HERBIVORY ON SHOALGRASS BY WINTERING REDHEADS IN TEXAS

An estimated 80% Of redheads (Aythya americana) winter On the Laguna M adre of south Texas and Mexico and feed almost exclusively on shoalgra ss (Halodule wrightii) rhizomes. Shoalgrass abundance has decreased by 60% over the past 30 years, and because the effects of shoalgrass los s on wintering redheads are unknown, we initiated a study to define ha bitat selection criteria and document the effect of wintering redheads on shoalgrass in the lower Laguna Madre, Texas. Redheads consumed an average of 75% of shoalgrass rhizome biomass at collection sites each winter. When rhizome biomass was grazed to a mean biomass of less-than -or-equal-to 0.18 g dry mass/core (approximately 10 g dry mass/M2), sh oalgrass did not recover to its previous level the following growing s eason. Thirty-three percent of the sites (10) were grazed below 0.18 g dry mass/core during both years of the study, while 64% (19) were gra zed below 0.18 g during 1 or the other of the 2 winters. Ramet number was positively correlated (P < 0.001, r2 = 0.54) with rhizome biomass; however, this relationship was influenced by grazing intensity. Heavy grazing reduced the amount of rhizome attached to each ramet compared with ungrazed ramets. Grazing had no effect on root biomass (P = 0.38 8), rhizome moisture content (P = 0.553), or soil magnesium, phosphoro us, and potassium (P = 0.102, 0.499, 0.162, respectively). Redhead pre sence increased (P = 0.042) soil nitrogen levels. Foraging areas selec ted by redheads within the lower Laguna Madre had lower (P = 0.026) sa linities (24 ppt) than areas not selected (35 ppt). Redheads did not s elect foraging areas in relation to crude protein levels in rhizomes. Shoalgrass habitat in the Laguna Madre should be protected from furthe r losses and enhanced where possible.