SEASONAL BELOWGROUND HERBIVORY AND A DENSITY REFUGE FROM WATERFOWL HERBIVORY FOR VALLISNERIA AMERICANA

Abstract

Studies of aquatic food webs rarely address the role of migratory waterfowl, even though they are ubiquitous and many are omnivorous. Several species of waterfowl consume belowground plant tubers during winter, yet the impact of such herbivory on future plant abundance is mostly unknown. To test the impact of wintering waterfowl on submerged vegetation, we installed waterfowl exclosures (N = 5) and reference plots (N = 5) open to grazing in Lake Mattamuskeet, North Carolina, USA, where Vallisneria americana (water celery, or tapegrass) dominated the plant community. Over the three-year study period, we sampled above- and belowground plant standing stock every 2–3 mo. Reference plots had a lower density (number per square meter) of Vallisneria tubers than ungrazed exclosures (repeated-measures ANOVA treatment P = 0.038, time × treatment P = 0.025). Percentage loss of tubers in reference plots was positively related to the density of tubers at the onset of winter (R2 = 0.294, P = 0.026). Despite significant reduction of tubers during winter, no detectable carryover effects of waterfowl exclosure on Vallisneria aboveground biomass occurred in subsequent growing seasons (rmANOVA treatment P = 0.342, time × treatment P = 0.539). Our study detected a threshold tuber density below which waterfowl herbivory on Vallisneria did not occur. Therefore, our results suggest that limits on waterfowl foraging efficiency provide a refuge for aquatic plants subject to intense, seasonal predation.