The importance of scales in ecology has been largely acknowledged for decades. Many modern ecologists are deeply influenced by he view that ecological processes act at a variety of spatial and temporal scales, and they generate patterns that may differ from those at which processes act. Today it is known that ecological dynamics tend to be stochastic at small scales, but variability is conditional on the resolution of description. Furthermore, there has been an increased recognition that the problem of scale at which ecological processes act, should be considered as critical if it is wanted to produce general predictions about patterns in space and time. Thus, modern ecological thinking agrees that in order to understand a system (e.g., a community), it is important to study it at the appropriate scale. It is clear that increasing consideration of scale is helping to address a key issue in ecology: the question of what influences the distribution and abundance of organisms. Species distributions depend on four important processes: (1) climate, (2) species interactions, (3) habitat structure and (4) dispersal capabilities, each one operating with different strength at a range of spatial scales. Generally, the presence or absence of organisms within a community may depend on rare or large-scale (regionspecific) dispersal and colonization events, while local abundance is more a function of frequent, fine-spatial scale processes such as biotic interactions and habitat heterogeneity. This implies that communities are structured by both abiotic and biotic factors nested along different spatial scales which often occur along environmental gradients. Concomitantly, the species richness of a community is also expected to be highly dependent on spatial scales evaluated.
In a paper published in the journal PeerJ on 4 May 2020, Emy Miyazawa of the Laboratorio de Ecología Experimental at the Universidad Simón Bolivar, Luis Montilla, also of the Laboratorio de Ecología Experimental at the Universidad Simón Bolivar, and of the Integrative Marine Ecology Departmen at Stazione Zoologica Anton Dohrn, Esteban Alejandro Agudo-Adriani, again of the Laboratorio de Ecología Experimental at the Universidad Simón Bolivar, and of the Department of Biology at the University of North Carolina at Chapel Hill, Alfredo Ascanio, again of the Laboratorio de Ecología Experimental at the Universidad Simón Bolivar, and of the Department of Biology at the Miami University of Ohio, Gloria Mariño-Briceño, again of the Laboratorio de Ecología Experimental at the Universidad Simón Bolivar, and Aldo Croquer, once again of the Laboratorio de Ecología Experimental at the Universidad Simón Bolivar, and of the Centro de Innovación Marina, at the Nature Conservancy, present the results of a multi-scale study of the distribution of Coral species on the coast of Venezuala.
Coral Reefs are one of the most complex and diverse ecosystems of the planet. Reef species diversity has been estimated from 600,000 to more than 9 million species worldwide. The habitat and shelter for the majority of these species is largely provided by Scleractinian Corals. There is compiling evidence indicating that ecological processes controlling the structure of Coral assemblages (e.g., substrate availability, recruitment, competition, and herbivory) are strongly dependent on spatial scales. In addition, oceanographic processes which partly define the environmental setting of a reef are also extremely variable within habitats, across sites, reef systems, and regions. Furthermore, biological and environmental factors may interact with each other to produce different patterns in species distribution across several spatial scales. In consequence, understanding the underlying factors controlling the Coral species richness in a reef is not a simple task for it is a multi-scale problem.
Total species richness of a region, frequently named gamma diversity (γ), can be partitioned in two components: (1) α -diversity (i.e., the number of species by site), and (2) β-diversity (i.e., the variation in the species identities from site to site). For decades, ecologist have debated ways to estimate and interpret α and β- diversity; but in recent years, the study of β-diversity has gained a lot of interest for it is what actually makes assemblages of species more or less similar to one another at different places and times. Many different measures of β -diversity have been introduced, but there is no overall consensus about which ones are most appropriate for addressing particular ecological questions. Some authors have distinguished two types of β-diversity: (a) turn-over and (b) variation. Turn-over refers to changes in community structure among sampling units distributed along well-defined environmental gradients, whereas variation portrays variability in species composition among sample units within a given spatial or temporal extent, or within a given category of a factor (such as a habitat type or experimental treatment). On the other hand, some have partitioned the total β-diversity into two components: (1) nestedness, i.e., when the species composition of sample units with low richness represent a subset of the species found in the richest sample units, and (2) species replacement, i.e., a turn-over of species. Regardless the point of view, the study of each of these components is relevant to understand processes that control ecological communities and a range of ecosystem functions.
While spatial patterns of γ and α -diversity of Coral assemblages have been studied extensively; only few studies have focused on measuring β-diversity. This is the case of Venezuela, where most of the papers published to date have only been focused on site descriptions based on species composition and abundance, whereas the influence of spatial variation across different scales on coral assemblages remains poorly explored. The Venezuelan coast is highly heterogeneous with clear longitudinal environmental gradients which are deeply influenced by up-welling regimes that play an important role for the distribution of marine biodiversity. In fact, Algal communities in rocky shores and sessile organisms associated to Mangrove roots have been found to vary at different spatial scales along the Venezuelan coast. Thus, it should not be surprising to find Coral assemblages to be extremely variable across spatial scales in Venezuela. Miyazawa et al. expected that greater changes in community structure and β-diversity of Coral assemblages will occur at scales of thousand of kilometers (i.e., between the eastern and western regions) and within sites (i.e., hundreds of meters). This is because of existing contrasting environmental settings driven by upwelling spots that have been described along the Venezuelan coast line. The goal of this study was two-fold: (1) to quantify spatial variation of Coral assemblages from hundreds meters to hundreds of kilometers, and (2) to determine if there are patterns of β -diversity across these scales.
Miyazawa et al. conducted a multi-scale sampling design comprising coastal areas as well as continental and oceanic islands. Specifically, seven localities were sampled along the Venezuelan territory encompassing three contrasting regions. The western region, included two localities: (1) Morrocoy National Park and (2) Ocumare de la Costa. The former is a continental reef system formed by a group of keys and lagoons surrounded by fringing and patch reefs located nearby Mangroves and Seagrass beds; whereas the latter is a small bay protected by reef barriers with Seagrass and Mangroves dominating the inner and shallower habitats. Likewise, the central region, entailed two localities: (3) Los Roques National Park which is an oceanic archipelago with a central lagoon, characterized by extensive reef banks/patches and two large coralline barriers located south and east of the archipelago; and (4) Chichirivivhe de la Costa, a location of rocky reefs with steep slopes and scattered coral assemblages. Finally, in the eastern region three localities were included: (5) Mochima National Park, (6) Los Frailes and (7) Cubagua. In Mochima, Seagrass beds and Mangroves border a rocky coastline with steep slopes and fringing reef communities. Los Frailes and Cubagua are islands lying at the continental shelf and dominated by small patch reefs with scattered Coral assemblages bordering their coastlines. The whole eastern coast of Venezuela and its continental islands are subjected to seasonal upwelling due to its connection with the Cariaco trench. The selection of these locations aimed to cover the vast majority of reef habitats described for Venezuela. Permits for taking pictures at marine protected areas was given by Ministerio del Poder Popular para el Ecosocialismo y Aguas.
Map of the Venezuelan coast with the seven locations used in this study. Western region, represented in blue, include: MOR D, Morrocoy National Park; and OCU D, Ocumare de la Costa. Central region, represented in red, include: ROQ D, Archipielago Los Roques National Park; and CHI D, Chichiriviche de la Costa. Eastern region, represented in green, included MOC, Mochima National Park; CUB D, Cubagua; and FRA D, Los Frailes. Miyazawa et al. (2020).
A fully-nested design encompassing three hierarchical-random factors (i.e., site, locality, and region) was used to determine spatial variation on Coral assemblage structure (i.e., absolute cover of Coral species) and β -diversity from hundreds of meters (sites) to hundreds of kilometers (region). The factor region encompassed three levels (West, Center, and East); nested within region there were two/three localities, four to seven reef sites within each locality, and four 30m-long transects within each site, understood as the operational unit.
At each reef site, benthic surveys were conducted during 2017 and 2018, following the guidelines outlined by the Global Coral Reef Monitoring Network-Caribbean with slight modifications. In order to increase the number of sampled sites, Miyazawa et al. surveyed four instead of five 30 m-long transects parallel to the shoreline following the bottom contour between 8 10 m depth. Transects were set randomly, with the first transect being always layout at the first spot of diving. From that point, each transect was moved up or down from the first transect. Distance among transects varied from 5 to 6 m, so each operational unit was inter-spaced across the sampled reef habitat. For each transect, fifteen 80 x 90 cm photos were taken every other meter to determine the benthic community structure (60 photos per site). A reference frame was used in the field to calibrate each photograph in the laboratory for further analysis of benthic cover.
The photo quadrat analysis was performed using PhotoQuad. For this, every coral was identified to species level and the percentage cover was estimated from 25 points randomly set in an area of approximately 7200 cm². From the analysis of photo quadrats, Miyazawa et al. obtained two matrices: (1) absolute cover of Coral species and (2) Coral species presence/absence. Data cleaning and quality control were performed using R. Thus, Coral cover estimates were done from a randomly-selected sample composed of 375 points per transect (15 photos x 25 points = 375).
The results show that species composition and abundance of corals in Venezuela varied across different spatial scales. The greatest variability was found at the scale of sites. The scale of hundreds of kilometers was the second most important source of variation in the analysis. This result indicates that Coral assemblages in Venezuela only vary by 21.35% at the scale of region. Also, Miyazawa et al. found statistical significance at the scale of locations within regions, explaining 11.42% of the total variance. Thus, their results indicate that Coral assemblages are much variable at small to medium scales (i.e., from hundreds of meters to tens of kilometers) rather than hundreds of kilometers (i.e., regions) alone.
Overall, Western region was largely composed of Orbicella faveolata reefs, whereas the eastern reefs were dominated by Pseudodiploria strigosa. On the other hand, across the Central regions which included Oceanic and Coastal Reefs, mixed Coral communities were found. These species accounted for more than 75% of dissimilarities across localities, sites and regions.
Reefs of the Mountainous Star Coral, Orbicella faveolata, were found to dominate the western region of Venezuela. Evan D'Alessandro/Rosenstiel School of Marine and Atmospheric Science/University of Miami.
Longitude was highly correlated with observed spatial patterns, in contradiction to latitude. This result indicates that the relative position of each site along the Venezuelan coast (i.e., longitudinal variation), is an important factor to determine the features of Coral assemblages in Venezuela, instead of the proximity to the coast (i.e., latitudinal variation).
When assessing β-diversity, Miyazawa et al. found the highest variation in species presence/absence occurring between transect of the same site and between sites of the same locality. On the other hand, at larger scales, they found no significant dispersion in species composition between localities of the same region. In addition, site and locality were the spatial scales with the largest dissimilarity, with turn-over component as the main contributor. Furthermore, at larger scales, dissimilarity decreased and the contribution of turn-over and nestedness became evener. Thus, Miyazawa et al.'s result clearly shows that in Venezuela it is more likely to find changes in Coral species composition at small to medium scales (i.e., hundreds of meters to tens of kilometers) than at larger scales (i.e., hundreds of kilometers). Finally, the results indicate that Coral species found between the western, central and eastern region of Venezuela can result either from species replacement or from species loss, which is interpreted as a subset of a total pool of species.
The Symetrical Brain Coral, Pseudodiploria strigosa, was found to dominate the eastern reefs of Venezuela. Dennis Mahle/Flikr/Wikimedia Commons.
While Coral assemblages have been extensively studied in Venezuela, this is the first multiscale assessment to show the importance of spatial scales in determining the structure of these communities. Overall, Miyazawa et al. found that Coral assemblages in Venezuela are variable from hundreds of meters and hundreds of kilometers. Additionally, the largest changes in the composition of coral species occurred at a small scale with a clear predominance of species turn-overs. Also, longitude and latitude are a good predictors of Coral assemblage structure (i.e., species composition and abundance) further indicating that large-scale processes are also important to determine the structure of these communities.
Previous studies have acknowledged the importance of spatial scales on Coral assemblages in the Caribbean and in Venezuela. Particularly, the effect of upwelling and other related oceanographic processes has been pinpointed as strong factors that shape Coral assemblage structure along the Venezuelan coast where at least 12 upwelling points have been targeted. Miyazawa et al.'s study shows that Coral assemblages in Venezuela are much variable within and between localities than we originally expected. They found two fold higher variability at small to medium scales when compared to regions. However, Coral assemblages between the western, central and eastern regions differed by 21.35%, further indicating that differences at scale of region cannot be neglected. Moreover, about 32% of the total variance in coral species composition and abundance was associated to the residual which indicates that other variables like levels of anthropogenic disturbance, oceanic influence or other intermediate scales between those taken into consideration might also be relevant to determine the structure of these assemblages.
In Venezuela, encrusting communities associated with Mangrove roots have been studied following a spatial hierarchical design. Similar to Miyazawa et al.'s results, higher variation for this assemblages were found at the smallest and biggest scales. Moreover, Algal assemblages associated rocky platforms have previously been found to be highly variable at tens of kilometers and not between localities or regions, further illustrating the importance of local processes in providing structure to different assemblages of sessile organisms in the country.
Only a few numbers of studies encompassing multiple hierarchical spatial scales have been conducted in the Caribbean. For example, a multi-scale study surveyed a series of reef sites across locations and different bio-regions in the Caribbean for three major Coral taxonomic groups: Corals, Sponges and Octocorals. This study concluded these faunas exhibited considerable biogeographical variability at broad spatial scales (hundreds of kilometers). However similar to Miyazawa et al.'s study, it uncovered a higher degree of variability within sites highlighting the relevance of local ecological drivers (e.g., rugosity and wave exposure) in structuring Coral assemblages. Other studies have also taken into account the importance of spatial scales for Coral assemblages, but must of them have focused on total live Coral cover and total abundance of colonies.
Coral formations at Madrisquí Island in the Los Roques National Parl. Anaurora Yranzo in Debrot et al. (2019).
It is widely acknowledged that behind patterns are ecological processes that shape communities. In Coral Reef ecosystems, processes such as predation (e.g., herbivory) and competition have profound impacts on species abundance and composition at scales of a few meters. For example, very high densities or the absence of the Black Sea Urchin, Diadema antillarum, can determine the composition of Corals in patches of few m². Also, reef Fish can preferentially prey on certain Coral species, thus decreasing their abundance or making them less competitive than other neighboring Corals. In addition, the presence of Vermetid Snails could potentially modify the survival rates of Coral species. Furthermore, factors such as structural complexity (e.g., rugosity and micro scale habitat heterogeneity) may influence competition and survival of colonies depending on their sizes. Coral Reefs in Venezuela are known to be highly variable within and between sites but the processes responsible for these patterns have not been firmly established. However, spatial variation of Coral assemblages in Los Roques has been associated to changes in reef slopes.
Various anthropogenic stressors can alter biological interactions thereby affecting the structure of Coral assemblages. For example, overfishing often leads to the dominance of Macroalgae which escape to herbivory control. The selective extraction of species of carnivorous Fish can lead to an increment in the abundance of Echinoderms, which may also steer to increasing erosion, loss of topographic complexity and live Coral cover. Also, high intensity of recreational activities represents an important disturbance to marine communities, for example, Coral cover and the proportion of massive Corals is being found to be lower in places with high recreational diving intensity Thus, spatial variability recorded within sites of Morrocoy and Los Roques National Parks could be explained by their differences in touristic use for not all sites within these Marine Protected Areas are
exposed to the same Human pressures.
Echinoderms such as the Black Sea Urchin, Diadema antillarum, can determine the composition of Corals in patches of few m². Alchetron.
In addition, experimental evidence shows that some Coral species differ in resistance to environmental stressors such as sedimentation which could explain the patterns observed within sites and between locations showing higher sedimentary regimes and river presence (e.g., coastal versus oceanic reef sites). Furthermore, local oceanographic events can generate mortality which leads to changes in the structure of Coral assemblages. Differences between Playa Caimán, Cayo Norte and Sombrero (Morrocoy National Park) represent a example of how abnormal oceanographic conditions can alter benthic communities by killing dominant species in specific sites while promoting stable alternative states which hampers recovery. Miyazawa et al.'s results seem to support that each site/location in Venezuela possess different communities because they may have been affected/unaffected by different stressors and/or mortality events in different times. Thus, high spatial variability on Coral assemblages in Venezuela could be related to the differences in the disturbance regime and local history as noticed in other studies.
Coral bleaching mortality events may be patchy and could potentially affect Coral assemblages at different spatial scales (e.g., within sites, localities and regions). For example, in 2010 an increase in seawater temperature in Los Roques National Park affected 72% of the colonies at the study sites, showing bleaching and prevalence of diseases such as Black Band and White Plague. Extensive mortality caused changes in the community structure a year later. Other bleaching events recorded in Venezuela since 1998 primarily affected reefs in several oceanic islands and the western and central coast of Venezuela, but these events did not produce extensive mortality events like the one reported in 2010.
It is likely that the eastern coast of Venezuela remained less affected by bleaching events because of seasonal upwelling and unique environmental conditions. In fact one study found differences in Coral mortality rates during bleaching events, according to local environmental settings (e.g., light intensity, penetration, temperature and currents). However it has been argued that upwelling does not necessarily guarantee a refuge for Corals. Thus, in Venezuela differences between geographical regions could be strongly influenced by factors such as nutrient input and temperature decrease associated with the upwelling season. In fact, it has been suggested that during these periods the assemblages of Macroalgae become more dominant, which could modify the Coral assemblage structure through competitive processes.
A bleached Coral colony in Venezuela. Tubuceo/Geographical.
The Venezuelan coast is characterised by an upwelling period that occurs between January and June, in particular, the eastern region of the country is characterised by a large area of upwelling. However, various studies did not find an effect of the upwelling on factors such as the reproductive behavior of spawn Corals and the growth rates of colonies. On the other hand, the dynamic of Black Band Disease, one of the most important factors producing rapid Coral mortality in Cubagua has been shown to be deeply influenced by upwelling events. Thus, Miyazawa et al.'s results indicate that upwelling alone is not sufficient to explain the extremely variable nature of Coral assemblage.
Miyazawa et al. found that differences in Coral species composition occurred at spatial scales of hundreds of meters to tens of kilometers. Although it is known that β -diversity depends on the spatial scale at which it is measured, in most studies β -diversity is assumed to be homogeneous at small spatial scales. Changes in species composition at scales of tens of meters often occurs in highly-heterogeneous habitats sampled with enough resolution to detect these changes. In Miyazawa et al.'s study, they found that there was more likely to be different species composition within transects of a single site than between localities belonging to different regions. Thus, their results clearly indicate that Coral habitats in Venezuela are extremely variable at local scales, suggesting significant environmental heterogeneity within reef habitats, with Coral species probably forming mosaics or patches within a single habitat. However, it is not clear what are the conditions favoring this heterogeneity within the reef sites. This variation at the smallest scale could mean that Venezuelan Coral assemblages are in good condition, although there are patches of mortality, with more or fewer species, reflected in a high turnover rate.
At larger scales (i.e., between the eastern, central and western Venezuelan coast) Miyazawa et al. found quite similar and homogeneous Coral species composition, which may be partly due to the reduced pool of species that exist in the Caribbean when compared to the Indo Pacific region. In regions with larger species pools such as the Indo Pacific, β-diversity tends to be higher at larger spatial scales because species represent a subset of a total species pool. In Venezuela, coral reefs located at the oceanic sites and the western coast are dominated by Orbicella, whereas in the eastern coast and the majority of sites located at the central coast Pseudodiploria and Colpophyllia become
more important.
The Boulder Brain Coral is an important species on the eastern and central coasts of Venezuela. Coralpedia/University of Warwick/Darwin Initiative.
Miyazawa et al.'s results indicate that at scales of tens of kilometers species nestedness (loss) becomes as important as species turnover (replacement). These two components arise from different ecological phenomena. Species nestedness occurs when the biotas of sites with smaller numbers of species are subsets of the biotas at richer sites, reflecting a non-random process of species loss as a consequence of any factor that promotes the orderly disaggregation of assemblages. On the other hand, species turn-over (replacement) occurs as a consequence of environmental sorting and spatial and historical constraints (i.e., stochastic process). For example, processes such as settlement selectivity of Coral larvae could explain species turn overs at tens of meters and kilometers. Coral larvae are known to select certain characteristics in the habitat to settle down, e.g., presence of certain species of Coralline Algae or sounds of the reef. Miyazawa et al.'s results therefore indicate that Coral assemblage structure in Venezuela is probably regulated by a series of interconnected processes acting alone and/or in combination at various spatial scales. This result highlights the importance of creating scale-adapted management actions in Venezuela since the smallest scales reflect the greatest variability. However, very small Marine Protected Areas are often ineffective in achieving their conservation goals, so they must necessarily be chained into a large-scale strategy.
In summary, Coral assemblage structure in Venezuela is highly variable at different spatial scales but within locality variability seem to be very important. The processes that could underlie these patterns are diverse and complex and little experimental efforts to untangle the specific contribution of each factor have been conducted. Longitude is a good predictor of Coral assemblages in Venezuela. Upwelling-related processes could be targeted as potential candidates to explain longitudinal variation of Coral assemblages, whereas oceanographic/coastal processes could explain latitudinal variability. Regarding β -diversity, Coral assemblages are fairly homogeneous across the Venezuelan coast, while increasing spatial resolution shows greater heterogeneity, with smaller scales revealing a greater change in species composition. In addition, the replacement of species is a relevant phenomenon to explain these diversity patterns. Results from this study highlights the importance of taking into account local variability during the design and implementation of specific conservation efforts.
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