Density-Dependence and Environmental Effects on the Long-Term Population Fluctuations of the Japanese Salamander, Hynobius tokyoensis (Amphibia: Hynobiidae)

Abstract

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Understanding population dynamics is critical for the management of animal populations. Comparatively little is known about the relative importance of endogenous (i.e., density-dependent) and exogenous (i.e., density-independent) factors in the population dynamics of amphibians with complex life cycles. A breeding population of the Japanese salamander, Hynobius tokyoensis, has been monitored at Habu in Hinode-machi, Tokyo, from 1975 to 2025. The number of breeding females was estimated annually by counting the total number of egg sacs deposited in a pond. Female abundance varied from year to year and ranged from 35 to 160 during this period. Time-series data of population abundance were analyzed using state-space modeling to estimate the strength of the effects of density dependence and climatic conditions on population dynamics, owing to the presence of plausible temporal autocorrelation. I used the exponential growth state-space model and the Ornstein-Uhlembeck state-space model as proxy models for density-independent and density-dependent population growth, selecting between them using the parametric bootstrap likelihood ratio test. The hypothesis of density independence was rejected for the study population (5,000 bootstraps, P=0.0026), and the Gompertz state-space (GSS) model, incorporating both density dependence and climate effects, was applied to the time-series data. The analysis using the GSS model also demonstrated a significant negative effect of climatic factors, such as relative humidity, in the preceding growth season. The present study shows the importance of both density-dependent and density-independent climate effects on the long-term dynamics of the breeding population of H. tokyoensis, and suggests the possibility that density dependence may buffer the salamander population against environmental stress.