Nutrient Transport by Wading Birds in the Everglades

Abstract

The effect of nutrient accumulation resulting from deposition of feces in colonies of colonially breeding and roosting wading birds is estimated in this chapter for breeding and nonbreeding ciconiiform birds in the Everglades ecosystem, by modeling energy consumption and feces deposition rates and by using existing measurements of size, energy, and nutrient content of prey items from the Everglades. Current populations of breeding and nonbreeding birds are estimated to consume 4.9 fewer tonnes of prey (dry mass) per year than the much larger populations of the 1930s and 1940s, equivalent to an estimated 14.6 million fewer prey items per year. This difference translates into 455 fewer tonnes of feces deposited in roosts and colonies per year, roughly equivalent to 59 fewer tonnes nitrogen and 5.6 fewer tonnes phosphorus. Nonbreeding birds are estimated to account for only 1.5% of the difference in nutrient flux attributable to birds between the two periods, indicating that the differences are due to reductions in energyintensive breeding attempts. Although even the largest historical populations are estimated to have redistributed only a very small fraction of the total annual deposition of phosphorus and other nutrients in the marsh, loading rates at colonies can be extremely high. Loading rates at historical colony sites could have been as high as 120 g phosphorus m_2 yr"1 (approximately 3000 times the estimated historic atmospheric deposition rate), while current colonies are estimated to have rates of only 0.9 g phosphorus irr2-yr_1 (more than 20 times the historic atmo­ spheric deposition rate). Evidence from the Everglades and other ecosystems suggests that high nutrient concentrations in the vicinity of colonies has a strong effect on the productivity and species composition of aquatic fauna and flora. This© St. Lucie Press CCC 0-9634030-2-8 l/94/$100/$.50 571may have strong feedback effects for survival of young wading birds, which characteristically develop foraging skills at or near colony sites. Recent relocation of large colonies from the estuarine zone to the freshwater Everglades implies that nutrient input to the estuary has decreased significantly. Nutrient-rich colonies probably serve as islands of refugia for nutrient-tolerant species in the oligotrophic Everglades and may serve to significantly affect the variability in biodiversity of the marsh. Sources of error tend to be in the direction of overestimation of nutrients transported, and in this regard, the amount of food required by nestlings is a central and poorly understood variable.

S.K. Morgan Ernest

Associate Professor of Wildlife Ecology and Conservation

My research interests include the dynamics of ecological systems and the integration of technology and field work