Forb‐Arthropod Food Chains in a One‐Year Experimental Field

Phosphorous—32 tracer studies of plant—arthropod food chains showed a significant difference in both consumer diversity and grazing pressure between the two dominant plant species on a first—year weed field in South Carolina. Where Heterotheca subaxillaris was the labeled plant ³ ²P was transferred readily to a number of phytophagous insects and secondarily to the predator fauna. Where only Erigeron canadensis was tagged, there was little transfer of the isotope to the consumer populations, with the exceptions of the ant Dorymyrmex and the tree cricket Oceanthus. Dorymyrmex is a predator—scavenger, but also tends the aphids present on Erigeron; this latter habit accounted for the high uptake and unusual retention pattern of ³ ²P by this ant where Erigeron was tagged. Oecanthus was primarily an herbivore, but the data suggest some predation on aphids from Erigeron plants. Very little ³ ²P was transferred to the detritus eaters during the 43 day period of the study. Further support is given for the use of radioisotope to study trophic position of organisms as well as grazing pressure and food chain diversity associated with single species of terrestrial primary producers.