BOTTOM-UP LIMITATION OF PREDACEOUS ARTHROPODS IN A DETRITUS-BASED TERRESTRIAL FOOD WEB

Knowing how an increase in the resource base of a food web produces effects that propagate through the web is central to developing a clearer understanding of food-web structure and dynamics. In a detritus-based terrestrial food web, we measured the responses of predaceous arthropods to increases in prey arthropods that occurred in response to experimentally enhancing the web's resource base. Open 2 × 5 m plots on the floor of a deciduous forest were randomly assigned to either a Food Enhancement or Control treatment. We supplemented the resource base of the arthropod community of the leaf litter layer for 3.5 mo by periodically adding chopped mushrooms, potatoes, and instant fruit fly medium to the Food Enhancement plots. Major taxa of detritivores and fungivores increased in response to added food. Densities of springtails (Collembola) were on average 3× higher in the Food Enhancement than Control plots. Numbers of adult fungus gnats (Diptera: Sciaridae and Mycetophilidae) did not differ significantly between treatments after 6 wk but were >2× higher in Food Enhancement plots at the end of the experiment. Total Diptera were twice as abundant in Food Enhancement plots on both census dates. Arthropod groups that include a range of feeding strategies also increased. Mites (Acarina), which include detritivores, fungivores, and predators, were twice as abundant in the experimental treatment. Staphylinid and carabid beetles (Coleoptera), which are primarily predaceous but include omnivorous species, were several times more numerous in the Food Enhancement plots. Effects of increasing the resource base propagated through the food web, leading to higher densities of the major strictly predaceous arthropod taxa. Centipedes (Chilopoda), pseudoscorpions (Pseudoscorpionida), and spiders (Araneae) were ∼2× as abundant in the Food Enhancement treatment. Thus, our experiment uncovered substantial bottom-up limitation in this detritus-based food web, expressed as responses by predaceous arthropods at least two trophic links removed from the experimentally elevated resource.