Insecticide Effects on Early Succession in an Old Field Ecosystem

An ecosystem approach was used to assess the effects of soil contamination with an organophosphate insecticide on ecological succession in 3—acre old field on the Piedmont of New Jersey. Diazinon was applied to one—half of the field in May 1967 and again in May 1967 and again in May 1968 at a rate of 12 lbs of active ingredient per acre. A latin square experimental design was used to analyze the effects of the insecticide on the density, diversity, and production of vegetation; the density and diversity of herb—stratum arthropods; and the trophic relationships that developed on treated and untreated areas. In 1967, insecticide contamination produced a significant enrichment in both density and diversity of vegetation as well as a consistent trend toward greater plant biomass on treated areas. The vegetation changes represented a summation of small but consistent differences in major producers common to both treated and untreated areas. Plant differences were believed due to a phytotoxic effect of diazinon on Convolvulus sepium, which was significantly reduced on treated areas. Other plant species normally inhibited by Convolvulus subsequently flourished on treated areas. Herb—stratum arthropod populations were similar on treated and untreated areas in 1967, despite the vegetation differences. Only a trend toward greater insect diversity on treated areas was present. The failure of indirect arthropod differences to develop was due to the similar availability of dominant producers and thus food web bases on treated and untreated areas. Radionuclide tracer studies indicated that ragweed (Ambrosia artemisiifolia) was the major food web base for resident consumers in the latter part of the growing season. The trophic transfer from ragweed was quite similar in treated and untreated areas. The lack of arthropod differences was thus attributed to similar availability of ragweed biomass in both areas. Vegetation differences were absent in 1968 following a second treatment with diazinon. Moisture appeared to be a major factor influencing the impart of the insecticide during the two summers. Vegetation differences developed in June 1967 at a time when rainfall was almost absent. In 1968, however, considerable rainfall occurred during the same period. The increased rainfall stimulated a greater seedling development than in 1967, thus apparently overcoming the effects of the insecticide. Environmental factors such as soil moisture can therefore interact with pollutants and seriously influence the degree of ecosystem modification.