IMPACT OF A DETRITAL SUBSIDY ON TROPHIC CASCADES IN A TERRESTRIAL GRAZING FOOD WEB

Theory predicts that allochthonous energy subsidies can modify the strength of trophic cascades in food webs. We tested the hypothesis that a detrital subsidy would enhance trophic cascades in the grazing food web of an agroecosystem in which generalist predators, wolf spiders and carabid beetles, feed both on detritivores and herbivores. We crossed detritus-addition with predator-removal treatments in replicated 8 × 8 m open and fenced plots of cucumbers and squash. Collembola, a major detritivore, became 2–4 times more abundant in detritus-addition than control plots; densities of other detritivores also increased. Although the detrital subsidy initially increased ground beetle numbers and produced a 2–3-fold increase in wolf spider densities, we found no consistent evidence of strong trophic cascades measured as changes in fruit yield in either detritus-addition or predator-removal treatments. This result contrasts with predator-removal experiments in previous years in the same study area, which revealed strong cascades affecting cucumber yield. We hypothesize that (1) competition between cucurbits and fungi for nitrogen in the detritus-addition plots, (2) unusually high herbivore densities in the spring cucumber crop, and low densities on the summer squash crop, (3) changes in patterns of intraguild predation in the detritus-addition plots, and (4) perhaps a switch by generalist predators to prey of the detrital food web all may have contributed to preventing the detrital subsidy from benefiting primary producers via an enhanced trophic cascade.