Aim: While geographical patterns of species richness are reasonably well explored for single wellstudied taxa, less is known about aggregate patterns of total richness for major biomes and their environmental correlates. Here we analyse continental-scale aggregate patterns of macrofaunal diversity for sandy beaches, a dominant habitat along the Atlantic and Pacific coasts of South America.Location: South American coastlines.Time period: Present day (data amassed from studies performed since 1971).Major taxa studied: Benthic macrofauna, including crustaceans, polychaetes and molluscs.Methods: We compiled richness information for all macrofaunal groups on 263 sandy beaches in South America using standard criteria. We further matched these data with environmental variables including sea surface temperature (SST), chlorophyll a, grain size, beach slope, tide range and various morphometrics. We used generalized linear mixed models to relate environmental factors to observed variation in total macrofaunal richness across all beaches, testing competing hypotheses about environmental correlates and possible drivers of latitudinal diversity.Results: Macrofaunal richness decreased from tropical to temperate beaches in the Pacific and followed a parabolic trend in the Atlantic, with the highest biodiversity found at tropical and midlatitudinal bands. Beach slope, tidal range and chlorophyll a mostly explained latitudinal trends in macrofaunal richness, followed by grain size, SST and ocean basin.Main conclusions: This study indicates that richness of macrofaunal species at a given beach is most closely related to characteristics of the physical habitat, such as beach slope, area and grain size. At this scale, planktonic food supply also appeared to be more important than temperature, which is a dominant explanatory variable of global-scale variation in species richness. K E Y W O R D S benthic macrofauna, environmental correlates, latitudinal diversity, macroecology, sandy beaches, South America
Summary: Log-spiral beaches display defined physical gradients alongshore. However, the majority of studies focus on the variability of a single population of macrofauna species. We aimed to investigate the variation in species distribution and in community structure along ten transects on a log-spiral beach. Principal component analysis indicated a clear physical gradient alongshore. Redundancy analysis showed that the sheltered end was related to smaller particle sizes, higher organic matter content and high densities of polychaetes. The exposed end was characterized by coarser sand, lower organic matter content and a high presence of crustaceans. Model selection indicated that the "best fit" to explain the variability in the number of individuals included grain size and beach slope. Variability of the polychaete Scolelepis squamata was best explained by grain size, slope and sediment sorting. The best model for the cirolanid Excirolana armata only included sediment sorting. The physical gradient in sediment texture and the beach slope explained more than one-third of the variability in community structure. The physical variables were also correlated with the distribution of the individual species. We showed that the physical gradient on log-spiral coasts may be an important driver of macrofauna variability, even at mesoscales and in dissipative conditions. Keywords: macrofauna assemblage; curved beaches; mesoscale; intertidal; morphodynamic; physical gradient.Variabilidad en la distributión de la macrofauna a lo largo de una playa disipativa arenosa en espiral en Rio de Janeiro, Sudeste de Brasil Resumen: Las playas en espiral muestran gradientes físicos definidos a lo largo de su recorrido. En este trabajo se investiga la relación entre la distribución en la estructura de comunidades de la macrofauna de una playa y sus gradientes físicos mediante el estudio de 10 transectos. El análisis de componentes principales reveló un claro gradiente físico a lo largo de la playa. El análisis de redundancia mostró que el extremo protegido se caracterizó con tamaños de grano menor, mayor contenido de materia orgánica y uma mayor densidad de anélidos poliquetos. El extremo expuesto se caracterizó por arena gruesa, bajo contenido de materia orgánica y una alta densidad de crustáceos. El mejor ajuste para explicar el número de individuos en una muestra se relacionó con el tamaño del grano de arena y la pendiente de la playa. La variabilidad del poliqueto Scolelepis squamata se explicó mejor en base al tamaño de grano, la pendiente y la selección de su sedimento. Por su parte, el mejor modelo para explicar la variabilidad del cirolánido Excirolana armata solo incluyó la selección del sedimento. El gradiente físico en la textura del sedimento y la pendiente de la playa explicó más de la tercera parte de la variabilidad en la estructura comunitaria. A su vez, las variables físicas también se correlacionaron con la distribución de especies individuales de la macrofauna. El gradiente físico en las playas en espiral puede explica...
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