Local scale processes drive long-term change in biodiversity of sandy beach ecosystems

Uptake of polycyclic aromatic hydrocarbons via high-energy water accommodated fraction (HEWAF) by beach hoppers (Amphipoda, Talitridae) using different sandy beach exposure pathways

Sandy beach ecosystems are highly dynamic coastal environments subject to a variety of anthropogenic pressures and impacts. Pollution from oil spills can damage beach ecosystems through the toxic effects of hydrocarbons on organisms and the disruptive nature of large-scale clean-up practices. On temperate sandy beaches, intertidal talitrid amphipods are primary consumers of macrophyte wrack subsidies and serve as prey for higher trophic level consumers, such as birds and fish. These integral organisms of the beach food web can be exposed to hydrocarbons by direct contact with oiled sand through burrowing and by the consumption of oiled wrack. We experimentally evaluated the primary polycyclic aromatic hydrocarbon (PAH) exposure pathway via high-energy water accommodated fraction (HEWAF) for a species of talitrid amphipod (Megalorchestia pugettensis). Our results indicated that tissue PAH concentrations in talitrids were six-fold higher in treatments that included oiled sand compared to those with only oiled kelp and the controls.

Effects of natural and anthropogenic storm-stranded debris in upper-beach arthropods: Is wrack a prey hotspot for birds?

Storm-stranded debris (i.e., wrack) are important components for the functioning of beach ecosystems. With the current increase in extreme storm events, beached wrack is expected to change globally. However, little is known about how different types of wrack can affect beach biodiversity. Here, we hypothesized that natural debris (algae and land-plant debris) would optimize the short-term aggregation of benthic arthropods on the beach ecosystem, while anthropogenic debris (plastics) would not perform this function. We also expected that short-term aggregations of arthropods in the natural debris would create a transient prey hotspot (i.e., points of high prey concentration) for birds on the beach. Thus, we performed manipulative field experiments with debris addition and predator exclusion by cage on a short temporal scale (maximum 20 days). We found that natural debris aggregated higher community abundances than anthropic debris and treatments without debris, while community richness was not affected by wrack. No differences were noted when comparing the community aggregation on plastic debris and treatments without debris. The coleopterans were the group responsible for this aggregation, mainly represented by Phaleria testacea, which aggregated on natural debris with abundances five times greater than those on plastic debris. Nevertheless, we did not find any evidence of increased predation by birds on the coleopterans aggregated in the natural debris. We conclude that arthropod aggregation in the wrack is a phenomenon primarily associated with natural debris, not occurring in plastic debris, although the role of this faunal aggregation as a prey hotspot for birds was not evident in the short term. These results showed that the wrack type matters in terms of consequences for beach arthropods, creating concerns against beach cleaning methods that are adopted indiscriminately, also signaling the need for long-term studies to proceed with investigating the wrack functions for top predators on sandy beaches.

The role of inputs of marine wrack and carrion in sandy-beach ecosystems: a global review

Sandy beaches are iconic interfaces that functionally link the ocean with the land via the flow of organic matter from the sea. These cross-ecosystem fluxes often comprise uprooted seagrass and dislodged macroalgae that can form substantial accumulations of detritus, termed 'wrack', on sandy beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on beaches form a rich food source ('carrion') for a diversity of scavenging animals. Here, we provide a global review of how wrack and carrion provide spatial subsidies that shape the structure and functioning of sandy-beach ecosystems (sandy beaches and adjacent surf zones), which typically have little in situ primary production. We also examine the spatial scaling of the influence of these processes across the broader land- and seascape, and identify key gaps in our knowledge to guide future research directions and priorities. Large quantities of detrital kelp and seagrass can flow into sandy-beach ecosystems, where microbial decomposers and animals process it. The rates of wrack supply and its retention are influenced by the oceanographic processes that transport it, the geomorphology and landscape context of the recipient beaches, and the condition, life history and morphological characteristics of the macrophyte taxa that are the ultimate source of wrack. When retained in beach ecosystems, wrack often creates hotspots of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. Nutrients are produced during wrack breakdown, and these can return to coastal waters in surface flows (swash) and aquifers discharging into the subtidal surf. Beach-cast kelp often plays a key trophic role, being an abundant and preferred food source for mobile, semi-aquatic invertebrates that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach-cast marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst deposits of organic matter on sandy-beach ecosystems underpin a range of ecosystem functions and services, they can be at variance with aesthetic perceptions resulting in widespread activities, such as 'beach cleaning and grooming'. This practice diminishes the energetic base of food webs, intertidal fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) are predicted to cause substantial reductions in the amounts of marine organic matter reaching many beach ecosystems, likely causing flow-on effects for food webs and biodiversity. Similarly, future sea-level rise and increased storm frequency are likely to alter profoundly the physical attributes of beaches, which in turn can change the rates at which beaches retain and process the influxes of wrack and animal carcasses. Conservation of the multi-faceted ecosystem services that sandy beaches provide will increasingly need to encompass a greater societal appreciation and the safeguarding of ecological functions reliant on beach-cast organic matter on innumerable ocean shores worldwide.

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.

Disturbance and nutrients synchronise kelp forests across scales through interacting Moran effects

Spatial synchrony is a ubiquitous and important feature of population dynamics, but many aspects of this phenomenon are not well understood. In particular, it is largely unknown how multiple environmental drivers interact to determine synchrony via Moran effects, and how these impacts vary across spatial and temporal scales. Using new wavelet statistical techniques, we characterised synchrony in populations of giant kelp Macrocystis pyrifera, a widely distributed marine foundation species, and related synchrony to variation in oceanographic conditions across 33 years (1987-2019) and >900 km of coastline in California, USA. We discovered that disturbance (storm-driven waves) and resources (seawater nutrients)-underpinned by climatic variability-act individually and interactively to produce synchrony in giant kelp across geography and timescales. Our findings demonstrate that understanding and predicting synchrony, and thus the regional stability of populations, relies on resolving the synergistic and antagonistic Moran effects of multiple environmental drivers acting on different timescales.

Species identity drives ecosystem function in a subsidy-dependent coastal ecosystem

Declines in species diversity carry profound implications for ecosystem functioning. Communities of primary producers and consumers interact on evolutionary as well as ecological time scales, shaping complex relationships between biodiversity and ecosystem functioning. In subsidized ecosystems, resource inputs are independent of consumer actions, offering a simplified view of the relationship between species diversity and function for higher trophic levels. With food webs supported by substantial but variable inputs of detritus from adjacent marine ecosystems, sandy beaches are classic examples of subsidized ecosystems. We investigated effects of consumer species diversity and identity on a key ecological function, consumption of kelp wrack from nearshore giant kelp (Macrocystis pyrifera) forests. We assessed effects of species richness on kelp consumption by experimentally manipulating richness of six common species of invertebrate detritivores in laboratory mesocosms and conducting field assays of kelp consumption on beaches. Consumer richness had no effect on kelp consumption in the field and a slight negative effect in laboratory experiments. Kelp consumption was most strongly affected by the species composition of the detritivore community. Species identity and body size of intertidal detritivores drove variation in kelp consumption rates in both experiments and field assays. Our results provide further evidence that species traits, rather than richness per se, influence ecosystem function most, particularly in detrital-based food webs with high functional redundancy across species. On sandy beaches, where biodiversity is threatened by rising sea levels and expanding development, our findings suggest that loss of large-bodied consumer species could disproportionally impact ecosystem function.

Multiple climate change-driven tipping points for coastal systems

As the climate evolves over the next century, the interaction of accelerating sea level rise (SLR) and storms, combined with confining development and infrastructure, will place greater stresses on physical, ecological, and human systems along the ocean-land margin. Many of these valued coastal systems could reach "tipping points," at which hazard exposure substantially increases and threatens the present-day form, function, and viability of communities, infrastructure, and ecosystems. Determining the timing and nature of these tipping points is essential for effective climate adaptation planning. Here we present a multidisciplinary case study from Santa Barbara, California (USA), to identify potential climate change-related tipping points for various coastal systems. This study integrates numerical and statistical models of the climate, ocean water levels, beach and cliff evolution, and two soft sediment ecosystems, sandy beaches and tidal wetlands. We find that tipping points for beaches and wetlands could be reached with just 0.25 m or less of SLR (~ 2050), with > 50% subsequent habitat loss that would degrade overall biodiversity and ecosystem function. In contrast, the largest projected changes in socioeconomic exposure to flooding for five communities in this region are not anticipated until SLR exceeds 0.75 m for daily flooding and 1.5 m for storm-driven flooding (~ 2100 or later). These changes are less acute relative to community totals and do not qualify as tipping points given the adaptive capacity of communities. Nonetheless, the natural and human built systems are interconnected such that the loss of natural system function could negatively impact the quality of life of residents and disrupt the local economy, resulting in indirect socioeconomic impacts long before built infrastructure is directly impacted by flooding.

Effects of Repeated Sand Replenishment Projects on Runs of a Beach-Spawning Fish, the California Grunion

Beach habitats are diminishing globally, particularly in urban areas, as sea-level rise, erosion, and shoreline hardening, along with reduced sediment inputs, combine to squeeze the coast. In California, USA an endemic marine fish, the California grunion, spawns on sandy beaches during late-night spring tides. Its unique recreational fishery is managed by the California Department of Fish and Wildlife. The City of Oceanside, CA contracts for annual harbor dredging and, after testing, places the sandy sediment on its public beach. The effects on local beach wildlife from this annual sand replenishment are not known. We examined the effect of this repeated activity as a case study over three years on the spawning runs of the California grunion. Some spawning runs occurred in all three years, but the fish avoided areas with high scarps in the intertidal zone that developed following sand placement activity. Grunion spawning runs have declined in the habitat range as a whole over the past two decades, and those in Oceanside have declined to an even greater extent. Increasing sandy beach habitat can be beneficial to wildlife, but the method of placement, timing of the project, and fate of the beach afterward can modulate or prevent beneficial effects. Frequent repetition of sand placement may accumulate impacts without allowing sufficient time for the ecosystem to recover. Rather than improving the habitat, these repeated projects in Oceanside may degrade the spawning habitat for the grunion. Alternative discharge methods and locations, slope and elevation designs, sediment volumes, and greater care in beach fill practices should be implemented to reduce future impacts.

Long-Term Ecological Research and Evolving Frameworks of Disturbance Ecology

Detecting and understanding disturbance is a challenge in ecology that has grown more critical with global environmental change and the emergence of research on social–ecological systems. We identify three areas of research need: developing a flexible framework that incorporates feedback loops between social and ecological systems, anticipating whether a disturbance will change vulnerability to other environmental drivers, and incorporating changes in system sensitivity to disturbance in the face of global changes in environmental drivers. In the present article, we review how discoveries from the US Long Term Ecological Research (LTER) Network have influenced theoretical paradigms in disturbance ecology, and we refine a framework for describing social–ecological disturbance that addresses these three challenges. By operationalizing this framework for seven LTER sites spanning distinct biomes, we show how disturbance can maintain or alter ecosystem state, drive spatial patterns at landscape scales, influence social–ecological interactions, and cause divergent outcomes depending on other environmental changes.

Functional over-redundancy and vulnerability of lichen communities decouple across spatial scales and environmental severity

According to the insurance hypothesis, high taxonomic diversity should ensure ecosystem stability because of functional redundancy, whereas reduced functional diversity that results from species loss should affect ecosystem sensitivity, resilience, and vulnerability. However, even in species-rich ecosystems, functional over-redundancy (FOR; i.e., the tendency of most species to cluster into a few over-represented functional entities) in some cases may result in under-representation of many functions, and the ecosystem might become highly vulnerable. Using a stratified random sampling design with nested spatial levels (nine land use strata, 70 plots, 435 trees/rock outcrops, and 9845 quadrats), we recorded the occurrence of over 350 species of epiphytic and rock-dwelling lichens in semi-arid ecosystems in western Sardinia, where solar radiation defines a wide environmental gradient. By accounting for species functional traits, such as growth form, photosynthetic strategies, and reproductive strategies, we obtained 43 functional entities (>60% of all possible combinations) and tested the scale-dependency of FOR and functional vulnerability (FV, i.e., the risk of losing functional entities) by generalized linear mixed models. We found that FOR increased and FV decreased with increasing spatial scale, which supports the hypothesis of a cross-scale functional reinforcement. Decoupling of FOR and FV was far more evident for rock-dwelling compared with epiphytic communities, which reflects differing environmental conditions associated with substrate type. Our results indicate that increased warming and climatic extremes could exacerbate species clustering into the most resistant functional entities and thus enhance FOR at the community level. Therefore, high taxonomic diversity may not ensure systematic buffering of climate change impacts.

Interdisciplinary knowledge exchange across scales in a globally changing marine environment

The effects of anthropogenic global environmental change on biotic and abiotic processes have been reported in aquatic systems across the world. Complex synergies between concurrent environmental stressors and the resilience of the system to regime shifts, which vary in space and time, determine the capacity for marine systems to maintain structure and function with global environmental change. Consequently, an interdisciplinary approach that facilitates the development of new methods for the exchange of knowledge between scientists across multiple scales is required to effectively understand, quantify and predict climate impacts on marine ecosystem services. We use a literature review to assess the limitations and assumptions of current pathways to exchange interdisciplinary knowledge and the transferability of research findings across spatial and temporal scales and levels of biological organization to advance scientific understanding of global environmental change in marine systems. We found that species-specific regional scale climate change research is most commonly published, and "supporting" is the ecosystem service most commonly referred to in publications. In addition, our paper outlines a trajectory for the future development of integrated climate change science for sustaining marine ecosystem services such as investment in interdisciplinary education and connectivity between disciplines.

Local scale processes drive long-term change in biodiversity of sandy beach ecosystems

Evaluating impacts to biodiversity requires ecologically informed comparisons over sufficient time spans. The vulnerability of coastal ecosystems to anthropogenic and climate change‐related impacts makes them potentially valuable indicators of biodiversity change. To evaluate multidecadal change in biodiversity, we compared results from intertidal surveys of 13 sandy beaches conducted in the 1970s and 2009–11 along 500 km of coast (California, USA). Using a novel extrapolation approach to adjust species richness for sampling effort allowed us to address data gaps and has promise for application to other data‐limited biodiversity comparisons. Long‐term changes in species richness varied in direction and magnitude among beaches and with human impacts but showed no regional patterns. Observed long‐term changes in richness differed markedly among functional groups of intertidal invertebrates. At the majority (77%) of beaches, changes in richness were most evident for wrack‐associated invertebrates suggesting they have disproportionate vulnerability to impacts. Reduced diversity of this group was consistent with long‐term habitat loss from erosion and sea level rise at one beach. Wrack‐associated species richness declined over time at impacted beaches (beach fill and grooming), despite observed increases in overall intertidal richness. In contrast richness of these taxa increased at more than half (53%) of the beaches including two beaches recovering from decades of off‐road vehicle impacts. Over more than three decades, our results suggest that local scale processes exerted a stronger influence on intertidal biodiversity on beaches than regional processes and highlight the role of human impacts for local spatial scales. Our results illustrate how comparisons of overall biodiversity may mask ecologically important changes and stress the value of evaluating biodiversity change in the context of functional groups. The long‐term loss of wrack‐associated species, a key component of sandy beach ecosystems, documented here represents a significant threat to the biodiversity and function of coastal ecosystems.