Abstract
The arroyo southwestern toad is a specialized and federally endangered amphibian endemic to the coastal plains and mountains of central and southern California and northwestern Baja California. It is largely unknown how long these toads live in natural systems, how their population demographics vary across occupied drainages, and how hydrology affects age structure. We used skeletochronology to estimate the ages of adult arroyo toads in seven occupied drainages with varying surface water hydrology in southern California. We processed 179 adult toads with age estimates between 1 and 6 years. Comparisons between skeletochronological ages and known ages of PIT tagged toads showed that skeletochronology likely underestimated toad age by up to 2 years, indicating they may live to 7 or 8 years, but nonetheless major patterns were evident. Arroyo toads showed sexual size dimorphism with adult females reaching a maximum size of 12 mm greater than males. Population age structure varied among the sites. Age structure at sites with seasonally predictable surface water was biased toward younger individuals, which indicated stable recruitment for these populations. Age structures at the ephemeral sites were biased toward older individuals with cohorts roughly corresponding to higher rainfall years. These populations are driven by surface water availability, a stochastic process, and thus more unstable. Based on our estimates of toad ages, climate predictions of extreme and prolonged drought events could mean that the number of consecutive dry years could surpass the maximum life span of toads making them vulnerable to extirpation, especially in ephemeral freshwater systems. Understanding the relationship between population demographics and hydrology is essential for predicting species resilience to projected changes in weather and rainfall patterns. The arroyo toad serves as a model for understanding potential responses to climatic and hydrologic changes in Mediterranean stream systems. We recommend development of adaptive management strategies to address these threats.
Highlights
Amphibians are declining rapidly on national and global scales; some species are declining more rapidly than others (Adams et al, 2013; Beebee & Griffiths, 2005; Lannoo, 2005; Stuart et al, 2004)
The surface hydrology of stream systems occupied by arroyo toads varies from being ephemeral, where surface water is present only in normal to high rainfall years, to seasonally predictable, where surface water is typically present during the breeding season every year
We used skeletochronology to estimate the ages of adult arroyo toads in seven occupied drainages with varying surface water hydrology in southern California. This technique is cost efficient in comparison with long-term capture–recapture studies because it allows for estimation of population age structures in a single year and with single captures of individuals
Summary
Amphibians are declining rapidly on national and global scales; some species are declining more rapidly than others (Adams et al, 2013; Beebee & Griffiths, 2005; Lannoo, 2005; Stuart et al, 2004). The arroyo southwestern toad is a specialized amphibian endemic to the coastal plains and mountains of central and southern California and northwestern Baja California (Ervin, Beaman, & Fisher, 2013; Jennings & Hayes, 1994) It primarily inhabits low gradient streams and rivers containing sandy soils with sandy streamside terraces (Barto, 1999; Sweet, 1992, 1993; Treglia, Fisher, & Fitzgerald, 2015). There is growing knowledge about habitat relationships and stressors (Mitrovich, Gallegos, Lyren, Lovich, & Fisher, 2011), there is still a lack of basic life history information important for modeling population viability and assessing species status These information gaps include how long toads live in natural systems and how their population structure varies across occupied drainages. This technique is cost efficient in comparison with long-term capture–recapture studies because it allows for estimation of population age structures in a single year and with single captures of individuals
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