Estimating the survival of unobservable life stages for a declining frog with a complex life history

Abstract

Demographic models enhance understanding of drivers of population growth and inform conservation efforts to prevent population declines and extinction. For species with complex life histories, however, parameterizing demographic models is challenging because some life stages can be difficult to study directly. Integrated population models (IPMs) empower researchers to estimate vital rates for organisms that have cryptic or widely dispersing early life stages by integrating multiple demographic data sources. For a stream‐inhabiting frog (Rana boylii) that is declining through much of its range in Oregon and California, USA, we collected egg‐mass counts and capture–mark–recapture data on adults from two populations in California to fit IPMs that estimate adult abundance and the survival rate of both marked and unobserved life stages. Estimates of adult abundance based on long‐term monitoring of egg‐mass counts showed that study populations fluctuated greatly inter‐annually but were stable at longer timescales (i.e., decades). Adult female survival during 5–6 yr of capture–mark–recapture study periods was nearly equal in each population. Survival rate of R. boylii eggs to the subadult stage is low on average (0.002) but highly variable among years depending on post‐oviposition stream flow. Population viability analysis showed that survival of adult and subadult life stages has the greatest proportional effect on population growth; the survival of egg and tadpole life stages, however, is more malleable by management interventions. For example, simulations showed head‐starting of tadpoles, salvaging stranded egg masses, and limiting aseasonal pulsed flows could dramatically reduce the threat of extirpation. This study demonstrates the value of integrating multiple demographic data sources to construct models of population dynamics in species with complex life histories.

Conservation Implications of Spatiotemporal Variation in the Terrestrial Ecology of Western Spadefoots

“…Our estimate of annual survival indicates that about half of adult spadefoots breeding in 1 year would be expected to survive until the next breeding season, at least under similar conditions to those at our study sites in a relatively wet year. The relative importance of adult survival to western spadefoot population growth could be further elucidated using population models as done for many other amphibians (Caswell 2001, Trenham and Shaffer 2005, Canessa et al 2014, Earl 2019, Rose et al 2021).…”

Cost-benefit evaluation of management strategies for an invasive amphibian with a stage-structured model

“…A positive effect of mortality preceding density dependence seems quite common in structured populations (Abrams 2009;Pardini et al 2009;Loppnow and Venturelli 2014;Schröder et al 2014;McIntire and Juliano 2018) and should be routinely considered in management planning (Zipkin et al 2009;Turner et al 2016), for instance, implementing removal only after density-dependent phenomena (but see Hilker and Liz 2013). The existence of overcompensatory density dependence might also explain why, in native amphibians, low or variable survival rates at early life stages (eggs and tadpoles) have only a minor effect on population growth or decline, in comparison with low post-metamorphic survival rates (Vonesh and De la Cruz 2002;Petrovan and Schmidt 2019;Rose et al 2021).…”

“…Multiple studies on amphibians have shown that variations in the survival rate of juveniles and sub-adults may have severe population-level effects (Vonesh and de la Cruz 2002;Beaty and Salice 2013;Petrovan and Schmidt 2019;Rose et al 2021). Therefore, it has been recently suggested that both adults and juveniles of guttural toads should be simultaneously removed (Davies et al 2020b).…”

Seasonal drought and its effects on frog population dynamics and amphibian disease in intermittent streams

“…Following the protocol detailed in Kupferberg et al (2012), We used autoregressive time series models to evaluate temporal trends in clutches detected at Alameda Creek. We included mean annual discharge 2 years prior as a predictor variable because population fluctuations of foothill yellow-legged frogs are often driven by the hydrologic conditions affecting recruitment (Rose et al, 2021); the cohort of females that would be breeding for the first time is age 2 in our focal populations (Gonsolin, 2010). We tested for serial autocorrelation in the number of clutches observed because the number of females breeding in a given year is often significantly correlated with the number that laid eggs the previous year (Kupferberg et al, 2012;Rose et al, 2021).…”