these modifications are important for decision support for coral reef biodiversity protection. The main objective of this study is to detect the spatiotemporal patterns of coral reef fish diversity during this transition using additive diversity-partitioning analysis. We examined α, β and γ fish diversity from 2000 to 2010, during which time a shift transition occurred at Mahahual Reef, located in Quintana Roo, Mexico. Data on coral reef fish and benthic communities were obtained from 12 transects per geomorphological unit (GU) in two GUs (reef slope and terrace) over six years (2000, 2005, 2006, 2007, 2008, 2010). Spatial analysis within and between the GUs indicated that the γ-diversity was primarily related to higher β-diversity. Throughout the six study years, there were losses of α, β and γ-diversity associated spatially with the shallow (reef slope) and deeper (reef terrace) GUs and temporally with the transition in cover from mound corals to fleshy macroalgae and boulder corals. Despite a drastic reduction in the number of species over time, β-diversity continues to be the highest component of γ-diversity. The shift transition had a negative effect on α, β and γ-diversity, primarily by impacting rare species, leading a group of small and less vulnerable fish species to become common and an important group of rare species to become locally extinct. The maintenance of fish heterogeneity (β-diversity) over time may imply the abetment of vulnerability in the face of local and global changes.

Additive Diversity Partitioning of Reef Fishes across Multiple Spatial Scales

Abstract. Reef fishes are critical functional groups for coral reefs, where the diversity of the former varies across different spatial scales. Multi-scale approaches are necessary to find this scale-dependence because the identification of such critical scales is fundamental to conserve coral reef biodiversity. For the first time, we present a study on fish diversity partition from the northern sector of the Mesoamerican Barrier Reef System (nsMBRS). In this area, coral reefs mainly present four habitats (e.g., lagoon, front, slope and terrace) and the spatial and environmental variables differ along a north-south gradient along the coast. This particular geomorphology provides an excellent opportunity to evaluate the reef fish diversity. Our objectives were to assess the inventory and differentiation diversity at habitat, reef and regional scales, and carry out additive diversity partitioning from sample to region scale. Total fish diversity (&egr;) was partitioned into its additive diversity components (&agr;s, &bgr;s, &agr;, &bgr;, &ggr; and &dgr;), which were evaluated using bootstrap and rarefaction procedures, non-parametric statistics, and non-linear and null models. We found that &agr; diversity was higher in the habitats front, slope and terrace, and &bgr;h diversity was highest between lagoons and fronts. The most developed reefs exhibited the highest &agr;, &bgr;h, &ggr; and &dgr;r diversity. The &bgr;s and &dgr;r diversity were essentials to keep &ggr; and &egr; diversity. Additive partition outcomes showed that total fish diversity is determined mostly by reef scale followed per sample and habitat scales. This supports the hypothesis that inter-habitat and reef differences seem to strongly regulate local and regional species richness. We conclude that reef scale was the most important level for conserving and keeping the biodiversity at nsMBRS.

Additive partitioning of reef fish diversity variation: a promising marine biodiversity management tool

Additive partitioning was applied to variation in reef fish spatial diversity at Isla Isabel National Park, Nayarit state, Mexico, and to identify the environmental and spatial variables that best explains it. Analyses included expected and observed species curves, rare species analysis, additive partitioning of alpha- and beta-diversity, and canonical redundancy analysis. A total of 10,517 individuals were recorded from 75 species and 33 reef fish families, representing 85% of expected richness. Species richness beta-diversity was dependent on the site scale, while the alpha-diversity of the Shannon diversity was most significant at the transect scale. Canonical partitioning showed species richness and Shannon diversity was explained by spatially-structured environmental components. Variation in species composition and abundance was explained by a purely environmental component. Therefore, elements of habitat structure (especially corals), topographic complexity, and refuge availability determine fish species diversity. Our results suggest that greater emphasis is required to conserve sites that promote β-diversity, increasing fish spatial diversity. In Isla Isabel, these sites would be mostly those located at eastern and southern of protected sides, where coral reef patches are well represented. The results of this multi-scale analysis are valuable and useful as an addition and complement to the holistic management strategies implemented at Isla Isabel.

Additive partitioning of coral reef fish diversity across hierarchical spatial scales throughout the Caribbean.

There is an increasing need to examine regional patterns of diversity in coral-reef systems since their biodiversity is declining globally. In this sense, additive partitioning might be useful since it quantifies the contribution of alpha and beta to total diversity across different scales. We applied this approach using an unbalanced design across four hierarchical scales (80 sites, 22 subregions, six ecoregions, and the Caribbean basin). Reef-fish species were compiled from the Reef Environmental Education Foundation (REEF) database and distributions were confirmed with published data. Permutation tests were used to compare observed values to those expected by chance. The primary objective was to identify patterns of reef-fish diversity across multiple spatial scales under different scenarios, examining factors such as fisheries and demographic connectivity. Total diversity at the Caribbean scale was attributed to β-diversity (nearly 62% of the species), with the highest β-diversity at the site scale. α¯-diversity was higher than expected by chance in all scenarios and at all studied scales. This suggests that fish assemblages are more homogenous than expected, particularly at the ecoregion scale. Within each ecoregion, diversity was mainly attributed to alpha, except for the Southern ecoregion where there was a greater difference in species among sites. β-components were lower than expected in all ecoregions, indicating that fishes within each ecoregion are a subsample of the same species pool. The scenario involving the effects of fisheries showed a shift in dominance for β-diversity from regions to subregions, with no major changes to the diversity patterns. In contrast, demographic connectivity partially explained the diversity pattern. β-components were low within connectivity regions and higher than expected by chance when comparing between them. Our results highlight the importance of ecoregions as a spatial scale to conserve local and regional coral reef-fish diversity.

A coral-algal phase shift in Mesoamerica not driven by changes in herbivorous fish abundance

Coral-algal phase shifts in which coral cover declines to low levels and is replaced by algae have often been documented on coral reefs worldwide. This has motivated coral reef management responses that include restriction and regulation of fishing, e.g. herbivorous fish species. However, there is evidence that eutrophication and sedimentation can be at least as important as a reduction in herbivory in causing phase shifts. These threats arise from coastal development leading to increased nutrient and sediment loads, which stimulate algal growth and negatively impact corals respectively. Here, we first present results of a dynamic process-based model demonstrating that in addition to overharvesting of herbivorous fish, bottom-up processes have the potential to precipitate coral-algal phase shifts on Mesoamerican reefs. We then provide an empirical example that exemplifies this on coral reefs off Mahahual in Mexico, where a shift from coral to algal dominance occurred over 14 years, during which there was little change in herbivore biomass but considerable development of tourist infrastructure. Our results indicate that coastal development can compromise the resilience of coral reefs and that watershed and coastal zone management together with the maintenance of functional levels of fish herbivory are critical for the persistence of coral reefs in Mesoamerica.

Coral biodiversity and bio-construction in the northern sector of the mesoamerican reef system

As the impact of anthropogenic activity and climate change continue to accelerate rates of degradation on Caribbean coral reefs, conservation and restoration faces greater challenges. At at this stage, of particular importance in coral reefs, is to recognize and to understand the structural spatial patterns of benthic assemblages. We developed a field-based framework of a Caribbean reefscape benthic structure by using hermatypic corals as an indicator group of global biodiversity and bio-construction patterns in eleven reefs of the northern sector of the Mesoamerican Barrier Reef System (nsMBRS). Four hundred and seventy four video-transects (50 m long by 0.4 m wide) were performed throughout a gradient of reef complexity from north to south (∼400 km) to identify coral species, families and ensembles of corals. Composition and abundance of species, families and ensembles showed differences among reefs. In the northern zone, the reefs had shallow, partial reef developments with low diversities, dominated by Acropora palmata, Siderastrea spp., Pseudodiploria strigosa and Agaricia tenuifolia. In the central and southern zones, reefs presented extensive developments, high habitat heterogeneity, and the greatest diversity and dominance of Orbicella annularis and Orbicella faveolata. These two species determined the structure and diversity of corals in the central and southern zones of the nsMBRS and their bio-construction in these zones is unique in the Caribbean. Their abundance and distribution depended on the reef habitat area, topographic complexity and species richness. Orbicella species complex were crucial for maintaining the biodiversity and bio-construction of the central and southern zones while A. palmata in the northern zones of the nsMBRS.

Scaling Up Models of the Dynamics of Coral Reef Ecosystems: An Approach for Science-Based Management of Global Change

Coral reefs around the world and the populations who directly and indirectly depend on them are facing a multitude of global, regional, and local threats. In face of these unprecedented global changes, it is critical to understand how coral reef ecosystems and the goods and services they provide will evolve. The problem is complex and its solution is difficult because of the nature of biophysical connectivity of coral reef systems, and their connection with human social and economic systems. In order to increase our knowledge and the predictive capacities necessary to determine how coral reef ecosystems will respond to global change, it is necessary to employ a combination of data synthesis and numerical simulation. The increase in the knowledge base and predictive capacities regarding the influence of the drivers: ocean circulation, climate, ocean-acidification, terrestrial run-offs driven by enhanced human activities such as the clearing of native vegetation and its replacement with intensive agriculture and coastal development, pollution, overfishing, and invasive species on marine coral reef ecosystems is central for the effective management and conservation of coral reef ecosystem services. Modeling at multiple scales has revealed to be a vital tool in meeting this challenge by providing important technology that allows managers, other decision makers, and users to see the dynamics of the whole system – including ecological, biophysical, socioeconomic, and restoration aspects. The aim of this review is to provide information concerning the studies of local and regional coral reef ecosystem models, the coupling of ecological and social system models and models based on ecosystems as well as to provide suggestions for future development and use of models for science-based management of global change