Title
Multiple sources of evidence for density dependence in the endangered Hawaiian stilt (Himantopus mexicanus knudseni)
Author(s)
van Rees, Charles B.;Surya, Gautam;Reed, J. Michael
Published
2020
Publisher
Population Ecology
Abstract
Hawaiian stilts (Himantopus mexicanus knudseni) are an endangered subspecies of the Black‐necked stilt endemic to the Hawaiian Islands. Despite long‐term study, the main drivers of Hawaiian stilt population dynamics are poorly understood. We tested for density dependence using two sources of evidence: a 30‐year time series of annual estimated range‐wide abundance, and two 15+ year time series of reproductive success. Using separate methods with independent data, sources allowed us to make up for the potentially positive bias of one approach with the more conservative nature of the second. We compared nonlinear density‐dependent and density‐independent population model fits to our time‐series data, using both frequentist and Bayesian state‐space approaches. Across both approaches, density‐dependent models best fit observed population dynamics, with lower AICc and cross‐validation statistics compared to density‐independent models. Among density‐dependent models, a conditional model in which density‐independent dynamics occur below a population size threshold (~850–1,000 birds), and then density‐dependent dynamics occur above that threshold, performed best across Bayesian and frequentist model comparisons, with the Ricker model ranked next or equivalently. Our analysis of reproduction data revealed a strong negative effect of local adult density on nest success (proportion of nests hatching at least one chick) at Kealia National Wildlife Refuge on Maui, where few alternative breeding habitats are available, but no such effect at another site where many nearby alternative wetlands are available. These congruent results across independent datasets and analytical approaches support the hypothesis that Hawaiian stilts exhibit density dependence across their range.
Keywords
Bayesian state‐space model;conservation reliance;Hawaiian waterbirds;island birds;population regulation

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