Predicted changes in foraging habitat of the Little Blue Heron (Egretta caerulea) in the Great White Heron National Wildlife Refuge, FL, USA, as a function of sea level rise

Background/Question/Methods

Wading birds are restricted to feeding in shallow water because of their leg-length constraint, making them sensitive to small changes in water depth. In coastal systems, this sensitivity is especially pronounced because tidal fluctuations control both the spatial and temporal extent of available foraging habitat. Baseline sea level rise in the Refuge is approximately 2 mm/year. Any increase in sea level can alter the timing and areal extent of feeding habitat for wading birds whose reproductive cycle is inextricably linked to hydrologic fluctuations. Our objective was to determine the risk of the Little Blue Heron (Egretta caerulea), a charismatic wading bird and species of concern, to sea level rise within the boundaries of the Great White Heron National Wildlife Refuge, in the Florida Keys, USA. Our approach to the problem was to develop a tide-driven simulation model to estimate foraging habitat availability (FHA). The FHA model incorporated fine-scale information on water depths used by the Little Blue Heron, predicted changes in habitat availability from the Sea Level Rise and Accretion Model, under 3 sea level rise scenarios. We validated the model's ability to predict available foraging habitat using locations of foraging Little Blue Herons (N=509) observed during 14 surveys (Dec 2010 - Jul 2011).

Results/Conclusions

Parameters to which the model was most sensitive were foraging water depth window, tide height, and the time of low tide. A sensitivity analysis revealed that variation in tide height (± 10cm) resulted in a 20-29% change in model output. The model performed moderately well (78% correct classification using survey-specific FHA estimates), to very well (94% correct classification using day-averaged FHA estimates), at predicting available foraging habitat. The majority (57%) of Little Blue Herons foraged at areas with tide-specific FHA values of >7hectare-minutes. Under all three sea level rise scenarios daily foraging habitat declined, with the most severe declines occurring between 2050 and 2075. However, our results may be liberal because we excluded mangrove islands as foraging habitat. An analysis of habitat preference was beyond the scope of our study, but we suspect that as mangrove habitats become inundated for longer periods of time they will become suitable foraging areas, if they are not already. The fine temporal scale of the FHA model (from a single-tide to days, months) makes it useful for addressing short- and long- term stressors to multiple wading bird species resulting from human disturbance or sea level rise.