Stability in Relation to Nutrient Enrichment in Arthropod Consumers of Old‐Field Successional Ecosystems

Abstract
Two adjacent, abandoned hayfields in central New York State were subjected to nutrient enrichment perturbation by means of a single application of 10—10—10 N, P, K fertilizer early in the growing season of 1970. Aboveground arthropod herbivores and carnivores were monitored with respect to net productivity (dB/dt) and diversity for one growing season (1970) in the younger field (6 yr old), and for two growing seasons (1970 and 1971) in the older field (17 and 18 yr old). Sampling of arthropods was done with a gasoline—powered suction sampler between 1200 and 1600 h, twice weekly. Stability of the old—field arthropods was defined as resistance to change imposed by external (fertilizer) perturbation and was measured in terms of (1) magnitude (amplitude of deflection from ground state, (2) rate of initial response to perturbation, and (3) rate of damping or return to ground state. The degree of deflection from ground state was determined by comparing fertilized (treated) plots with unfertilized (control) plots in each field. During the first growing season the older, more species—rich field was less stable with respect to magnitude and rate of productivity deflection than the younger, less species—rich field at both arthropod consumer levels. In addition, the magnitude of productivity deflection from ground state was lower in the carnivore level than in the herbivore trophic level. By the second growing season, the productivity of the arthropod consumers in the treated plots of the older field had not yet returned to control levels. The productivity of arthropod herbivores in the older field exhibited a relatively lower magnitude of response in 1971 than in 1970, while the arthropod carnivores in the older field exhibited a significantly greater difference from ground state in 1971 than in 1970. It was concluded that (1) the magnitude of deflection caused by the enrichment perturbation was decreased up the trophic levels from the herbivore to the carnivore level, (2) the rate of response to perturbation decreased up the trophic levels, and (3) although in control plots diversity increased and productivity decreased with increasing successional age for both trophic levels, stability was not positively related to either higher diversity or to increasing age in either consumer trophic level in this old—field successional ecosystem.