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Ecology, management and conservation in natural and modified habitats
RESEARCH ARTICLE (Open Access)

Hot, wet and rare: modelling the occupancy dynamics of the narrowly distributed Dixie Valley toad

Jonathan P. Rose https://orcid.org/0000-0003-0874-9166 A * , Patrick M. Kleeman https://orcid.org/0000-0001-6567-3239 B and Brian J. Halstead https://orcid.org/0000-0002-5535-6528 C
+ Author Affiliations
- Author Affiliations

A US Geological Survey, Western Ecological Research Center, Santa Cruz Field Station, 2885 Mission Street, Santa Cruz, CA 95060, USA.

B US Geological Survey, Western Ecological Research Center, Point Reyes Field Station, 1 Bear Valley Road, Point Reyes Station, CA 94956, USA.

C US Geological Survey, Western Ecological Research Center, Dixon Field Station, 800 Business Park Drive, Suite D, Dixon, CA 95620, USA.

* Correspondence to: jprose@usgs.gov

Handling Editor: Jonathan Webb

Wildlife Research 50(7) 552-567 https://doi.org/10.1071/WR22029
Submitted: 19 February 2022  Accepted: 23 July 2022   Published: 29 August 2022

© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution 4.0 International License (CC BY)

Abstract

Context: Small population sizes and no possibility of metapopulation rescue put narrowly distributed endemic species under elevated risk of extinction from anthropogenic change. Desert spring wetlands host many endemic species that require aquatic habitat and are isolated by the surrounding xeric terrestrial habitat.

Aims: We sought to model the occupancy dynamics of the Dixie Valley toad (Anaxyrus williamsi), a recently described species endemic to a small desert spring wetland complex in Nevada, USA.

Methods: We divided the species’ range into 20 m × 20 m cells and surveyed for Dixie Valley toads at 60 cells during six primary periods from 2018 to 2021, following an occupancy study design. We analysed our survey data by using a multi-state dynamic occupancy model to estimate the probability of adult occurrence, colonisation, site survival, and larval occurrence and the relationship of each to environmental covariates.

Key results: The detection probabilities of adult and larval toads were affected by survey length and time of day. Adult Dixie Valley toads were widely distributed, with detections in 75% of surveyed cells at some point during the 3-year study, whereas larvae were observed only in 20% of cells during the study. Dixie Valley toad larvae were more likely to occur in cells far from spring heads with a high coverage of surface water, low emergent vegetation cover, and water temperatures between 20°C and 28°C. Adult toads were more likely to occur in cells with a greater coverage of surface water and water depth >10 cm. Cells with more emergent vegetation cover and surface water were more likely to be colonised by adult toads.

Conclusions: Our results showed that Dixie Valley toads are highly dependent on surface water in both spring and autumn. Adults and larvae require different environmental conditions, with larvae occurring farther from spring heads and in fewer cells.

Implications: Disturbances to the hydrology of the desert spring wetlands in Dixie Valley could threaten the persistence of this narrowly distributed toad.

Keywords: Anaxyrus williamsi, aquatic conservation, Bayesian hierarchical model, desert springs, endangered species, geothermal energy, Great Basin, multi-state occupancy model.


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