Exploring Macroinvertebrate Species Distributions at Regional and Local Scales across a Sandy Beach Geographic Continuum

Lastra M (2012) Exploring Macroinvertebrate Species Distributions at Regional and Local Scales across a Sandy Beach Geographic Continuum. PLoS ONE 7(6): e39609. doi:10.1371/journal.pone.0039609 Exploring Macroinvertebrate Species Distributions at Regional and Local Scales across a Sandy Beach Geographic Continuum Iva n F. Rodil 0 Tanya J. Compton 0 Mariano Lastra 0 Brian R. MacKenzie, Technical University of Denmark, Denmark 0 1 Departamento de Ecolog a y Biolog a Animal, Universidad de Vigo , Vigo , Spain , 2 National Institute of Water and Atmospheric Research , Hamilton , New Zealand Exposed sandy beaches are highly dynamic ecosystems where macroinvertebrate species cope with extremely variable environmental conditions. The majority of the beach ecology studies present exposed beaches as physically dominated ecosystems where abiotic factors largely determine the structure and distribution of macrobenthic communities. However, beach species patterns at different scales can be modified by the interaction between different environmental variables, including biotic interactions. In this study, we examined the role of different environmental variables for describing the regional and local scale distributions of common macrobenthic species across 39 beaches along the North coast of Spain. The analyses were carried out using boosted regression trees, a relatively new technique from the field of machine learning. Our study showed that the macroinvertebrate community on exposed beaches is not structured by a single physical factor, but instead by a complex set of drivers including the biotic compound. Thus, at a regional scale the macrobenthic community, in terms of number of species and abundance, was mainly explained by surrogates of food availability, such as chlorophyll a. The results also revealed that the local scale is a feasible way to construct general predictive speciesenvironmental models, since relationships derived from different beaches showed similar responses for most of the species. However, additional information on aspects of beach species distribution can be obtained with large scale models. This study showed that species-environmental models should be validated against changes in spatial extent, and also illustrates the utility of BRTs as a powerful analysis tool for ecology data insight. - Exposed sandy beaches dominate the worlds open coastlines shaping one of the most valuable coastal ecosystems. Increasing recreational and financial demands are placing escalating pressures on sandy beaches at unprecedented scales [1,2]. Although exposed beaches have been considered marine deserts, they are teeming with life that provides numerous goods and services [3]. Specifically, macrobenthos are an integral component of beaches that process large quantities of organic material, recycle nutrients, and are also a fundamental food resource for higher vertebrates of both, commercial importance and conservation significance [3,4]. Macroinvertebrates in exposed beaches show specific adaptations to the dynamic environment provided by the interactions of sand, waves, and tides, also called beach morphodynamics [3]. An understanding of the relationships between beach morphodynamics and the macroinvertebrate community has been critical in beach ecology studies [48]. Studies on beach macroinvertebrate have mainly focused on the elucidation of spatial and temporal patterns along a gradient of contrasting morphodynamic types from harsh reflective (coarse sands, steep slopes) to benign dissipative (fine sands, gentle slopes) beaches [3,4,6,7,9]. These studies showed that physical variables inevitably drive the composition of the macroinvertebrate community. Therefore, the number of macroinvertebrate species, their abundance and/or their biomass, generally increases towards the more benign morphodynamic beach type [4]. Beach type thus represents a dynamic spectrum of environmental gradients where both, local and regional factors constrain and modify macroinvertebrate distributions. However, the processes that explain macroinvertebrate species distribution patterns across different spatial scales using several morphodynamic conditions and beach characteristics have not been examined. There are a number of environmental and biological variables that influence the distribution and abundance of beach macroinvertebrates, including sediment grain size, hydrodynamic regime, biotic interactions and food supply [6,7,1012]. Some of these variables affect beach species patterns at a local scale (e.g. slope), other variables have an influence over a broader scale (e.g. food availability); while other variables have both local and large scale effects (e.g. sediment characteristics). Thus, species-environment relationships can change with the scale of observation, because at different spatial scales different processes may influence species distributions [13]. For example, a species may be locally limited to beaches with fine grain sizes, but at a larger geographical scale species can also be limited by other variables, such as food availability. Therefore, over a broad spatial scale, the same species may show preferences for beaches with coarser sediment types, but also beaches that are food-enriched. The analysis of scale dependent relationships can help to distinguish whether specific variables are direct drivers of species patterns or are mediated by other variables [13]. In this study, we examined the role of different environmental variables for describing the regional and the local scale distributions of representative macrobenthic species across 39 beaches along the North coast of Spain. This shoreline is characterised by extensive, intermediate morphodynamic beaches that differ in size, orientation and physical characteristics across more than 2000 km of coastline. This large continuum of beaches provides an opportunity to assess the drivers of species distributions at contrasting-scales, i.e., regional versus local responses of macroinvertebrates over a gradient of intermediate beaches. To identify which environmental variables explain the regional distribution of the macrobenthic species we used estimates of chlorophyll a, temperature and morphodynamic variables that broadly characterise a beach. We expect that at a regional scale, the macroinvertebrate abundance and richness would follow a longitudinal gradient related to primary productivity [14]. On the other hand, for each beach a specific abiotic variable might be the driver limiting or shaping a local species response. To test whether the responses of individual species to the same environmental variable differ at a local scale, we examined the fitted responses of species within a beach to local variables (e.g., slope, sediment). Different techniques have been used for testing beach species environment relationships (e.g., [4,6,7,15,16]), but these have mainly focused on contrasting morphodynamic gradients at a s (...truncated)