Inter-year carryover effects of a nutrient pulse on Spartina plants, herbivores, and natural enemies

Pulsed resources, such as natural and anthropogenic inputs of nutrients into natural communities, can have important consequences for primary productivity, as well as for consumers and their predators. Little, however, is known of how a resource pulse influences community assemblages for periods longer than one growing season. This study, repeated twice and in two different habitats on a mid-Atlantic Spartina alterniflora salt marsh, showed that a one-time nitrogen (N) subsidy can have not only significant within-year effects on plant primary production and herbivore and natural enemy abundance, but can also have long-term effects on plants and arthropod food web structure. Spartina plants exhibited significantly greater biomass for 2-3 yr and elevated percentage of N for up to two years following the nutrient pulse, an effect that had direct positive effects on herbivores that were sensitive to variation in plant N content. Populations of Prokelisia planthoppers (Hemiptera: Delphacidae) were elevated in fertilized plots for up to three years following the nutrient addition. However, Trigonotylus uhleri, an herbivorous mirid (Hemiptera: Miridae), was more abundant on previously fertilized plants only for a brief time, either within the same season as the nutrient addition, or for only one year after the pulse. The carryover effect of the nutrient pulse on plants and herbivores also propagated to the third trophic level. Both non-lycosid spiders (mostly linyphiids) and a specialist egg predator of Spartina planthoppers (Tytthus vagus) were more abundant in previously fertilized plots for up to two years following the nutrient pulse. In contrast, wolf spider (Lycosidae) densities were elevated in fertilized plots only within the same season as the initial fertilization pulse. Food web structure, as indexed by herbivore: predator ratio, was altered in plots that received a one-time N subsidy, with the arthropod food web in fertilized Spartina taking three years to resemble that of unperturbed Spartina. The response of particular herbivores and predators to the nutrient pulse was in part habitat dependent (contiguous Spartina meadows vs. isolated islets), suggesting that habitat heterogeneity combined with the differential dispersal ability of arthropods can influence the long-term response of a food web to a nutrient pulse.