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

Abstract

Factors regulating the numbers and species of herbivorous 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.

Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment

“…However, the estimates of species richness from individual-based rarefaction did not increase with fertilization for any trophic group. Numerous studies have examined the diversity of herbivorous or other arthropods in response to fertilization, but generalizations have been elusive (e.g., Hurd and Wolf 1974;Kirchner 1977;Kajak 1981;Vince et al 1981;Prestidge 1982;Strauss 1987;Sedlacek et al 1988;Siemann 1998;Haddad et al 2000). However, several of these studies observed higher abundance of sap-sucking Auchenorryncha and other herbivorous groups, but decreasing evenness and diversity as a small number of species emerged as dominant (Prestidge 1982;Sedlacek et al 1988;Haddad et al 2000).…”

Impact of Fertilization on a Salt Marsh Food Web in Georgia

“…Nitrogen additions likely do not only affect primary producers, but also flow through the food web to affect other trophic levels (Vince et al 1981, Tober et al 1996, Rosemond et al 2001. Although almost all trophic levels are potentially limited by nitrogen (Sterner and Elser 2002), whether particular populations actually respond to added nitrogen in the field depends on what additional factors constrain their response.…”

“…Although almost all trophic levels are potentially limited by nitrogen (Sterner and Elser 2002), whether particular populations actually respond to added nitrogen in the field depends on what additional factors constrain their response. For example, responses of salt marsh herbivores to nitrogen, although generally positive, have been suggested to be limited by competition, predation or life-cycle constraints (Vince et al 1981, Stiling et al 1991, Moon and Stiling 2002. Similar constraints might limit the responses of decomposers or detritivores to nutrient additions.…”

Relationship between anthropogenic sewage discharge, marsh structure and bird assemblages in an SW Atlantic saltmarsh

“…Specifically, the number of 332 herbivore insects increased when the nutritional quality of salt marsh plants improved as 333 a result of higher nutrient loading increased (Vince et al 1981). All of this evidence 334 suggests that we might expect increased arthropod availability for birds with an increase 335 of nutrient loadings to marshes.…”