Allochthonous aquatic insects increase predation and decrease herbivory inriver shore food webs

Rivers produce an abundance of aquatic insects that traverse land, where th ey can have bottom-up effects on predators, who, in turn, can have top-down effects on terrestrial herbivores. This effect can cascade down to plants. These trophic relationships were demonstrated in a field of stinging nettl es, Urtica dioica, along a river in Germany. At the shore compared to simil ar microhabitats 30-60 m away the abundance and biomass of: midges were hig hest, spiders were also highest, while herbivorous leafhoppers were lowest. At the shore, nettle plants were less damaged by herbivores and thus had l ess regrowth. Spiders regularly captured both aquatic midges as well as ter restrial leafhoppers and they captured more individuals of both groups at t he shore than further away. Midges supported high densities of shore spider s. This was inferred From correlation of distribution and diet in the absen ce of other environmental gradients. Removal of spiders from experimental p lots caused leafhoppers to increase at the shore, causing more plant damage . These effects were not evident at spider-removal sites away from the shor e. This demonstrated that spiders depressed leafhoppers and decreased herbi vory on plants only at the shore. It is concluded that aquatic insects had a bottom-up effect on spiders and that this subsidy facilitated a top-down effect that cascaded From spiders to leafhoppers to plants. Similar effects would explain the distribution of arthropods along many rivers. Allochthon y connects river food webs with shore food webs, making both components ess ential for each other.