An Experimental Study of the Structure of Herbivorous Insect Communities in a Salt Marsh

Factors regulating the numbers and species of herbivores insects in a salt marsh were experimentally investigated by chronic fertilization of plots. All biweekly nutrient enrichments (urea or three dosages of a mixed fertilizer) resulted in increased grass standing crop in low and high marsh habitats. Plant nitrogen content increased only in those plots receiving the two highest levels of mixed fertilizer. Grass responses were most rapid where the highest nutrient load was applied. In all plots, fertilization resulted in increased standing crop of low and high marsh insect herbivores. However, the greatest herbivore loads occurred where grass nitrogen content as well as standing crop had increased. Life history data and laboratory experiments indicated that the herbivore increases were largely due to in situ changes in survivorship and fecundity in response to higher plant nitrogen content. The herbivore response to fertilization was rapid, tracking in a single summer the nutrient and biomass changes of the grasses. The rate of increase, however, differed among herbivore species, and was positively related to the annual number of generations. Nutrient additions did not alter the diversity of the high marsh herbivore community, but led to increased diversity in low marsh. This was achieved primarily through greater equitability of the species abundances. All of the herbivore species collected in the fertilized plots occurred in the undisturbed salt marsh. The low marsh herbivore community responded to fertilization mainly with greater numbers of mirids, cicadellids, and grasshoppers, although normally dominated by the delphacid Prokelisia marginata. Potential increases in number of P. marginata in response to fertilization were apparently checked by the increased abundance of spiders. The delphacids were more susceptible to spider predation than other salt marsh herbivores, due to their small size and low position in the grass canopy. Plant nutritional quality is of major importance in governing the abundance of salt marsh herbivores, with life history characteristics and predation interacting to structure the community.