Review and Synthesis of Estimated Vital Rates for Terrestrial Salamanders in the Family Plethodontidae
Population models depend on reliable estimates of vital rates, yet for many taxa, such estimates and how they vary in response to spatial or temporal environmental gradients are lacking. The goal of this review was to determine whether existing estimates of vital rates for temperate, direct-developing plethodontid salamanders (subfamily Plethodontinae) could be used to reasonably project values for populations or species where such estimates are lacking, or whether current estimates are biased in a manner that limits their utility. We synthesized current knowledge of stage-specific survival rates, age- and size-at-maturity at first clutch, and clutch frequency. We tested for expected correlations among published vital rates (e.g., age at maturity and survival) and between vital rates and factors such as body size or latitude. We used matrix projection models to judge whether published estimates were reasonably possible for stable salamander populations. The largest number of published vital rates were for clutch size, clutch frequency (proportion of females with clutches), size at maturity or first clutch, and age at maturity or first clutch, though the latter vital rate is primarily inferred from size distribution and growth rate data. Among these vital rates, we found expected correlations with body size and latitude suggesting these rates were reasonable and somewhat predictable among species or populations. In contrast, there were few estimates of egg hatch rate or juvenile or adult survival. Hatch and survival rate estimates were widely variable; estimates seldom included measures of uncertainty, but when uncertainty measurements were included, they were generally high. Based on projection models, few survival estimates were likely unbiased or realistic for stable populations given other salamander vital rates. Additionally, few studies quantified how vital rates vary with spatial or temporal environmental gradients. We outline the key knowledge gaps that limit basic demographic modeling of these remarkably common, influential, and otherwise well-studied salamanders, and make recommendations for future research efforts.
Across all three plots, genera are coded symbolically: Aneides = triangle, Desmognathus = square, Plethodon = circle, Speleomantes = ‘
,' and the one combined estimate for Plethodon and Ensatina = diamond. The fitted regression lines are black, while the upper and lower 95% confidence limit lines are gray. Plot A shows the relationship between estimated annual adult or non-age-specific survival probability and study duration for temperate, direct-developing salamanders in the subfamily Plethodontinae. In this plot, open symbols denote estimates from studies in ‘undisturbed' habitats, and black symbols denote estimates from studies in ‘disturbed' habitats. Plot B shows the relationship between estimated adult or non-age-specific survival probability and snout–vent length. Gray points indicate estimates from count data, white points indicate estimates from capture–mark–recapture studies, and error bars indicate standard errors if reported by the study. Plot C shows the relationship between estimated adult or non-age-specific survival probability and age at maturity (in years). Point colors indicate study type, and error bars are standard errors, as in plot B.
Regression relationships between vital rates for temperate, direct-developing salamanders in the family Plethodontidae. Across plots, genera are coded by shapes: Aneides by triangles, Desmognathus by squares, Ensatina by diamonds, Phaeognathus by a ‘+' symbol, Speleomantes by a ‘
' symbol, and Plethodon by circles. Gray lines represent 95% confidence intervals; the black line is the fitted regression line. Plots A and B show the two sub-relationships included in the multivariate regression analysis for age at maturity, respectively: age at maturity and snout–vent length, and age at maturity and latitude of the studied population. Plot C shows the relationship between frequency of reproductive females, which is interpreted as the annual probability of female reproduction, and latitude of the studied population. Plot D shows the relationship between clutch size and body size. The solid lines represent the fitted regression line and 95% confidence limits when all points are included in the analysis, while the dashed lines represent the fitted regression line and 95% confidence limits when the outlying point, a single estimate for Phaeognathus hubrichti, is excluded. Phaeognathus hubrichti is the longest species but has one of the smallest reported clutch sizes. When Phaeognathus hubrichti was removed, the slope increased from 0.204 to 0.317, and the adjusted R2 increased from 0.264 to 0.447.
Stable population growth (λ = 1) isoclines for a generic, temperate, direct-developing salamander in the subfamily Plethodontinae. Each isocline represents different combinations of female age at first reproduction, adult (age 5+) and juvenile (age 1) annual survival, annual probability (or proportion) a female lays a clutch, and probability an egg hatches. Egg hatch rates are organized by columns increasing from left to right, and age at maturity is arranged as rows increasing from top to bottom. Probability of laying a clutch is represented by the different isoclines within each plot. Each point represents a pair of published survival estimates for a given species (Supplementary Table 1; see Data Accessibility), with the genus Aneides represented by triangles, Desmognathus by squares, Plethodon by circles, and the single study that produced a combined survival estimate for Plethodon and Ensatina by diamonds. Dark gray points are those that fall to the left/below all isoclines indicating that those survival rate estimates would result in a declining population (λ < 1) under all estimates of probability of laying a clutch at that age at maturity and egg hatch rate, and are likely unrealistically biased estimates assuming the population was stable.
Contributor Notes
Associate Editor: C. Bevier.