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15 pages; International audience; How food webs are structured has major implications for their stability and dynamics. While poorly studied to date, arctic food webs are commonly assumed to be simple in structure, with few links per species. If this is the case, then different parts of the web may be weakly connected to each other, with populations and species united by only a low number of links. We provide the first highly resolved description of trophic link structure for a large part of a high-arctic food web. For this purpose, we apply a combination of recent techniques to describing the links between three predator guilds (insectivorous birds, spiders, and lepidopteran parasitoids) and their two dominant prey orders (Diptera and Lepidoptera). The resultant web shows a dense link structure and no compartmentalization or modularity across the three predator guilds. Thus, both individual predators and predator guilds tap heavily into the prey community of each other, offering versatile scope for indirect interactions across different parts of the web. The current description of a first but single arctic web may serve as a benchmark toward which to gauge future webs resolved by similar techniques. Targeting an unusual breadth of predator guilds, and relying on techniques with a high resolution, it suggests that species in this web are closely connected. Thus, our findings call for similar explorations of link structure across multiple guilds in both arctic and other webs. From an applied perspective, our description of an arctic web suggests new avenues for understanding how arctic food webs are built and function and of how they respond to current climate change. It suggests that to comprehend the community-level consequences of rapid arctic warming, we should turn from analyses of populations, population pairs, and isolated predator-prey interactions to considering the full set of interacting species.

In the High Arctic, the species richness of spiders is typically low, but abundances can be very high. Thus, how the few spider species occurring in the region choose their prey, and what prey taxa they focus on, may significantly affect the community structure of arctic arthropods. Here we estimate the ecological imprint of adult spiders of three large-bodied species coexisting in Northeast Greenland: the morphologically similar crab spiders Xysticus deichmanni and X. labradorensis (Thomisidae) and the wolf spider Pardosa glacialis (Lycosidae). To describe an important part of these spiders’ diet in detail, we amplified and sequenced DNA from prey remains in their guts, selectively focusing on two of the most abundant prey orders in the area (Diptera and Lepidoptera). By comparing the resultant sequences to a reference library including most taxa encountered in the region, we assigned the prey to species. Among the spider taxa occurring in the region, the wolf spider Pardosa glacialis is dominant in terms of both biomass and density. All three spider species proved to be wide generalists, with no detectable differences in prey choice among either the two crab spiders, or among these crab spiders and the wolf spider. This lack of dietary differentiation among species may be caused by the limited prey availability in the Arctic, forcing the predators to both generalism and opportunism. Given the substantial abundance of spiders and the lack of other predatory arthropods in the region, the opportunistic prey choice observed implies that these High-Arctic spider species have the potential for inflicting a strong influence on their prey community.

The species status of Hyphydrus anatolicus Guignot, 1957 and H. sanctus Sharp, 1882, previously often confused with the widespread H. ovatus (Linnaeus, 1760), are tested with molecular and morphological characters. Cytochrome c oxidase subunit 1 (CO1) was sequenced for 32 specimens of all three species. Gene-trees were inferred with parsimony, time-free bayesian and strict clock bayesian analyses. The GMYC model was used to estimate species limits. All three species were reciprocally monophyletic with CO1 and highly supported. The GMYC species delimitation analysis unequivocally delimited the three species with no other than the three species solution included in the confidence interval. A likelihood ratio test rejected the one-species null model. Important morphological characters distinguishing the species are provided and illustrated. New distributional data are given for the following species: Hyphydrus anatolicus from Slovakia and Ukraine, and H. aubei Ganglbauer, 1891, and H. sanctus from Turkey.