Marine resource flows to terrestrial arthropod predators on a temperate island: the role of subsidies between systems of similar productivity

Abstract

Marine-terrestrial resource flows can subsidies recipient consumers at various trophic levels. Theory suggests that the importance of such spatial subsidies depends on the productivity gradient between adjacent systems; however, the empirical data required to test this assumption are scarce. Most studies of marine-terrestrial subsidies have been performed in arid coastal habitats of low productivity surrounded by productive ocean waters. We examined the importance of marine resource inputs for terrestrial consumers on a temperate, productive forest island surrounded by a marine system of similar productivity. The importance of marine resources for the dominant arthropod consumers was estimated using stable isotopes and linear mixing models. We compared isotopic signatures of spiders and ants captured along a gradient from shore to inland to estimate how far marine-derived energy penetrates the island. We evaluated the distribution of ground-dwelling arthropods using pitfall-trap transects extending from the supratidal-forest boundary to the middle of the island. The contribution of marine-derived energy assimilated by arthropod consumers differed both among taxa and location. Marine-derived resources contributed >80% to the assimilated C of intertidal spiders and 5-10% for spiders at the forest edge and further inland. Ants assimilated 20% of their C from marine-derived resources and this proportion was not affected by distance from shore. Spiders, ants, and all arthropods combined exhibited no spatial aggregation towards the shore. Our results indicate that on temperate islands marine-terrestrial subsidies might be predominantly an edge effect, confined to intertidal consumers. Mobile consumers that opportunistically forage in intertidal habitats play an important role in transferring marine-derived energy further inland. This suggests that the importance of the productivity gradient for spatial subsidies can be modified by the mobility traits of the recipient consumers and their degree of specialization on the interface habitat.

The use of DNA barcodes in food web construction—terrestrial and aquatic ecologists unite!

“…Of key interest is the extent to which local productivity at different trophic layers, population dynamics, and community structure is affected not only by local processes and energy accumulation but also by the inflow of energy and biomass from the other realm (Polis et al 1997;Sabo and Power 2002a;Hoekman et al 2011;Dreyer et al 2012). Nutrient-rich wetlands, lakes, and marine areas often provide productive habitats not only for aquatic but also for terrestrial species consuming taxa emerging from the water (Sabo and Power 2002b;Paetzold et al 2008;. Not surprisingly, densities of terrestrial predators are typically higher close to various water bodies and depend on the amount of inflow of prey on the shoreline (Paetzold et al 2008;Dreyer et al 2012).…”

“…Nutrient-rich wetlands, lakes, and marine areas often provide productive habitats not only for aquatic but also for terrestrial species consuming taxa emerging from the water (Sabo and Power 2002b;Paetzold et al 2008;. Not surprisingly, densities of terrestrial predators are typically higher close to various water bodies and depend on the amount of inflow of prey on the shoreline (Paetzold et al 2008;Dreyer et al 2012). DNA barcodes (and other molecular tools; e.g., Collier et al 2002;Kolb et al 2010;) allow us to trace these flows with new accuracy, and to thus understand the real links between terrestrial and aquatic food webs.…”

Associations in ephemeral systems: the lack of trophic relationships between sandhoppers and beach wrack

“…These results suggest alternative links in sandy beach food webs, with amphipods obtaining their carbon indirectly from wrack via bacterial communities that are specific to different types of imported material. Stapp & Polis 2003, Paetzold et al 2008, but marine inputs to terrestrial systems seem generally to be weaker (Paetzold et al 2008), with the possible exception of isolated islands (Smith & Froneman 2008). Cross-boundary exchanges are common within marine systems, like subtidal kelp inputs to intertidal grazers (Bustamante et al 1995), and rocky shores dominated by filter-feeders or macroalgae may act respectively as sinks or sources of detrital food (McQuaid & Branch 1985).…”

“…Flow of material between markedly separate systems, such as land and water, is the most extreme example of cross-boundary trophic subsidisation (Polis et al 1997, Paetzold et al 2008, Schlacher & Connolly 2009). Smaller-scale differences in subsidisation are also common within systems, such as the different proportions of autochthonous and allochthonous inputs within estuaries (Mann 2000), which are characterised by an upstream to downstream gradient of terrestrial and marine inputs (Vannote et al 1980).…”

Terrestrial trophic subsidy in sandy beaches: evidence from stable isotope analysis in organic matter sources and isopod Excirolana armata

“…We are aware that differences between TOM and isopod signatures indicate at least 3 to 5 trophic levels between the carbon source and the animals. However, previous studies using stable isotopes have reported close trophic relationships between sandy-beach invertebrates and terrestrial subsidies (Paetzold et al 2008, Colombini et al 2011. Moreover, intertidal isopods, by scraping the sediment surface, consume a wide variety of resources ranging from macroalgae to animal matter and detritus (Arrontes 1990, McLachlan & Brown 2006, Schlacher et al 2008; they also show the ability to hydrolytically digest cellulose and oxidatively break down lignins, both of which are common components of leaf litter (Zimmer at al.…”