Marine subsidies likely cause gigantism of iguanas in the Bahamas

Abstract

We utilized natural experiment opportunities presented by differential conditions (presence/absence of seabirds and invasive species) on cays in the Bahamas to study whether interisland variations in food resources contributed to gigantism in Allen Cays Rock Iguanas (Cyclura cychlura inornata). We analyzed the stable carbon (δ13C) and nitrogen (δ15N) isotope values from iguana tissues and resources from each island food web to test the predictions that (1) food webs on islands with seabirds exhibit the influence of marine subsidies from seabird guano, whereas those from non-seabird islands do not, and (2) size differences in iguanas among cays were due to either (a) supplemental food availability from mice and/or seabird carcasses killed by barn owls (Tyto alba) and/or (b) access to more nutrient-rich vegetation fertilized by seabird guano. Food web components from the seabird island (Allen Cay) had 5–9‰ higher δ15N values than those on the other cays and Allen Cay plants contained nearly two times more nitrogen. Bayesian stable isotope mixing models indicated that C3 plants dominated iguana diets on all islands and showed no evidence for consumption of mice or shearwaters. The iguanas on Allen Cay were ~ 2 times longer (48.3 ± 11.6 cm) and ~ 6 times heavier (5499 ± 2847 g) than iguanas on other cays and this was likely from marine-derived subsidies from seabird guano which caused an increase in nitrogen concentration in the plants and a resultant increase in the δ15N values across the entire food web relative to non-seabird islands.

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Acknowledgements

We thank W. Mackin, S. Buckner, and C. Knapp for assistance, field support, and transport or storage of samples, C. Kelleher and M. McCartha for laboratory assistance, and the Kurle lab for input and guidance in manuscript preparation. Thanks to E. Cleland, J. Kohn, D. Holway, B. Wolf, and two anonymous reviewers for their comments in improving this manuscript. Funding for KMR and CMK was provided by UC San Diego. Funding for JBI was provided by the National Fish and Wildlife Foundation, Zoo Atlanta, the International Iguana Foundation, the late Ned and Sally Test, and the Cope Museum Fund of Earlham College. Seven C’s Charters provided transportation and housing in the field, and Earlham students and alumni provided critical field assistance. Samples were collected and processed under permits to JBI from the Bahamas Environment Sciences and Technology (BEST) Commission and the Bahamas Department of Agriculture. All animal capture, handling, and sampling was performed following the American Society of Ichthyologists and Herpetologists (ASIH) guidelines for use of reptiles and amphibians in research and is approved under an IACUC permit from Earlham College (20150420-3JI), and tissues transported to UCSD were done so under a UCSD approved Tissue Transfer Permit (13132).

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Contributions

KMR, JBI, and CMK conceived of and designed the experiments, KMR and JBI collected the data in the field in the Bahamas, KMR did the laboratory work, KMR and CMK analyzed the data, and KMR, JBI, and CMK wrote and edited the manuscript.

Corresponding author

Correspondence to Carolyn M. Kurle.

Additional information

K. Richardson’s novel contribution to the Island Rule literature provides mechanistic evidence of insular gigantism caused by marine subsidies with applications to the wider field of community ecology.

Communicated by Blair Wolf.

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Richardson, K.M., Iverson, J.B. & Kurle, C.M. Marine subsidies likely cause gigantism of iguanas in the Bahamas. Oecologia 189, 1005–1015 (2019). https://doi.org/10.1007/s00442-019-04366-4

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Keywords

  • Stable isotope analysis
  • Allochthonous inputs
  • Audubon’s Shearwaters (Puffinus ihermineiri ihermineiri)
  • Allen Cays Rock Iguana (Cyclura cychlura inornata)
  • Invasive species