Nestedness patterns and the role of morphodynamics and spatial distance on sandy beach fauna: ecological hypotheses and conservation strategies

An analysis of human impact on sandy coasts of the Costa Rican Pacific

Increased human activities on sandy beaches have significantly impacted ecosystems. This study aims to determine whether beaches with higher tourism and human activities exhibit greater impact on various ecological indicators compared to less-visited beaches. Ecological monitoring was performed across three sandy beaches, where sand samples were collected during three visits. Sediment characteristics, macrofauna presence, crab burrows abundance, and solid waste were analyzed. Fourteen benthic macrofauna taxa, 1209 crab burrows, and 7645 intertidal organisms were recorded. Notably, 98.8 % of the solid waste consisted of plastic items. Differences in organic matter, carbonates, the number of crab burrows, and macrofauna were observed, indicating that increased tourism and human activity influence these ecological indicators. These findings represent an initial step toward more rapid and comprehensive ecological monitoring of beaches, contributing to coastal environmental management and decision-making.

The efficacy of benthic indices to evaluate the ecological quality and urbanization effects on sandy beach ecosystems

Benthic indices have been widely used across different coastal ecosystems to assess ecological quality and detect anthropic impacts, but very few studies investigated their effectiveness on sandy beaches. Here, we evaluated and compared the efficacy of 12 assemblage-based benthic indices in assessing ecological quality in beaches, across a gradient of anthropic pressure and natural variability in 90 sandy beach sites. Overall, when sandy beaches were considered collectively, benthic indices had a poor performance in identifying decreases in ecological quality with increasing urbanization. However, when each morphodynamic type was evaluated separately, a few indices, especially those that were calibrated by reference conditions (i.e., M-AMBI, BAT, and BEQI-2), showed promising results for dissipative, and to a lesser extent, intermediate beaches. For reflective beaches, indices performed poorly, likely a reflection of the stronger natural disturbance these beaches are subjected to. Among functional indices, richness was found to be lower in urbanized beaches, but only in dissipative ones. Overall, our results show that benthic indices have the potential to be incorporated in sandy beach management and monitoring programs, especially for dissipative and intermediate beaches. For reflective beaches, given the early stage of studies with benthic indices in beaches, more research is needed to corroborate the observed patterns.

Does coastal armoring affect biodiversity and its functional composition on sandy beaches?

Sandy beaches are increasingly squeezed due to the construction of backshore man-made structures (i.e., coastal armoring) and current global changes. Coastal armoring impacts beach sediment dynamics, inducing erosion and habitat loss, threatening biodiversity processes and the functional roles of sandy beach organisms. Here, we examine how the abundance, taxonomic richness, and functional richness of sandy beach fauna are affected by coastal armoring. We compared macrobenthic infaunal communities on five armored beaches (with backshore urban structures) and five vegetated beaches (not-armored). We also evaluated the abundance and biomass of upper-beach arthropods using pitfall traps, comparing armored and vegetated segments within the beaches. Infaunal richness and abundance were lower at armored beaches, mainly in the subtidal zones, because of a reduction in polychaete and molluscan abundance. There was no difference in overall functional richness between the armored and vegetated beaches. Nevertheless, we found that functional groups such as small suspension feeders were more associated with armored beaches, while large-bodied species and predators were more frequent at vegetated beaches. Pitfall traps showed that coastal armoring also reduced the abundance of the upper-beach coleopteran Phaleria testacea, leading to a loss of biomass. Therefore, our data suggest that coastal armoring can influence the functional composition of sandy beach biodiversity and significantly impact macrobenthic abundance and biomass.

Evaluation of the effects of urbanization and environmental features on sandy beach macrobenthos highlights the importance of submerged zones

To assess how sandy beach biodiversity is influenced by the effects of environmental features and urbanization, we sampled the macrofauna in the whole across-shore gradient of 90 beach sites in Southeast Brazil. We found that morphodynamic features were the main drivers of macrobenthos, but urbanization-related variables, such as number of beachgoers and number of constructions on the upper shore, decreased species richness and biomass. We also found that submerged zones sustained the highest number of species and biomass in the across-shore gradient, but were the most impacted by human activities. By demonstrating the ecological importance of submerged zones, our results show that beach management practices, which are mainly focused on the upper shore, are missing important components of beach biodiversity. To secure the sustainability of beach ecosystems, management initiatives should include both their social and ecological components and consider the entire Littoral Active Zone as the proper management unit.

Assessing the benthic quality status of three fine sand tropical beaches from the Andaman Islands through the application of marine biotic indices

The rapid coastal development in the Andaman Islands has resulted in littoral habitat degradation. Understanding the performances of marine biotic indices and the interpretation and translation of those results into coastal health assessment could become an integral tool in future monitoring and management policies. In this line of efforts, the ecological quality status of three sandy beaches, two urban and one nonurban, was evaluated by using three marine biotic indices. The faunal community belonged to moderately well-sorted fine sand biocenosis. The relatively high species richness (15.9 ± 0.80 taxa sample^-1) and moderate abundance (563 ± 38.8 ind.m^-2) were features of the benthic fauna. The urban beaches (Aberdeen Bay and Carbyn's Cove) corresponded to tolerant benthic communities. Malacoceros indicus, Grandidierella megnae and Scolelepis squamata (tolerant species), and Ampelisca diadema (indifferent species) were the major constituents of urban beaches, while Scoloplos capensis, Urothoe grimaldii, and Urothoe platydactyla (sensitive species) were important at the nonurban beach (Wandoor). The high-good quality status prevailed across the spatial and temporal scales except for Carbyn's cove beach, where good-poor status was noticed. The M-AMBI appeared to be the most robust measure in distinguishing the impact between the urban and nonurban beaches. The constrained ordinations revealed a gradient of disturbance across the beaches. The distinct patterns of sample segregation were the result of the ecological response. This attempt should be considered a comprehensive measure of quality assessment of beaches under human pressure and draw a parallel line of evidence to global studies on sandy beaches.

Locally developed models improve the accuracy of remotely assessed metrics as a rapid tool to classify sandy beach morphodynamics

Classification of beaches into morphodynamic states is a common approach in sandy beach studies, due to the influence of natural variables in ecological patterns and processes. The use of remote sensing for identifying beach type and monitoring changes has been commonly applied through multiple methods, which often involve expensive equipment and software processing of images. A previous study on the South African Coast developed a method to classify beaches using conditional tree inferences, based on beach morphological features estimated from public available satellite images, without the need for remote sensing processing, which allowed for a large-scale characterization. However, since the validation of this method has not been tested in other regions, its potential uses as a trans-scalar tool or dependence from local calibrations has not been evaluated. Here, we tested the validity of this method using a 200-km stretch of the Brazilian coast, encompassing a wide gradient of morphodynamic conditions. We also compared this locally derived model with the results that would be generated using the cut-off values established in the previous study. To this end, 87 beach sites were remotely assessed using an accessible software (i.e., Google Earth) and sampled for an in-situ environmental characterization and beach type classification. These sites were used to derive the predictive model of beach morphodynamics from the remotely assessed metrics, using conditional inference trees. An additional 77 beach sites, with a previously known morphodynamic type, were also remotely evaluated to test the model accuracy. Intertidal width and exposure degree were the only variables selected in the model to classify beach type, with an accuracy higher than 90% through different metrics of model validation. The only limitation was the inability in separating beach types in the reflective end of the morphodynamic continuum. Our results corroborated the usefulness of this method, highlighting the importance of a locally developed model, which substantially increased the accuracy. Although the use of more sophisticated remote sensing approaches should be preferred to assess coastal dynamics or detailed morphodynamic features (e.g., nearshore bars), the method used here provides an accessible and accurate approach to classify beach into major states at large spatial scales. As beach type can be used as a surrogate for biodiversity, environmental sensitivity and touristic preferences, the method may aid management in the identification of priority areas for conservation.

Cumulative stressors impact macrofauna differentially according to sandy beach type: A meta-analysis

Many studies have demonstrated human impacts on sandy beach macroinvertebrates. However, little is known about causative drivers among multiple co-occurring stressors and how these interact with natural habitat conditions to yield specific faunal responses. We performed a global meta-analysis to shed light on how interactions between human disturbances and beach morphodynamics shape macroinvertebrate populations. We found that supralittoral forms (Talitridae and Ocypodidae) responded more negatively to the Human Modification Metric (a proxy for urbanization) on dissipative beaches, whereas intertidal organisms (Hippidae and Cirolanidae) showed more negative responses on non-dissipative beaches. Based on these findings we propose the Cumulative Harshness Hypothesis (CHH), which predicts higher sensitivity of beach macroinvertebrates to human disturbances when inhabiting a harsher physical environment according to their life histories. Secondly, we compared the response of macroinvertebrates to urbanization levels from local to larger scales (from 500 to 50000 m). Supralittoral families responded more negatively to local urbanization, which leads to habitat loss due to removal or reduction of upper beach zones. Conversely, intertidal organisms with planktonic larval stages were more affected by urbanization at the largest spatial scales, which we hypothesize disrupts metapopulation dynamics by impacting the supply of larvae that could colonize human-disturbed beaches. The differential effects of human disturbances on macroinvertebrates according to beach morphodynamics suggest that the efficiency of these ecological indicators for beach monitoring is context-dependent. Focusing on multiple stressors rather than on a single one is also critical to mitigate human impacts on these threatened ecosystems.