Document: JUR-3-52-22

Testing the multichannel omnivory model: Impact of detrital subsidies on top-down control in a terrestrial grazing food web.

HALAJ, J.* and D.H.WISE

University of Kentucky, Lexington, Kentucky ¹

Abstract:
The multichannel omnivory model of Polis and Strong predicts that allochthonous energy subsidies increase the strength of trophic cascades in grazing food webs by enhancing the extent to which omnivorous feeders suppress their in situ resources. This study tested the hypothesis that top-down control by generalist arthropod predators in a grazing food web varies with the energy subsidies that these predators receive from feeding on soil-litter detritivores. In replicated 8x8-m open and fenced plots of cucumbers and squash, we crossed a detritus-addition with a predator-reduction treatment. Half of both open and fenced plots received shredded horse manure and straw compost; the other plots (detritus-controls) received no extra detrital input. In the fenced plots we altered rates of predator immigration, and also removed predators, to establish reduced and ambient densities of ground spiders and predacious ground beetles. Springtails (Collembola), a major detritivore, became >10x more abundant in detritus-addition than control plots. The detrital subsidy also increased densities of other detritivores, resulted in a 2-3x increase in wolf spiders throughout the experiment, and, increased ground beetle numbers early in the season. However, the detrital subsidy failed to intensify trophic cascades in the grazing food web, because production of cucumbers and squash did not differ between detritus-addition and control plots. Direct manipulation of predator densities in the fenced non-detrital treatments uncovered no evidence of trophic cascades in the absence of a subsidy, a result contrary to previous findings in this system. We suggest that preferential feeding by predators on detritivores and/or intensified intraguild predation, in combination with unusual drought conditions and higher population densities of herbivores than previous years, may explain the failure of the detrital subsidy to impact primary producers via an enhanced trophic cascade in our experiment.

Keywords: trophic cascade, food web, generalist predator, detrital subsidy, omnivory, spiders, carabid beetles, cucumber, squash

This abstract is being presented at: 11:45 AM in session:
Oral Session #44: Terrestrial Invertebrates: Foodwebs and Plant Responses.