Introduction
Shorelines and other riparian habitats are often described as hotspots for arthropod predators, and spiders in particular seem to thrive in these habitats (Polis & Hurd 1995; Mellbrand & Hambäck 2010; Batzer & Wu 2020). 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 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. In fact, the spatial variability of arthropod communities in these habitats is poorly documented compared to inland habitats. 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, Gravier & Rothhaupt 2017), whereas prey communities on marine shorelines are more dominated by species developing in rotting wrack beds (Schlacher et al. 2017; Hyndeset al. 2022). Similarly, what is the relative importance from the inflow of dead organic matter versus prey that developed in the water for shoreline predators? Previous studies suggest that the importance of these different resources for spiders and other shoreline predators may vary both between sites, between life stages and over time (Mellbrandet al. 2011; Verschut et al. 2019). The diet analysis of spiders across the season by Verschut et al. (2019) indicated that adult wolf spiders during early summer on marine shorelines feed largely on terrestrial dipterans such as dung flies, which have developed in wrack beds, whereas juvenile wolf spiders later in season had fed more on aquatic dipterans such as chironomids, where the larvae had fed on algae or detritus in the water.
To approach these questions, we studied prey communities, spider diets and spider community structure in a salinity gradient along the Swedish coast. Using this gradient allows us to explore effects from comparatively small salinity differences, as the salinity along the gradient changes continuously from freshwater (<1‰) in the inner parts of the Bothnian Bay to oceanic conditions (>30‰) on the western coastlines. In this study, we included two coastal regions with 5 ‰ and 7 ‰ respectively, where previous studies have indicated shifting dominance of spider species (Hambäck et al. 2016; Verschut et al. 2019). We focus our attention on wolf spiders because these typically dominate the shoreline predator community in the area (Mellbrand & Hambäck 2010). To account for the role of marine inflow, we aimed to include sites with and without thick wrack beds in each region. We also needed to control for climatic effects because the salinity gradient for our study is also a latitudinal gradient. For this reason, we used a similarly collected data set of spider communities on shores by inland waters along the same latitudinal gradient. Finally, to examine the role of a changing prey community and spider diet, we estimate prey densities using SLAM traps and collected spiders for gut metabarcoding in the same sites. Prey densities and spider diets were estimated two times, to cover seasonal changes in prey availability and diet differences between adult and juvenile spiders (cf. Verschut et al. 2019).