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Responses of Small Herons and Wood Storks to a Changing Prey Base

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Responses of Small Herons and Wood Storks to a Changing Prey Base Responses of small herons and Wood Storks to a changing prey base Betsy A. Evans, Ashley E. Jackson, Jessica A. Klassen, & Dale E. Gawlik 2019 Greater Everglades Ecosystem Restoration @evanbe01 @FAUAvianEcology Alteration of Everglades hydrology Historic Current • Human development has fragmented the landscape Landscape change • Half its original size, with 70% less water Evergladesplan.org Anthropogenic water bodies Rise of non-native fishes Photos: Alessandro Abate LBHE Kevan Sunderland WOST Kline et al. 2014 Wading bird response 14000 Threatened Legend GA+SC+NC 12000 Florida marsh Endangered Florida urban 10000 What we don’t know: • Population decline due in part to food 8000 How are wading birds resource limitations 6000 responding to these • Storks listed as Endangered in 1984 Number of nests 4000 changes? • Acute decline in small heron nest 2000 numbers over past decade 0 • Uncertainties surrounding decline • However, today, wading birds are moving into urban environments Robertson and Kushlan 1974, Ogden 1994, Hafner 1997, Loftus and Kushlan 1987, Gawlik 2002 Hypotheses In order to examine the influence of non-native fishes and creation of alternative foraging habitats, we examined the diets of three small heron species and Wood Storks: For all wading bird species, we expected the use of non-native species would be highest during suboptimal hydrologic/foraging conditions in the natural marsh Likely due to accessing alternative foraging habitats where non-native species are more prevalent For Wood Storks, we expected that non-native species and other alternative food sources provided in urban areas would influence the reproductive performance of urban nesters Methods DIET Loxahatchee Wood Stork N = 643 (nests = 160) Little Blue Heron N = 142 (nests = 60) Snowy Egret N = 83 (nests=48) Wading bird colonies Tricolored Heron N = 183 (nests = 84) Major roadways Hidden PRODUCTIVITY # of chicks fledged per nest N = 150 Hydrologic conditions 2015-2017 HYDROLOGIC CONDITIONS Optimal 2017 Below average dry season rainfall, allowed for a steady water level recession Moderate 2015 Moderate water levels with steady water level recession; hydrologic reversal at end of season Suboptimal 2016 Unseasonably high rainfall with minimal water level recession Diet differences among small herons 2D Stress: 0.14 Similarity ANOSIM Results 2016 2016 2015 45 Global R-statistic: 0.401 60 Different w/some overlap 65 2015 Bird Pairwise Tests LBHE 2016 SNEG SNEG -TRHE: 0.203 2016 2015 TRHE Similar w/high overlap 2016 2016 2015 LBHE - SNEG: 0.448 2015 2017 Different 2015 2015 2017 LBHE - TRHE: 0.669 2017 Different 2017 Small heron diet differences Collected 6,650 prey items from 191 nests Non-native fish Amph & Rept CrustaceansCustaceans Insects Native fish 90.0 n=2,134 81.4 82.8 80.0 70.0 60.0 50.0 n=2,070 40.0 % Biomass % 32.4 30.0 26.4 20.0 16.4 15.5 12.9 11.9 11.7 10.0 3.6 3.3 0.0 0.7 0.7 0.4 n=2,446 0.0 Little Blue Heron Snowy Egret Tricolored Heron Wood Stork diet shift Ogden et al. 1976 72%72% 2015-2017 7%7% 14% 11% 14% 11% 11%11% 44% 44% 1% 0.5%0.5% 1% 0.5%0.5% 1% 12%12% 7%7% Flagfish Sailfin Marsh Sunfish Mosquito- Bullhead Non-native molly killifish fish species Ogden et al. 1976 Use of non-native fish increases during suboptimal foraging conditions Available 2017 % non-natives found in dry season throw-traps Below average Available 2016 dry season Tricolored Heron 2017 2017 rainfall, allowed Tricolored Heron 2016 for a steady Snowy Egret 2017 water level Where are wading birds finding recession Snowy Egret 2016 non-native fish in such high quantities? Little Blue Heron 2017 Little Blue Heron 2016 Unseasonably high rainfall with Wood Stork 2017 2016 minimal water Wood Stork 2016 level recession 0 10 20 30 40 50 % non-native species in diet Non-native fish and anthropogenic water bodies Natural wetland sites 2D Stress: 0.17 Permanently-inundated anthropogenic sites 3D Stress: 0.13 Ephemeral roadway anthropogenic sites Global R = 0.50 Bolus samples Natural Permanently-inundated wetland anthropogenic water features and stork boluses are most Wood Stork Boluses similar Ephemeral anthropogenic Permanently-- inundated anthropogenic Non-native fish and anthropogenic water bodies Non-native prey 2D Stress: 0.17 3D Stress: 0.13 Global R = 0.50 Natural wetland Non-native prey are more characteristic of permanently-inundated anthropogenic water features and boluses Wood Stork boluses Permanently- Ephemeral inundated anthropogenic anthropogenic Do urban and natural wetland birds differ in their reproductive responses? For Wood Storks, we expected that non-native species and other alternative food sources provided in urban areas would influence the reproductive performance of urban nesters HYDROLOGIC CONDITIONS Optimal 2017 Below average dry season rainfall, allowed for a steady water level recession Moderate 2015 Moderate water levels with steady water level recession; hydrologic reversal at end of season Suboptimal Major roadways 2016 Unseasonably high rainfall with minimal water level recession COLONY LOCATION Urban Natural wetland Productivity Natural wetland birds pay a greater penalty during suboptimal conditions Diet breadth During suboptimal During optimal conditions, conditions, diet breadth is diet breadth is similar greatest in urban colonies (P=0.724) Response of wading birds to a changing landscape and prey base The increase of non-native fishes in the diets of small herons and Wood Storks during suboptimal conditions suggests that these species have behavioral flexibility to deal with the fluctuations of the natural marsh system. We found that non-native fishes are more common in anthropogenic water bodies than natural wetlands, suggesting that birds may be accessing these areas when conditions in the marsh are suboptimal. Furthermore, increased productivity of urban storks may be explained by alternative food sources that buffer urban birds from unpredictable conditions in the natural system. These patterns suggest that the responses of small herons and storks to natural hydrologic conditions may be mediated by foraging habitat and prey species in urban environments. Implications • Responses of wetland fauna to natural processes are mediated by both native and non-native animals that inhabit nearby urban areas. • Wading birds in the Everglades are able to 80 KM max distance Wood Stork observed foraging from colony use urban areas during suboptimal foraging conditions in the natural system. 50 KM avg distance Wood Stork observed foraging from colony • Some wetland animals are able to exploit urban animals as a buffer to food shortages. • Wood Storks and Little Blue Herons in particular, are using non-native fish that are more commonly found in urban created wetlands. • These responses should be considered when evaluating restoration progress. Acknowledgements Funding for this study was provided by Florida Department of Transportation and US Army Corps of Engineers. We are grateful to members of the FAU Avian Ecology Lab and undergraduate student volunteers. Contact Information: Betsy Evans: [email protected] @evanbe01 Dale Gawlik: [email protected] @FAUAvianEcology.
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