Estructura poblacional del árbol Avicennia germinans (Acanthaceae) en un manglar alterado del Golfo de México / Population structure of Avicennia germinans trees (Acanthaceae) in a disturbed mangrove in the Gulf of Mexico

Resumen Introducción: Los manglares están en declive en el mundo, conocer su estructura y regeneración en diferentes niveles de alteración puede ayudar a comprender la dinámica poblacional ante situaciones de estrés. Objetivo: La meta principal del estudio fue analizar la estructura comunitaria y estimar la trayectoria de las poblaciones de Avicenia germinans en diferentes sitios de manglar que presentan tres niveles de alteración. Métodos: Ocho transectos de aproximadamente 500 m de largo y colocados de manera perpendicular a la laguna de Tampamachoco, Veracruz, interceptaron tres niveles de alteración en los sitios estudiados del bosque de manglar. En cada transecto, cuadrantes de 10 x 10 m, 5 x 5 m, y 1 x 1 m sirvieron para contar todos los árboles de mangle por especie y estadio de madurez en 2017 y 2018. Resultados: La especie más abundante fue: A. germinans seguida de Rhizophora mangle y Laguncularia racemosa. Con base en su abundancia, se encontraron diferencias significativas según los tres grados de alteración (AM R = 0.6, P < 0.001), ilustrado en un análisis de ordenación no métrico (nMDS). Una tabla de vida para A. germinans permitió un análisis poblacional que mostró una tasa intrínseca de incremento poblacional (r) negativa para los sitios que presentan alteración (-0.0027 u -0.0774) y positiva (0.0289) en el sitio sin alteración aparente. La supervivencia del estadio de plántula a juvenil varia de 50 % en el sitio no alterados a 5 y 4 % en los otros dos sitios. Conclusiones: Se necesitan medidas para incrementar la sobrevivencia de las plántulas y revertir la disminución de las trayectorias en los sitios con alteración, de otra forma el manglar tendrá una viabilidad reducida en el término medio.

Abstract Introduction: Globally, mangroves are declining; characterizing their structure and regeneration at different disturbance levels can help understand their responses to stressful situations. Objective: The study's primary goal was to analyze the mangrove community structure and to estimate the populations' trajectories of Avicenia germinans at different mangrove sites that present three levels of disturbance. Methods: Eight transects of approximately 500 m in length and laid perpendicular to the Tamapamchoco lagoon, Veracruz, intersected three levels of disturbance currently present on the mangrove forest. On each transect, 10 x 10 m, 5 x 5 m, and 1 x 1 m quadrants served to account for all mangrove trees by species and stage of maturity in 2017 and 2018. Results: The most abundant species was: A. germinans followed by Rhizophora mangle and Laguncularia racemosa. Based on their abundance, we found significant differences for the three levels of disturbance (ANOSIM R = 0.6, P < 0.001) presented in a non-metric ordination (nMDS). A life table for A. germinans allowed a population analysis that showed an intrinsic rate of population increase (r) that was negative at the disturbed sites (-0.0027 and -0.0774) and positive (0.0289) at the apparently undisturbed site. Seedling to juvenile stage survival ranged from 50 % at the undisturbed site to 5 and 4 % at the disturbed sites. Conclusions: Measures to increase the survival of seedlings are necessary to reverse the decreasing population trajectories at the disturbed sites; otherwise, the mangrove will have reduced viability in the medium term.

Coral disease prevalence estimation and sampling design.

In the last decades diseases have changed coral communities' structure and function in reefs worldwide. Studies conducted to evaluate the effect of diseases on corals frequently use modified adaptations of sampling designs that were developed to study ecological aspects of coral reefs. Here we evaluate how efficient these sampling protocols are by generating virtual data for a coral population parameterized with mean coral density and disease prevalence estimates from the Caribbean scleractinian Orbicella faveolata at the Mexican Caribbean. Six scenarios were tested consisting of three patterns of coral colony distribution (random, randomly clustered and randomly over-dispersed) and two disease transmission modes (random and contagious). The virtual populations were sampled with the commonly used method of belt-transects with variable sample-unit sizes (10 × 1, 10 × 2, 25 × 2, 50 × 2 m). Results showed that the probability of obtaining a mean coral disease prevalence estimate of ±5% of the true prevalence value was low (range: 11-48%) and that two-sample comparisons achieved rather low power, unless very large effect sizes existed. Such results imply low statistical confidence to assess differences or changes in coral disease prevalence. The main problem identified was insufficient sample size because local mean colony size, density and spatial distribution of targeted coral species was not taken into consideration to properly adjust the sampling protocols.

Does Dark-Spot Syndrome Experimentally Transmit among Caribbean Corals?

Over the last half-century, coral diseases have contributed to the rapid decline of coral populations throughout the Caribbean region. Some coral diseases appear to be potentially infectious, yet little is known about their modes of transmission. This study experimentally tested whether dark-spot syndrome on Siderastrea siderea was directly or indirectly transmissible to neighboring coral colonies. We also tested whether open wounds were necessary to facilitate disease transmission. At the completion of the experiments, we sampled bacterial communities on diseased, exposed, and healthy coral colonies to determine whether bacterial pathogens had transmitted to the susceptible colonies. We saw no evidence of either direct or waterborne transmission of dark-spot syndrome, and corals that received lesions by direct contact with diseased tissue, healed and showed no signs of infection. There were no significant differences among bacterial communities on healthy, exposed, and diseased colonies, although nine individual ribotypes were significantly higher in diseased corals compared with healthy and exposed corals, indicating a lack of transmission. Although our experiments do not fully refute the possibility that dark-spot syndrome is infectious and transmissible, our results suggest that in situ macroscopic signs of dark-spot syndrome are not always contagious.

Controls on coral-ground development along the northern Mesoamerican Reef tract.

Coral-grounds are reef communities that colonize rocky substratum but do not form framework or three-dimensional reef structures. To investigate why, we used video transects and underwater photography to determine the composition, structure and status of a coral-ground community located on the edge of a rocky terrace in front of a tourist park, Xcaret, in the northern Mesoamerican Reef tract, Mexico. The community has a relatively low coral, gorgonian and sponge cover (<10%) and high algal cover (>40%). We recorded 23 species of Scleractinia, 14 species of Gorgonacea and 30 species of Porifera. The coral community is diverse but lacks large coral colonies, being dominated instead by small, sediment-tolerant, and brooding species. In these small colonies, the abundance of potentially lethal interactions and partial mortality is high but decreases when colonies are larger than 40 cm. Such characteristics are consistent with an environment control whereby storm waves periodically remove larger colonies and elevate sediment flux. The community only survives these storm conditions due to its slope-break location, which ensures lack of burial and continued local recruitment. A comparison with similar coral-ground communities in adjacent areas suggests that the narrow width of the rock terrace hinders sediment stabilization, thereby ensuring that communities cannot escape bottom effects and develop into three-dimensional reef structures on geological time scales.