Spatial subsidies in spider diets vary with shoreline structure: Complementary evidence from molecular diet analysis and stable isotopes

Abstract

Inflow of matter and organisms may strongly affect the local density and diversity of organisms. This effect is particularly evident on shores where organisms with aquatic larval stages enter the terrestrial food web. The identities of such trophic links are not easily estimated as spiders, a dominant group of shoreline predator, have external digestion. We compared trophic links and the prey diversity of spiders on different shore types along the Baltic Sea: on open shores and on shores with a reed belt bordering the water. A priori, we hypothesized that the physical structure of the shoreline reduces the flow between ecosystem and the subsidies across the sea–land interface. To circumvent the lack of morphologically detectable remains of spider prey, we used a combination of stable isotope and molecular gut content analyses. The two tools used for diet analysis revealed complementary information on spider diets. The stable isotope analysis indicated that spiders on open shores had a marine signal of carbon isotopes, while spiders on reedy shores had a terrestrial signal. The molecular analysis revealed a diverse array of dipteran and lepidopteran prey, where spiders on open and reedy shores shared a similar diet with a comparable proportion of chironomids, the larvae of which live in the marine system. Comparing the methods suggests that differences in isotope composition of the two spider groups occurred because of differences in the chironomid diets: as larvae, chironomids of reedy shores likely fed on terrestrial detritus and acquired a terrestrial isotope signature, while chironomids of open shores utilized an algal diet and acquired a marine isotope signature. Our results illustrate how different methods of diet reconstruction may shed light on complementary aspects of nutrient transfer. Overall, they reveal that reed belts can reduce connectivity between habitats, but also function as a source of food for predators.

Seasonally varying marine influences on the coastal ecosystem detected through molecular gut analysis

“…Spiders are evidently predators with very broad diets (Foelix, ; Wise, ), and similar to previous studies using very general primers for molecular gut content analyses of spiders (Hambäck et al., ; Wirta, Weingartner, Hambäck, & Roslin, ; Wirta et al., ), we found a very high diversity of prey species in the gut. We analysed the guts of 505 individuals of two dominant wolf spiders ( Pardosa agricola and P. prativaga ) and recorded a minimum of 169 prey species.…”

Dietary shifts across biogeographic scales alter spatial subsidy dynamics

“…Biological features along ecotones such as changing plant structure may act as a physical barrier that alters the ability of large-bodied predators to easily move through a given habitat, thus inhibiting the flow of energy from one habitat to an adjacent one. For example, spiders foraging along the shore of the Baltic Sea tend to have lower aquatic-based energy subsidies on shorelines with reeds than on shorelines with no plant structure (Hamb€ ack et al 2016). On bare shoreline, spiders are able to forage upon aquatic larvae (with marine-derived energy) but on reedy shorelines the spiders can no longer access this prey resource, thus shifting its diet to more terrestrial-based prey items.…”

“…For example, spiders foraging along the shore of the Baltic Sea tend to have lower aquatic‐based energy subsidies on shorelines with reeds than on shorelines with no plant structure (Hambäck et al. ). On bare shoreline, spiders are able to forage upon aquatic larvae (with marine‐derived energy) but on reedy shorelines the spiders can no longer access this prey resource, thus shifting its diet to more terrestrial‐based prey items.…”

Inferring origins of migrating insects using isoscapes: a case study using the true armyworm, Mythimna unipuncta, in North America

“…However, due to the generally small adult body mass, these techniques offer few opportunities for the study of insect migration (Bridge et al, 2013;Dingle, 2014;Hallworth & Marra, 2015; but see Kissling et al, 2013;Wikelski et al, 2006). For example, insect tissues, especially chitinous wing material, can be a faithful isotopic recorder of larval diets (Hamback et al, 2016;reviewed by Hyodo, 2015; see also Ponsard et al, 2004;Schallhart et al, 2009), and strong isotopic differences between stable isotopes of carbon ( 13 C) have been used to infer local movement among crop types. Hobson, 1151 Richmond St., London, Ontario, N6A 5B7, Canada, E-mail: khobson6@uwo.ca isotopes has been used successfully to infer natal origins and movements, especially at local scales, of several insect taxa.…”