Estimating the survival of unobservable life stages for a declining frog with a complex life history (original) (raw)
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Population Ecology, 2010
The development of methods providing reliable estimates of demographic parameters (e.g., survival rates, fecundity) for wild populations is essential to better understand the ecology and conservation requirements of individual species. A number of methods exist for estimating the demographics of stage-structured populations, but inherent mathematical complexity often limits their uptake by conservation practitioners. Estimating survival rates for pond-breeding amphibians is further complicated by their complex migratory and reproductive behaviours, often resulting in nonobservable states and successive cohorts of eggs and tadpoles. Here we used comprehensive data on 11 distinct breeding toad populations (Bufo calamita) to clarify and assess the suitability of a relatively simple method [the Kiritani-Nakasuji-Manly (KNM) method] to estimate the survival rates of stage-structured populations with overlapping life stages. The study shows that the KNM method is robust and provides realistic estimates of amphibian egg and larval survival rates for species in which breeding can occur as a single pulse or over a period of several weeks. The study also provides estimates of fecundity for seven distinct toad populations and indicates that it is essential to use reliable estimates of fecundity to limit the risk of under-or overestimating the survival rates when using the KNM method. Survival and fecundity rates for B. calamita populations were then used to define population matrices and make a limited exploration of their growth and viability. The findings of the study recently led to the implementation of practical conservation measures at the sites where populations were most vulnerable to extinction.
2010
I am indebted to the large number of people who have been instrumental to my progress through graduate school. First, I would like to thank Bryce Maxell, who introduced me to the Columbia spotted frog and his study site in the Bitterroot Mountains. Thanks to Bryce's tireless work setting up and developing this demographic monitoring, I have been able to ask questions I never would have been able to in a typical graduate project. I would like to thank Jeanne Franz for all her assistance with questions great and small over the years, and Dan Pletscher for his tireless devotion to the Wildlife Biology program. Thanks to the College of Forestry and Conservation, the Graduate School, and
Use of integrated modeling to enhance estimates of population dynamics obtained from limited data
2007
Demographic data of rare and endangered species are often too sparse to estimate vital rates and population size with sufficient precision for understanding population growth and decline. Yet, the combination of different sources of demographic data into one statistical model holds promise. We applied Bayesian integrated population modeling to demographic data from a colony of the endangered greater horseshoe bats (Rhinolophus ferrumequinum). Available data were the number of subadults and adults emerging from the colony roost at dusk, the number of newborns from 1991 to 2005, and recapture data of subadults and adults from 2004 and 2005. Survival rates did not differ between sexes, and demographic rates remained constant across time. The greater horseshoe bat is a long-lived species with high survival rates ( first year: 0.49 [SD 0.06]; adults: 0.91 [SD 0.02]) and low fecundity (0.74 [SD 0.12]). The yearly average population growth was 4.4% (SD 0.1%) and there were 92 (SD 10) adults in the colony in year 2005. Had we analyzed each data set separately, we would not have been able to estimate fecundity, the estimates of survival would have been less precise, and the estimate of population growth biased. Our results demonstrate that integrated models are suitable for obtaining crucial demographic information from limited data. Resumen: A menudo, los datos demográficos de especies raras y en peligro son demasiado escasos para estimar las tasas vitales y el tamaño poblacional con precisión suficiente para comprender el crecimiento y declinación poblacional. Sin embargo, la combinación de diferentes fuentes de datos demográficos en un modelo estadístico es prometedora. Aplicamos un modelo poblacional Bayesiano integrado a datos demográficos de una colonia de murciélagos en peligro (Rhinolophus ferrumequinum). Los datos disponibles eran el número de subadultos y adultos emergentes del dormidero colonial al atardecer, el número de neonatos de 1991 a 2005, y datos de recaptura de subadultos y adultos de 2004 y 2005. Las tasas de supervivencia no fueron diferentes entre sexos, y las tasas demográficas permanecieron constantes en el tiempo. Esta especie de murciélago es longeva, tiene altas tasas de supervivencia (primer año: 0.49)[DS 0.06]; adultos: 0.92[DS 0.02] y fecundidad baja (0.74[DS 0.12]). El crecimiento poblacional medio anual fue de 4.4% (DS 0.1%) y hubo 92 (DS 10) adultos en la colonia en 2005. De haber analizado cada conjunto de dados por separado, no hubiéramos podido estimar la fecundidad, las estimaciones de supervivencia hubieran sido menos precisas y la estimación del ‡ Integrated Population Modeling of Bats Schaub et al. crecimiento poblacional hubiera sido sesgada. Nuestros resultados demuestran que los modelos integrados son adecuados para la obtención de información demográfica crucial a partir de datos limitados. Palabras Clave: conservación de especies, datos de historia de vida, demografía, modelo estado-espacio, modelo poblacional integrado, monitoreo, murciélagos, Rhinolophus ferrumequinum
Bulletin of Mathematical Biology, 2012
We derive point and interval estimates for an urban population of green tree frogs (Hyla cinerea) from capture-mark-recapture field data obtained during the years 2006-2009. We present an infinite-dimensional least-squares approach which compares a mathematical population model to the statistical population estimates obtained from the field data. The model is composed of nonlinear first order hyperbolic equations describing the dynamics of the amphibian population where individuals are divided into juveniles (tadpoles) and adults (frogs). To solve the least-squares problem, an explicit finite difference approximation is developed. Convergence results for the computed parameters are presented. Parameter estimates for the vital rates of juveniles and adults are obtained, and standard deviations for these estimates are computed. Numerical results for the model sensitivity with respect to these parameters are given. Finally, the above-mentioned parameter estimates are used to illustrate the long-time behavior of the population under investigation.
Partial Life-Cycle Analysis: A Model for Birth-Pulse Populations
Ecology, 2001
Matrix population models have become standard tools for the demographic analysis of age-or stage-structured populations. Although age-classified (Leslie) matrix models make maximum use of age-specific demographic data, age at first reproduction, which has been suggested to be an important life-history variable, does not appear as an explicit parameter in these models. Consequently, the sensitivity of population growth rate to changes in age at first reproduction cannot be calculated using standard techniques. Agespecific demographic data to parameterize age-structured models are difficult to collect, and models that can be parameterized with partial demographic data (''partial life-cycle models'') have been developed. Partial life-cycle models are based on life-history stages, and these models can also be used to calculate sensitivity of population growth rate to changes in various life-history variables, including ages at first and last reproduction. Here, we present a partial life-cycle model appropriate for situations where demographic data are collected immediately after the birth pulse (post-breeding census). We present methods of parameterizing the partial life-cycle model, and derive formulas for calculating the sensitivity of the population growth rate to changes in model parameters, including ages at first and last reproduction. We analyzed life-table data for several species of mammals using the partial life-cycle model and found that results of our partial life-cycle model compare favorably with those obtained from the corresponding age-classified models.
Oikos, 2008
Long-lived species show delayed maturity and generally skip breeding sites on a given year, causing difficulties in estimating demographic parameters. A novel multi-state capture Á recapture model (model G for general) is proposed to estimate survival and recruitment. Model G considers long and short periods of non-attendance at the breeding site. Model G is compared against a reduced model (model R) to test if pre-breeders skip the studied site a given year, potentially prospecting other breeding habitats.
Canadian Journal of Zoology, 1995
Syntopic populations of bullfrogs (Rana catesbeiana), green frogs (Rana clamitans), and mink frogs (Rana septentrionalis) were monitored between May and October in each of 1985 through 1987 and 1991 through 1993 in Algonquin Provincial Park, Ontario, Canada. We assessed the descriptive and predictive utility of a dichotomous system for classification of anuran life histories by testing the hypothesis that large body size and large clutch size are associated with a survivorship curve in which mortality is highest for very small individuals. Ages of individuals were estimated from size-frequency and recapture data. Survivorship and longevity were estimated from standing age distributions smoothed with the log-polynomial method. Survivorship was also estimated by comparing the number of animals in an age-class in a given year with the number in the next age-class in the next year. Age distributions were unstable in all three species. The strengths and weaknesses of both methods of estimation of survivorship are discussed.
Temporal correlations among demographic parameters are ubiquitous but highly variable across species
Ecology Letters
Temporal correlations among demographic parameters can strongly influence population dynamics. Our empirical knowledge, however, is very limited regarding the direction and the magnitude of these correlations and how they vary among demographic parameters and species’ life histories. Here, we use long‐term demographic data from 15 bird and mammal species with contrasting pace of life to quantify correlation patterns among five key demographic parameters: juvenile and adult survival, reproductive probability, reproductive success and productivity. Correlations among demographic parameters were ubiquitous, more frequently positive than negative, but strongly differed across species. Correlations did not markedly change along the slow‐fast continuum of life histories, suggesting that they were more strongly driven by ecological than evolutionary factors. As positive temporal demographic correlations decrease the mean of the long‐run population growth rate, the common practice of ignori...
Empirical and theoretical studies of population trends and extinction risks
2010
Empirical and theoretical approaches are needed to solve the current problem of increased extinction risk for many species. Thus, this thesis focuses on: (1) ways to estimate population trends for a large number of species, and (2) a predictive framework for identifying vulnerable populations from species traits or life history traits to allow for more proactive conservation actions. I estimated long-term population trends and range-abundance dynamics of longhorn beetles using Natural History Collections. In general, negative population trends were not accompanied by declines in range, but range increased among species with increasing populations. The analysis also exemplified how the results can be used in the red listing process. Linking life history traits and two metrics of extinction risk (population trend and red list classification) in long horn beetles showed that generation time, overwintering stage, larval host plant specialisation, adult activity period and body size were related to extinction risk, often with interaction effects between predictor variables.
Assessing Long-Term Population Trends of Wood Frogs Using Egg-Mass Counts
Journal of Herpetology, 2011
In North America, most efforts to monitor pond-breeding anurans have focused on call surveys. Egg-mass counts offer an alternative monitoring strategy that has been used extensively in Europe because this technique can produce precise and accurate estimates of annual reproductive effort at many study sites. We surveyed egg masses of Wood Frogs (Lithobates sylvaticus) at 18 ponds for up to 16 years from 1993-2008 in the largest contiguous forest tract in southern New England. We detected an average of 441.5 ± 343.7 egg masses per pond. Based on annual egg-mass counts, coefficients of variation (CV) were slightly higher than previous estimates for this species. We detected no relationship between mean annual population size and CV or between length of time series and CV. Population fluctuations in these ponds exhibited evidence of annual synchrony, in part because annual fluctuations at individual ponds were large enough that it was difficult to assess differences in population trends among ponds. However, the overall trend suggests this population was probably increasing slightly, which was expected because ponds were located in contiguous forest that remained intact during the study. Egg-mass counts appear to represent a feasible technique to monitor Wood Frog populations, given that all local breeding ponds are monitored.