Potential alteration of cross‐ecosystem resource subsidies by an invasive aquatic macroinvertebrate: implications for the terrestrial food web

Abstract

Summary Biodiversity is globally threatened by the replacement of native species by invasive species and ensuing changes in ecosystem functioning. Although trophic linkages between aquatic and terrestrial ecosystems have received attention, effects of aquatic invasive species on the flow of resource subsidies have been considered only recently. We examined how the effects of one of the most invasive macroinvertebrate species in European waterways, the amphipod Dikerogammarus villosus, extend from streams to the terrestrial food web. We quantified aquatic emergence and the contribution of aquatic resources to the diets of two riparian spider taxa in relation to the density of D. villosus. Our results indicated that the effects of this invasive species carry over to the terrestrial system via cross‐ecosystem flow of resource subsidy. The contribution of aquatic resources to the diet of the terrestrial web‐building spider Tetragnatha decreased from 60% at low densities of D. villosus to 10% at a D. villosus density >5000 individuals m−2. This correlates with a decreasing emergence rate of merolimnic midges (species with an aquatic larval phase) from 12 to <3 mg dry biomass m−2 day−1 at the respective densities of D. villosus. The magnitude of biomass flow from the aquatic to the terrestrial ecosystem is most likely decreased by D. villosus, and this decrease extends to the diet of riparian web‐building spiders. Effects of this aquatic invader may also extend to a decoupling of the terrestrial ecosystem from the aquatic ecosystem in terms of subsidy flux.