Edge effect on a Neritina virginea (Neritimorpha, Neritinidae) population in a black mangrove stand (Magnoliopsida, Avicenniaceae: Avicennia germinans) in the Southern Caribbean * VIVIANA AMORTEGUI-TORRES , ALEXANDER TABORDA-MARIN & JUAN F. BLANCO *University of Antioquia, Faculty of Natural Sciences, Institute of Biology. *Corresponding author: [email protected] Abstract. Mangroves in the Caribbean and particularly in the Urabá Gulf (Colombia) are strongly threatened by selective logging and conversion to pastures and croplands. Specifically, extensive Avicennia germinans- basin stands were converted to pastures during the twentieth century, thus exposing benthic fauna to an edge effect. We measured this effect on the population of a numerically dominant gastropod (Neritina virginea). Despite its resistance to natural disturbances, it is sensitive to extreme anthropogenic disturbances, and it would therefore be a good biological indicator of basin-mangrove conversion to pastures. Forest structure variables, soil texture, porewater properties and snail density and size were measured in quadrats placed in pastures, pasture-mangrove edges, and mangrove interiors. Snail abundance sharply decreased from the mangrove interior to the edge and then gradually towards the pastures. Individuals in the pasture were predominantly >10mm, and they frequently exhibited shell corrosion compared to individuals found in the interior. There were increases in soil temperature and pH (but oxygen) from interior to pasture consistent with canopy openness. The occurrence of the mangrove edges has led to a marked ecosystem-wide deterioration; however, N. virginea (abundance, size, shell corrosion) could be used as a reliable short to midterm indicator of microhabitat and microclimatic differences observed across mangrove-pasture edge. Key words: basin mangrove, pasture, gastropods, Urabá Gulf Resumen. Efecto de borde sobre la población de Neritina virginea (Neritimorpha, Neritidae) en un rodal de mangle negro (Magnoliopsida, Avicenniaceae: Avicennia germinans) en el sur del Caribe. Los manglares del Caribe, particularmente del golfo de Urabá (Colombia) están fuertemente amenazados por la tala selectiva y la conversión a cultivos y potreros. Específicamente, grandes extensiones de rodales de cuenca de Avicennia germinans fueron convertidos a potreros durante el siglo veinte, exponiendo así a la fauna béntica a un efecto de borde. Medimos este efecto sobre la población de un gasterópodo numéricamente dominante (Neritina virginea). A pesar de su resistencia a las perturbaciones naturales, es sensible a perturbaciones extremas antrópicas, y por lo tanto, puede ser un buen indicador biológico de la conversión del manglar de cuenca a potreros. Variables de la estructura del bosque, textura del suelo, propiedades del agua intersticial y densidad y tamaño del caracol fueron medidas en cuadrantes ubicados en los potreros, en el límite potrero-manglar, y en los interiores del bosque. La abundancia del caracol disminuyó bruscamente desde el interior del manglar hacia el borde y gradualmente hacia el potrero. Los individuos en el potrero fueron predominantemente >10mm, y frecuentemente exhibieron desgaste en la concha, comparados con los encontrados en el interior. Hubo un incremento en la temperatura y el pH del suelo (pero no del oxígeno) desde los interiores hacia los potreros, consistente con la apertura del dosel. La presencia del borde ha llevado a un marcado deterioro del ecosistema, sin embargo, N. virginea (abundancia, tamaño, desgaste concha) puede ser usado como un indicador confiable a corto y mediano plazo de las diferencias en microhábitat y microclima observadas a través del límite manglar-potrero. Palabras clave: manglar de cuenca, potrerización, gasterópodos, golfo de Urabá Pan-American Journal of Aquatic Sciences (2013), 8(2):68-78 Edge effect in mangrove gastropods 69 Introduction clearing and conversion to other land covers, and Anthropogenic edges, as a straightforward therefore, anthropogenic edges have become a consequence of the widespread deforestation and dominant landscape feature. As an example, we fragmentation (FAO 2011), are key drivers of the recently proposed that the Urabá Gulf, located in the worldwide loss of species, especially in the Tropics (e. southwestern Colombian Caribbean coast, in the g. Murcia 1995, Fox et al. 1997, McIntyre & Hobbs vicinity of the border with Panama, is a deforestation 1999, Ries et al. 2004, Cavalcanti & Rodal 2010), but hotspot where coastal floodplain forests and no study has focused on mangroves. Anthropogenic mangroves were decimated during the second half of edges are formed after forest clearing to establish a the twentieth century (Blanco 2009, Blanco et al. different vegetation cover for different uses such as 2012). Although it is located within a worldwide agriculture and cattle ranching, thus changing known biodiversity hotspot (Chocó-Darién microclimatic and soil conditions not only at the open Ecoregion), overexploitation of natural coastal land but also several meters inside the untouched resources has increased extensive banana and plantain forest, and ultimately contributing to decimating or crops at the expense of the native forests during the eliminating sensitive populations of plants and animals last six decades. This situation has decimated and (Murcia 1995, Ries et al. 2004). Anthropogenic edges fragmented native forests and created long and marked are also evident in mangrove forests because this edges with crops and pastures (Arroyave et al. 2012). ecosystem is globally one of the most threatened due Mangroves, in particular the basin type, have been to deforestation and reclamation (Farnsworth & encroached by pastures in some sectors of the Eastern Ellison 1997, Duke et al. 2007, Alongi 2008). Coast of the Urabá Gulf, where the formerly Nowadays, although many studies have focused on Rhizophora mangle (Linnaeus 1753) and Avicennia mapping and quantifying mangrove loss, there are a germinans (Linnaeus 1764) –the dominant stands– few comprehensive assessments on the impacts of have been replaced by Laguncularia racemosa stands mangrove deforestation (Blanco et al. 2012). (Gaertner 1807) as a consequence of both, selective However, in contrast to the Old World mangroves, over-logging and rapidly expanding cattle ranching, there is little understanding about spatial patterns, thus causing clearly-defined anthropogenic edges rates and ecological impacts of deforestation in the (Corpourabá 2003, Taborda 2008, Blanco et al. 2012). Neotropics (Álvarez-Leon & Polanía 1996, Ellison & The ecological consequences of these edges have not Farnsworth 1996). The available studies in African been evaluated, although recent studies suggested a and Asian mangroves (Fondo & Martens 1998, decline in epifauna due to mangrove conversion to Sasekumar & Chong 1998, Diop et al. 2001) have pastures (Blanco & Castaño 2012). demonstrated that without mangrove exploitation, the Within the above mangrove-pasture transition, tree coverage offers spatio-temporal stability in it is noteworthy that Neritina virginea (Linnaeus physicochemical pore-water variables and therefore it 1758) (Neritimorpha: Neritidae) is one of the most weakens the correlations between gastropod abundant benthic macroinvertebrates (García & microdistribution and those variables. On the contrary, Palacio 2008, Ortiz & Blanco 2012). It is numerically correlations with physicochemical variables effects dominant on the sediments, and on mangrove prop- such as temperature and pH are strengthened in roots and trunks (Ortiz & Blanco 2012), and it is deforested areas (Fondo & Martens 1998). This widespread in the Caribbean, colonizing mangroves negative influence of deforestation on epifauna, could and seagrasses in coastal lagoons, bays and gulfs, but be present in Neotropical mangroves, given the high also rivers up to several kilometers inland (Blanco & clearing rates documented in some locations (e.g. Scatena 2006). In this paper, we used N. virginea as 0.85%/yr in Quintana Roo, Mexico, Hirales-Cota et al. possible bio-indicator of mangrove-pasture edge effect 2010). In these mangroves, forest structure, and because individuals exhibit: a) a significant resistance therefore epifauna, may respond in a complex way to to natural and anthropogenic disturbances and the anthropogenic forest-gaps and edges as proposed fluctuations in water level and microhabitat, thus by individual-based models (Berger et al. 2008). quickly recovering from minor disturbances (Blanco Mangroves in the Caribbean coast of & Scatena 2006, Blanco & Arroyave 2009, Blanco & Colombia, as most forests in this region (e. g. Etter & Castaño 2012), and b) they show a widespread Wyngaarden 2000, Gómez et al. 2005, Etter et al. distribution and strong tolerance to wide variations in 2006), are experiencing widespread fragmentation, salinity, pH, dissolved oxygen concentration, and soil Pan-American Journal of Aquatic Sciences (2013), 8(2):68-78 70 AMORTEGUI-TORRES, V. ET AL. texture (Blanco & Scatena 2006, Blanco & Castaño located in the middle part of the Eastern Coast, where 2012). These features are shared with other mangrove quaternary sedimentary sandflats have promoted the gastropods reported as bioindicators elsewhere (e.g. establishment of mangroves (Corpourabá 2003, Vermeij 1971, Cantera et al. 1983). This paper was Taborda 2008). Currently, there are 145ha of specifically aimed at determining: 1) how snail density mangroves, where red mangrove fringes