SELECT RESEARCH SUMMARIES
Integral Projection Modeling for California Tiger Salamanders
The California tiger salamander (CTS; Ambystoma californiense; Figure 1) is listed as federally Threatened under the US Endangered Species Act. Though we still have much to learn about the ecology of this species, we know more about CTS than most other amphibians in the US thanks to a legacy of long-term field studies. Dr. Chris Searcy and I have applied the current knowledge on CTS to the development of a stochastic integral projection model (IPM) for the species (Messerman et al., 2023). In contrast to traditional amphibian matrix models, the IPM approach allows vital rates (e.g., survival, growth, and reproduction) to be estimated for both discrete life stages and as a function of continuous variables like body size. Accounting for body size is likely to improve model predictions, given mounting evidence that body size correlates positively with terrestrial amphibian survival and lifetime reproductive output. Constructing the IPM under a Bayesian statistical framework facilitates the incorporation of uncertainty into model predictions. We have used the IPM for a population viability analysis that reveals minimum upland habitat requirements for CTS under distinct pond size and climate scenarios (Messerman et al., 2023).
Figure 1. A California tiger salamander (Ambystoma californiense) from Santa Cruz County, California.
Describing Metamorph Ecology of an Endangered Salamander
The Santa Cruz long-toed salamander (SCLTS; Ambystoma macrodactylum croceum; Figure 2) has been under federal protection since 1967, prior to the creation of the US Endangered Species Act. This Endangered subspecies is only found in portions of Santa Cruz and Monterey Counties, California, where it is increasingly threatened by habitat fragmentation and saltwater intrusion into breeding ponds. Despite growing anthropogenic pressures and over 50 years of recognized conservation need, little is understood about the ecology of SCLTS. This lack of information hinders species recovery efforts by obscuring both optimal management strategies and trends in population dynamics. As a postdoc in the lab of Dr. Chris Searcy, I have established a long-term drift fence study around 10 SCLTS breeding ponds. By monitoring newly metamorphosed salamanders (Figure 2A) each year throughout the metamorph emergence period, we will make some of the first estimates of the natural metamorph body size distribution, habitat preferences, and emergence phenology of SCLTS during this important transitional life stage. Larger metamorph body size correlates with increased juvenile survival, reduced age at maturation, and greater reproductive output (i.e., greater fitness). Identifying the mean and variance of body size among free-living metamorphs will establish benchmarks for determining the 'health' of metamorphs when monitoring other populations and when assessing captive propagation efforts. Moreover, by combining metamorph cohort number and body size data in an integral projection model built for the Threatened California tiger salamander (CTS; A. californiense; see above), we will calculate a unified metric of metamorph productivity for each study pond in each year. Using this metric as a response variable and numerous biotic and abiotic measures as predictors, we will identify optimal habitat characteristics for SCLTS population recruitment. Lastly, documenting annual dates of SCLTS metamorph emergence will indicate minimum required hydroperiods for extant and constructed breeding ponds to achieve SCLTS metamorphosis, and will guide the timing of future monitoring efforts.
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Figure 2. Metamorph (A) and adult (B) life stages of the Santa Cruz long-toed salamander (Ambystoma macrodactylum croceum).
Estimating Survival Rates of Juvenile Salamanders
The vital rates (e.g., survival, growth, and reproduction) of distinct life stages within a species are known to influence the growth and persistence of populations. Studies describing stage-specific vital rates, and the factors that shape variation in these rates across species and populations, can help to improve our understanding of population dynamics and species distributions. Moreover, the insights gained from such studies can guide the prioritization of populations for conservation efforts and inform the selection of management strategies under predicted climate and land use changes. Yet life stage-specific vital rate estimates, and characterizations of the physiological responses that influence demographic rates under variable conditions, are lacking for most species.
Amphibians are experiencing drastic population declines across the globe. As amphibians exhibit complex life cycles, wherein unique life stages rely on divergent resources and habitat types, examinations of distinct life stages may be particularly critical for enhancing amphibian conservation efforts. Specifically, juveniles are known to play a critical role in amphibian population dynamics, but are relatively understudied compared with other life stages due to their small body size and often elusive life histories.
In collaboration with Dr. Ray Semlitsch and Dr. Manuel Leal at the University of Missouri, I estimated juvenile survival rates among three ambystomatid species (A. annulatum, A. maculatum and A. texanum) by conducting an 11-month capture-mark-recapture study within semi-natural enclosures (Figure 3; Messerman et al., 2020). I found juvenile survival rates to be constant through time and comparable among species. These similarities indicated that vital rate estimates from congeneric, ecologically similar species can serve as robust place-holder information to examine the population dynamics of the many amphibian species for which stage-specific data are lacking.
Figure 3. Photograph of a 4-m^2 pen used to study juvenile ambystomatid survival rates at the Botany Greenhouse, Research Park, Columbia, Missouri, USA, featuring ultraviolet-resistant baffles and edging, 6 artificial burrows, 4 pitfall traps, leaf litter, and shade cloth that has been opened to allow for recapture protocols.
Identifying Factors that Influence Juvenile Survival
Juvenile ambystomatids must maintain net energetic and hydric balance to survive the novel terrestrial environment once they leave the pond. To start understanding how juveniles accomplish this, I've examined inter- and intraspecific patterns in juvenile ambystomatid physiological regulation of water loss and metabolism among five species (A. annulatum, A. maculatum, A. opacum, A. talpoideum and A. texanum) across an ~200 km latitudinal gradient in Missouri, USA (Messerman & Leal, 2020). By performing flow-through respirometry on juveniles, I found respiratory surface area water loss (RSAWL) and standard metabolic rates (SMR) to differ between species. Though SMR showed no relationship with locality, RSAWL was weakly positively correlated with latitude. This suggested that juvenile ambystomatids exposed to warmer average conditions at more southern latitudes, and thus a higher desiccation risk, may demonstrate the locally adaptive regulation of RSAWL compared with juveniles from northern populations. Given common rearing, it is likely that differences among species and populations had a genetic basis, and were not solely the result of phenotypic plasticity.
I next identified relationships between likelihoods of A. maculatum and A. opacum juvenile survival and individual body mass, physiological rates of water loss, and standard metabolic rates under variable semi-natural environmental conditions (Figure 3; Messerman & Leal, 2022). Examining known survival at three time points during the seven-month study, I found juveniles with higher initial body mass and/or lower SMR to have a higher likelihood of survival, particularly under warm initial conditions. There was no effect of RSAWL on survival. Acclimation experiments with surviving salamanders revealed that thermal tolerances and SMR demonstrated plastic responses to warming (Messerman et al., 2022). Further, a simulation of juvenile survival following high temperatures suggested that the two study species may demonstrate diverging juvenile survival rates after being thermally challenged due to distinct acclimation strategies.
Assessing Metapopulation Dynamics in Two Vulnerable Salamanders
The Santa Cruz long-toed salamander (SCLTS; Ambystoma macrodactylum croceum) co-occurs with the California tiger salamander (CTS; A. californiense; Figure 4) across ~50% of the SCLTS range. While we know little about the ecology of SCLTS, numerous studies have been conducted to understand CTS ecology and population dynamics. I am working with Dr. Chris Searcy to establish long-term population monitoring at 5 ponds with SCLTS only, 5 ponds with CTS only, and 5 ponds with both species. Multi-year and multi-population data collection will allow us to adapt a single population model for CTS (see above) into a metapopulation viability assessment tool by estimating spatial and temporal synchrony in population dynamics among the 10 populations of each study species. Once this tool is completed for CTS, we will begin applying it to understand and recover metapopulations of SCLTS. Further, our study design will allow us to examine whether CTS and SCLTS interact where they co-occur such that the presence of the congener alters the degree of synchrony among populations, changes metapopulation productivity, or shifts the phenology of either species.
Figure 4. PhD Candidate Leyna Stemle collects body size data from a CTS captured at a drift fence in Santa Cruz County, California in June 2021.
