The influence of environmental variation on the snail kite population in Florida

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)



First Committee Member

Donald DeAngelis - Committee Chair

Second Committee Member

Wiley Kitchens - Committee Member


The purpose of restoration is to restore and maintain a healthy, balanced, and functioning landscape. In order to adhere to this proposed goal, it is important to determine to the ways in which a population is adapted to its environment, particularly the inherent variability of the environment. The snail kite is an endangered species of the central and southern Florida wetlands. This species is highly dependent on the mosaic of wetland units in this landscape. Research questions relevant to conservation and management of this species include what factors influence temporal variations in snail kite population and how the spatial heterogeneity of the central and southern Florida wetland units affect this population.To address these questions, I studied the population of snail kites in central and southern Florida. I examined the magnitude of temporal and spatial variation in life history traits; in particular, the effects of hydrology on temporal and spatial variation in reproduction; and how the variation in life history traits affected aspects of population dynamics. I used a components of variance analysis to partition sampling variation from process variation and used a model selection approach to estimate life history parameters using mark-resighting and random effects models. Of the reproductive components examined, hydrology explained most of the temporal variation in juvenile survival. This is likely the result of the temporal variation in hydrologic levels in the wetlands of central and southern Florida exhibiting the same pattern of temporal variation as juvenile survival. An interaction of temporal and spatial variability explained most of the variation in nest success, for cases in which hydrology was not a principal source of variation. I modified mark-resighting approach to provide a statistical valid estimate in population size and a level of precision in the population size estimates. The population size estimates from 1997--1999 were substantially higher than previous reports and, exhibited both temporal variation and an increasing trend. Survival varied more than recruitment and had the most influence on the annual rates of population change (lambda). A forecast of annual lambda, estimated from life history traits, suggests that the snail kite population is relatively stable. I postulated that the magnitude of lambda is determined by both density-dependent factors (i.e., food, disease) and density-independent factors (i.e., hydrology, vegetation structure), whereas the variation around lambda is also influenced by density-dependent and density-independent factors. However, as habitat quality deteriorates, the variation in lambda should become more pronounced.


Biology, Ecology

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