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

Abstract

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 eco...

Species composition of shoreline wolf spider communities vary with salinity but their diets vary with wrack inflow

“…There are several reasons underlying these high spider densities, but an important factor seems to be the high prey density in these near-water habitats (Polis & Hurd 1995;Sanchez-Ruiz et al 2018). Prey densities are high in these sites both because of a direct inflow of insects from the aquatic environment, such as midges with aquatic larvae and terrestrial adults, and because large inflows of organic material are deposited on shorelines providing food for detritivores and fertilizing plants (Colombini & Chelazzi 2003;Baxter, Fausch & Saunders 2005;Hyndes et al 2022). At the same time, shorelines are often harsh environments due to flooding and wave disturbance, and on marine shorelines due to a high salinity and a high turnover of organic material (Defeo & McLachlan 2013;Barboza & Defeo 2015).…”

Species composition of shoreline wolf spider communities vary with salinity, but their diets vary with wrack inflow

“…For instance, what differences in the species composition between limnic and marine shorelines are due to direct effects from a saline environment and what differences are rather due to differences in prey communities? Prey communities on limnic shorelines are often dominated by midges and a range of other taxa (Benke, 1998; Delettre & Morvan, 2000; Salvarina et al, 2017), whereas prey communities on marine shorelines are more dominated by species developing in rotting wrack beds (Hyndes et al, 2022; Schlacher et al, 2017). Similarly, what is the relative importance of the inflow of dead organic matter versus prey that developed in the water for shoreline predators?…”

“…There are several reasons underlying these high spider densities, but an important factor seems to be the high prey density in these near‐water habitats (Polis & Hurd, 1995; Sanchez‐Ruiz et al, 2018), which also reduces intraguild predation (Wise, 2006). Prey densities are high in these sites both because of a direct inflow of insects from the aquatic environment, such as midges with aquatic larvae and terrestrial adults, and because large inflows of organic material are deposited on shorelines providing food for detritivores and fertilizing plants (Baxter et al, 2005; Colombini & Chelazzi, 2003; Hyndes et al, 2022). At the same time, most shorelines are harsh environments due to flooding and wave disturbance, and on marine shorelines due to a high salinity and a high turnover of organic material (Barboza & Defeo, 2015; Defeo & McLachlan, 2013).…”

“…Despite these general patterns, there is a lack of understanding on how physical processes and prey availability interact in shaping coastal arthropod communities (Hyndes et al, 2022). In fact, the spatial variability of arthropod communities in these habitats is poorly documented compared with inland habitats.…”

“…Species diversity on shorelines may therefore be poor, particularly on marine shorelines where communities often consist of a range of habitat specialists that can endure high salinity levels (Cheng, 1976;Irmler et al, 2002). Despite these general patterns, there is a lack of understanding on how physical processes and prey availability interact in shaping coastal arthropod communities (Hyndes et al, 2022). In fact, the spatial variability of arthropod communities in these habitats is poorly documented compared with inland habitats.…”