Radionuclide Tracer Analysis of Trophic Relationships in an Old‐Field Ecosystem

Wild radish (Raphanus raphanistrum) and ragweed (Ambrosia artemisiifolia), the dominant producers during initial succession in an old field, were labeled with ³ ²P in several trophic—transfer studies. Feeding relationships were determined by correlating the population dynamics and ³ ²P uptake by consumer species with the phenological changes in producer components and isotope distribution in plant tissues. The relative magnitude of consumer pathways was estimated by using trophic—transfer indices. The first year of old—field succession was characterized by rapid phenological changes in plant—organ availability with related shifts in consumer trophic relationships. The food web of wild radish, the early summer dominant producer, changed rapidly over time resulting in successive pulses of leaf, pollen, and seed feeders. Grasshoppers were the major leaf feeders. Aphids consumed internal juices within stems while flea beetles were concentrated and feeding solely on nutrient—rich seed tissue. Leafhoppers and tarnished plant bugs also consumed internal fluids during wild radish development. The high densities and consumption by plant bugs and aphids, the major consumers of wild radish, resulted in a low food—web diversity for wild radish. Predator—prey relationships during early summer were governed mainly by the aphid infestation. Young ragweed plants and the litter and seed crop of wild radish were major food—web bases in midsummer. Generalized herbivores such as grasshoppers and crickets utilized radish seeds and ragweed tissue during this period. A second phase of specialized feeders entered the old field as ragweed became available. Food—web diversity was highest at this time when many species were present and at low densities. Ragweed was the dominant producer late in the growing season. Some herbivores utilized ragweed leaves while several species of plant bugs became abundant and were consuming the nutrient—rich fluids being channeled into pollen tissue. Plant bugs were the major ragweed herbivores. The importance of these plant bugs resulted in a decline in food—web diversity near the end of the growing season. Most consumers decreased in numbers and ³ ²P activity as ragweed died back in September. Ragweed litter, however, supported crickets and other components of the detrital food web over the fall season. Wild radish and ragweed were the major food—web bases during the first year of succession. Over 90% of the arthropods sampled in the old field were food—web components of these producers. Many of these consumers were specialists and sucking forms which peaked in density and ³ ²P activity as nutrient—rich fluids were entering their specific food sources. The synchronal occurrence of consumer population peaks and successive nutrient pulses in host—plant organs thus appears to maximize the energy utilization by these consumers while minimizing interspecific competition for sites of trophic transfer. Little foliar damage was evident for wild radish or ragweed because of the importance of herbivores with sucking mouthparts. Tracer studies were thus critical in evaluating the role of both producers as food—web bases. The similarity of results in two replicate studies confirmed the repeatability and accuracy of tracer techniques for trophic—transfer analysis. However, temporal aspects of food—web structure should be considered in future studies where rapid changes in plant phenology are present.