Arthropod food webs in the foreland of a retreating Greenland glacier: Integrating molecular gut content analysis with Structural Equation Modelling

Combining DNA gut content analysis with Structural Equation Modelling (SEM) based on arthropod activity-density at a Greenland glacier foreland give us the opportunity to combine qualitative data from the metabarcoding technique with an analysis technique based quantitative data. SEM shows bottom-up and top-down controlled food chains as bottom-up control was important for spider and harvestman populations while top-down control was important for ground beetle populations. These mechanisms are closely related to hunting strategies of the predators as a bottom-up mechanism are connected to a sit and-wait behavior while top-down mechanisms are related to active-search behavior. Bottom-up controlled population developments were important in the early phase of the vegetation development while top-down prevail in the later phase of the vegetation development away from the glacier snout. The shift from bottom-up to top-down cascades between arthropod predators and their potential prey populations were mainly driven by increasing temperatures away from the glacier. A consequence of the strong top-down cascades in the later phase of the succession is high rates of intra-guild predation (IGP) among all arthropod predators. Particularly in the guts of the linyphiid spider, Collinsia holmgreni Thorell 1871, trophic linkages to other linyphiid and lycosid spiders were detected. The IGP-ratio of C. holmgreni was negatively correlated with the activity-density of available ground living prey. Probably as a consequence of the high IGP among the linyphiid spiders, cold adapted linyphiid species like C. holmgreni decreased in numbers downhill and became extinct in the warmer climax vegetation, where lycosid spiders dominated.